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6 Frequency responses of grooved media
6.1 The problem stated

The subject of this chapter raises emotions varying from deep pedantry to complete lack of understanding. Unfortunately, there has never been a clear explanation of all the issues involved, and the few scraps of published material are often misquoted or just plain wrong, whilst almost-perfect discographical knowledge is required to solve problems from one recording-company to the next. Yet we need a clear understanding of these issues to make acceptable “service copies”, and we are compelled to apply the lessons rigorously for “objective copies”. (My research shows we now have the necessary level of understanding for perhaps seventy-five percent of all grooved media before International Standards were developed). But for the “warts-and-all” copy, it isn’t an issue of course.

Grooved disc records have never been recorded with a flat frequency response. The bass notes have always been attenuated in comparison with the treble, and when electrical methods are used to play the records back, it is always implied that the bass is lifted by a corresponding amount to restore the balance. You may like to demonstrate the effect using your own amplifier. Try plugging a good quality microphone into its “phono” input, instead of a pickup-cartridge. If you do, you will notice a boomy and muffled sound quality, because the phono circuitry is performing the “equalisation” function, which will not happen when you use a “Mic” input.

The trouble is that the idea of deliberately distorting the frequency response only took root gradually. In the days of acoustic recording (before there was any electronic amplification), it was a triumph to get anything audible at all; we shall be dealing with this problem in Chapter 11. Then came the first “electrical recording” systems. (I shall define this phrase as meaning those using an electronic amplifier somewhere - see Ref. 1 for a discussion of other meanings of the phrase, plus the earliest examples actually to be published). At first, these early systems were not so much “designed”, as subject to the law of “the survival of the fittest.” It was some years before objective measurements helped the development of new systems.

This chapter concentrates on electrical recordings made during the years 1925 to 1955, after which International Standards were supposed to be used. I shall be showing equalisation curves the way the discs were recorded. If you are interested in restoring the sound correctly, you will have to apply equalisation curves which are the inverse of these; that is, the bass needs to be boosted rather than cut.

The major reason for the importance of this issue is different from the ones of restoring the full “power-bandwidth product” that I mentioned in Chapter 1. Incorrect disc equalisation affects sounds right in the middle of the frequency range, where even the smallest and lowest-quality loudspeaker will display them loud and clear – usually at a junction between a modern recording and an old one. The resulting “wooliness” or “harshness” will almost always seem detrimental to the archived sound.

6.2 A broad history of equalisation

Electrical recording owed its initial success to the Western Electric recording system. Although this was designed using scientific principles to give a “flat frequency response,” it had at least one undefined bass-cut which needs correction today, and other features if we are ever to achieve “high fidelity” from its recordings. So its success was partly accidental. The recording equipment dictated the equalisation, rather than the other way round.

During the next twenty years the whole process of making an acceptable record was a series of empirical compromises with comparatively little scientific measurement. During the Second World War accurate methods of measurement were developed, and after the war the knowledge of how to apply these to sound reproduction became more widely known. Thus it became possible to set up “standards”, and modify equipment until it met those standards. Thus professionals (and, later, hi-fi fanatics) could exchange recordings and know they would be reproduced correctly.

This last phase is particularly troublesome. There were nine “standards” which users of disc recording equipment were invited to support between 1941 and 1953, and the ghastly details will be listed in sections 6.62 onwards. If you put your political thinking-cap on, and conclude that such chaos is typical of a Free Market Economy, I reply that State Monopolies could be just as bad. For example, between 1949 and 1961 the British Broadcasting Corporation had three “standards” used at once, none of which were International ones!

Most record manufacturers had different recipes which we can describe in scientific language. The number of recipes isn’t just because of the “Not Invented Here” syndrome, but there was at least one manufacturer who kept his methods a trade secret because he feared his competitive advantage would be harmed! Two international standards were established in 1955, one for coarsegroove records and one for microgroove records. The latter has several names, but most people call it by the name of the organisation which promoted it, the Recording Industries Association of America. It is on any present-day “Phono Input,” and I shall call it “RIAA” from now on.

So if you are interested in the faithful reproduction of pre-1955 records, you should at least know that an “equalisation problem” may exist.

6.3 Why previous writers have gone wrong

This section is for readers who may know something already. It summarises three areas in which I believe previous writers have got things wrong, so you can decide whether to read any more.

  • (1) Equalisation is largely independent of the make of the disc. It depends only upon who cut the master-disc and when. (I shall be using the word “logo” to mean the trademark printed on the label, which is something different again!) I’m afraid this implies you should be able to detect “who cut the master-disc and when” by looking at the disc, not the logo. In other words, you need discographical knowledge. I’m afraid it’s practically impossible to teach this, which may explain why so many previous writers have made such a mess of things.
  • (2) It is best to define an equalisation curve in unambiguous scientific language. American writers in particular have used misleading language, admittedly without committing gross errors along the way. I shall be using “microseconds”, and shall explain that at the end of section 6.7 below.
  • (3) The names of various “standards” are themselves ambiguous. For instance, when International Standards became operational in 1955, most old ones were re-named “the new Bloggs characteristic” or words to that effect. I recently found a microgroove nitrate dated 24-1-57 whose label bore the typed message: “Playback: New C.C.I.R., A.E.S., Orthoacoustic.” (This was clearly RIAA, of course!) Similar considerations apply to curves designed for one particular format (for example, American Columbia’s pioneering long-playing disc curve of 1948), which may be found on vintage pre-amplifiers simply called “LP” only - or worse still “Columbia” only - when neither name is appropriate, of course.


6.4 Two ways to define “a flat frequency response”

Equalisation techniques are usually a combination of two different systems, known for short as “constant velocity” and “constant amplitude.” The former, as its name implies, occurs when the cutting stylus vibrates to and fro at a constant velocity whatever the frequency, provided the volume remains the same. This technique suited an “ideal” mechanical reproducing machine (not using electronics), such as the Orthophonic Victrola and its HMV equivalent gramophone of 1926. These scientifically-designed machines approached the ideal very closely. On such records, as the frequency rises the amplitude of the waves in the grooves falls, so the high frequencies are vulnerable to surface noise. On the other hand low frequencies cause high amplitudes, which have the potential for throwing the needle out of the groove (Fig. 1a). Thus all disc records are given some degree of bass cut compared with the idealised constant-velocity technique.

Constant-amplitude recording overcomes both these difficulties. Given constant input, if varying frequencies are cut, the amplitude of the waves in the groove stays the same (Fig. 1b). Thus the fine particulate matter of the record is always overshadowed and the hiss largely drowned, while the low notes are never greater than the high notes and there is less risk of intercutting grooves. Unfortunately, the result sounded very shrill upon a clockwork gramophone.

Most record-companies therefore combined the two systems. In the years 1925-1945, most European record companies provided constant-velocity over most of the frequency range to give acceptable results on acoustic gramophones, but changed to constant-amplitude for the lower frequencies (which were generally below the lower limit of such machines anyway) to prevent the inter-cutting difficulty. The scene was different in America, where the higher standard of living encouraged electrical pickups and amplifiers, and it was possible to use a greater proportion of constant-amplitude thanks to electronic compensation. From the mid-1930s, not only did many record manufacturers use a higher proportion of constant-amplitude, but another constant-amplitude section may have been added at high frequencies, which is usually called “pre-emphasis.” More high-frequency energy is recorded than with the “constant-velocity” system. Thus the wanted music dominates hiss and clicks, which are played back greatly muffled without the music being touched. An equivalent process occurs today on FM broadcasts, TV sound, and some digital media.

In principle, magnetic pickups (not crystal or ceramic ones) give a constant voltage output when playing constant-velocity records. But this can be converted to the equivalent of playing a constant-amplitude record by applying a treble cut (and/or a bass boost) amounting to 6 decibels per octave. This can be achieved in mono with just two simple components - a resistor and a capacitor - so it is a trivial matter to convert an electronic signal from one domain to the other.

What exists on most electrically-recorded discs can be defined by one or more frequencies at which the techniques change from constant-amplitude to constant-velocity, or vice versa. “Phono” equalisers are designed to apply 6dB/octave slopes to the appropriate parts of the frequency spectrum, so as to get an overall flat frequency response. And graphs are always drawn from the “velocity” viewpoint, so constant-velocity sections form horizontal lines and constant-amplitude sections have gradients.

If you wish to reproduce old records accurately, I’m afraid I shan’t be giving you any circuit diagrams, because it depends very much on how you propose to do the equalisation. Quite different methods will be needed for valves, transistors, integrated circuits, or processing in the digital domain; and the chronology of the subject means you will only find circuits for valve technology anyway. Personally, I don’t do any of those things! I equalise discs “passively,” using no amplification at the equalisation stage at all; but this implies neighbouring circuitry must have specific electrical impedances. This technique automatically corrects the relative phases (section 2.11), whether the phase changes were caused by acoustic, mechanical, or electronic processes in the analogue domain.

Next we have the problem of testing such circuitry. Over the years, many record companies have issued Frequency Test Discs, documenting how they intended their records to be reproduced. (Sometimes they published frequency response graphs with the same aim, although we don’t actually know if they were capable of meeting their own specifications!). Such published information is known as “a Recording Characteristic,” and I follow Terry’s definition of what this means (Ref. 2): “The relation between the R.M.S electrical input to the recording chain and the R.M.S velocity of the groove cut in the disc.” This gives rise to the following thoughts.

6.5 Equalisation ethics and philosophy

In the late ’twenties, a flat frequency response seems to have been the dominant consideration in assessing fidelity. Regardless of any other vices, a piece of equipment with a wide flat frequency range was apparently described as “distortionless.” (Ref. 3).

Unless there is definite evidence to the contrary, we should therefore assume that 1920s engineers wanted their recordings to be reproduced to a flat frequency response, with the deficiencies of their equipment reversed as far as possible. This would certainly be true until the mid-thirties, and is sometimes true now; but there is a counter-argument for later recordings.

I mentioned how empirical methods ruled the roost. At the forefront of this process was the session recording engineer, who would direct the positions of the performers, choose a particular microphone, or set the knobs on his control-desk, using his judgement to get the optimum sound. As archivists, we must first decide whether we have the right to reverse the effects of his early microphones or of his controls.

The engineer may have had two different reasons behind his judgements, which need to be understood. One is an aesthetic one - for example, using a cheap omnidirectional moving-coil microphone instead of a more expensive one on a piano, because it minimises thumping noises coming from the sounding-board and clarifies the “right hand” when it is mixed with other microphones. Here I would not advocate using equalisation to neutralise the effects of the microphone, because it was an “artistic” judgement to gain the best overall effect.

The other is to fit the sound better onto the destination medium. In 78rpm days, the clarity would perhaps be further enhanced to get it above the hiss and scratch of the shellac. For reproduction of the actual 78 this isn’t an issue; but if you are aiming to move the sound onto another medium, e.g. cassette tape or compact disc, it might be acceptable to equalise the sound to make it more faithful, so it suits today’s medium better. (After all, this was precisely how the original engineer was thinking). This implies we know what the original engineer actually did to fit the sound onto his 78. We either need industrial archaeology, or the original engineer may be invited to comment (if he’s available). I am very much against the principle of one lot of empirical adjustments being superimposed on someone else’s empirical adjustments.

Personally I take the above argument to its logical conclusion, and believe we should only compensate for the microphone and the controls if there were no satisfactory alternatives for the recording engineer. Thus, I would compensate for the known properties of the 1925 Western Electric microphone, because Western Electric licencees had only one alternative. It was so conspicuously awful that contemporary engineers almost never used it. But listeners soon discovered that the silky sound of Kreisler’s violin had been captured more faithfully by the acoustic process, despite the better Western Electric microphone being used. Subsequent measurements discovered the reason for its “acid sound.” Therefore I consider it reasonable to include the effects of this microphone in our equalisation. It is debatable whether the microphone counts as part of the “recording chain” in Terry’s definition of a “recording characteristic” - it wouldn’t be considered so today, because of the engineer’s conscious judgements - but when there were no alternatives, I think it may be reasonable to include it.

6.6 Old frequency records as evidence of characteristics

The concept of Frequency Records providing hard evidence of how old recording equipment performed is a modern idea, not considered by engineers in the past. Their concern was (usually) for calibrating reproducing equipment, not to provide us with concrete evidence of their weaknesses! Making test records is something this writer has attempted, and I can tell you from experience it isn’t easy. It is one thing to claim a flat frequency response to 15kHz; it is quite another to cut a flat frequency record to prove it. History shows that several “kludges” were adopted to achieve such records with the technology of the time. I shall be pointing out some of these kludges, and indicating where they might mislead a modern playback operator.

Given a frequency record, does it document the objective performance of the equipment, or does it document what the manufacturer fondly hoped his equipment should be doing? To use the word I employed above, do we know whether the disc has been “kludged” or not? I have stumbled across several ways of answering this question.

The first is to study the evolving performance of professional recording equipment through history, and put the frequency record in its chronological context. (Could a machine have cut 10kHz in 1944?). The next step is to study other sound recordings made by such equipment. (Do any of them carry frequencies as high as 10kHz?).

Another guide is to look at the physical features of the frequency disc. Does it appear that it was intended to be used as a standard of reference? If it carries a printed label, there would presumably be many copies, and that implies it was meant to be used; if someone has taken the trouble to label the frequencies by scrolls or announcements, that implies the disc was meant to be used; and if we have two contemporary records made by the same manufacturer and one is technically better than the other, then there is a higher probability that the “good” one was meant to be used. Thus we can say that the one intended to be used as a frequency disc is more likely to have been “kludged” to make it fit an intended characteristic and render it relatively “future-proof,” while the others are more likely to document the actual everyday performance of the machine.

Many frequency records which were “meant to be used” carry tones at discrete frequencies. Unless the label says something specifically to the contrary, these cannot be used to assess the frequency response of the recorder, since there is no evidence that someone hasn’t made adjustments between the different frequencies. If, however, the documentation makes it clear that the frequencies are recorded to some characteristic, it is possible to plot them on a graph (joining up several sides in the process if necessary) and achieve an overall frequency curve (or the intended frequency curve) for the recording-machine.

Even better evidence comes from a “sweep” frequency run, when a variable oscillator delivers frequencies covering a considerable range in a continuous recording. However, this too can be “cheated”. A “sweep” recorded under laboratory conditions might not be representative of the hurly-burly of practical recording life, although it might very well document the intended performance. Other kludges were sometimes made to a machine to make it record to a particular characteristic, and we shall meet at least one definite example of such cheating in Section 6.11.

6.7 Two common characteristics

I shall now talk about the two most important equalisations I consider you need for making “service copies”. The majority of European disc records made between 1925 and about 1952 had constant-velocity above about 300Hz and constant-amplitude below about 300Hz. This shape is known conversationally as a “Blumlein shape”, after the EMI engineer who made several significant sound recording inventions. (He employed, but did not actually invent, this shape). The exact turnover frequency may vary somewhat, for example it might be 250Hz. Restoration operators call this “Blumlein 250Hz”, even though the record in question was not cut with Blumlein’s equipment, and there may be no evidence that Blumlein ever used this particular frequency. It is a useful shorthand expression to describe the shape, nothing more. (Fig. 2)

The other important equalisation is the International Microgroove or “RIAA” one, which you probably have anyway. British citizens could find the 1955 version in British Standard 1928: 1955. This dealt with both coarsegroove and microgroove records. Since then there have been a number of revisions, and the current standard is BS 7063: 1989 (which is the same as IEC 98: 1987). This has one minor amendment to the original microgroove characteristic. I shall try to describe them both in words.

The international coarsegroove characteristic retained constant-velocity between 300Hz and 3180Hz, so most of the range of acoustic gramophones remained on the ideal curve, but the sections from 50 to 300Hz and from 3180Hz upwards were made constant-amplitude. But because it dates from 1955 when the coarsegroove format was nearly dead, and the subject matter was nearly always mastered on tape first, you should hardly ever need it.

For international standard microgroove, the constant-velocity section ran only from 500Hz to 2120Hz, so more of the frequency range approached constant-amplitude, and surface noise was further diminished. This was actually a compromise between the practices of several leading record companies, all slightly different. But it was found this type of bass cut caused too little amplitude at extremely low frequencies, and turntable rumble was apt to intrude. So the 1955 standard specified constant-velocity for all frequencies below 50Hz. A further development occurred in 1971, when some American record companies were trying the dbx Noise Reduction System on discs (section 9.7). The standard was changed so there should be decreased levels below 25Hz, because it was found that unevenness in this region had dramatic side-effects with such noise reduction systems. But noise reduction never “took off” on disc records in the way that it did on cassettes, and relatively few discs carried significant amounts of sound energy below 25Hz anyway.

Engineers therefore define nearly all characteristics in terms of intersecting straight lines with “flat” or “6dB/octave” sections. But in practice electronic circuits cannot achieve sharp corners, and everyone accepts that the corners will be rounded. The straight line ideals are called “asymptotes”. If there are only two intersecting asymptotes, a practical circuit will have an output three decibels away at the corner, and this intersection is therefore known conversationally as the “three dB point.” Thus we may say “This Blumlein shape has a 3dB point at 250Hz” (and you will see why by referring back to Fig. 2.

Another way of defining the intersection is to specify the ratio of resistance to reactance in an electronic circuit - in other words, to specify the actual circuit rather than its result. For reasons it would take too long to explain here, this method gives an answer in microseconds. (It’s related to the time it would take for the reactance to discharge through the resistance). Earlier, I said the international microgroove curve changed from constant-velocity to constant-amplitude at 2120Hz. This corresponds to a time constant of 75 microseconds. These are just two different ways of saying the same thing.

You could, of course, say the same thing in yet a third way, for example “Recorded level = +13.5dB at 10kHz.” In other words, you could describe the characteristic by its effect at some extreme frequency, rather than at the intersection of the asymptotes. You may frequently encounter this style of description (favoured in America); but I shan’t be using it, because it’s fundamentally ambiguous. Decibels are a method of defining the relation between two things, and if you’re an engineer you immediately ask “plus 13.5dB with respect to what?” A mid-frequency tone, such as 1kHz? Or the asymptote (which, in many cases, is approached but never reached)? Or an idealised “low frequency” (in which case, what about the 500Hz bass-cut - do we “count” it, or don’t we)? And so on. To eliminate the ambiguities, and to prove I’ve done my homework properly, I shall be describing equalisation curves in microseconds only.

6.8 Practical limits to equalisation

As I hinted above, asymptotes of 6dB/octave are easy to achieve in electronics; but I want to draw your attention to some practical limitations. A specification which says a characteristic should be constant-amplitude for all frequencies above 75 microseconds is, of course, impossible to achieve. Quite apart from the obvious mechanical limitations of a cutting stylus doubling its velocity every time the frequency doubles, the electronics cannot achieve this either. If the recording amplifier has to double its output every time the frequency doubles, sooner or later it is going to “run out of amplification.” No practical equipment exists in which this type of asymptote can be achieved.

In practice, equipment was designed to be approximately correct throughout the audio frequency-range, and to terminate at ultrasonic frequencies. It is quite normal for a couple of decibels to separate the theoretical and practical characteristics under these conditions. Unfortunately the differences depend upon the exact compromises made by the designer, and with the exception of an equalisation system built by myself, I do not know of any that have ever been documented! They are probably not very significant to the listener. But you should remember it is easy to destroy an infinite number of decibels of sound during playback, while it is never possible to boost an infinite number of decibels when recording, so there is sometimes an inherent asymmetry between the two processes.

This seems to be the occasion to say that digital techniques are (in general) unsuitable for reversing such equalisation. Slopes of 6dB’s per octave imply responses up to infinity (or down to zero), and digital techniques are inherently incapable of handling infinities or zeros. And if they do, side-effects are generated which defeat the “anti-aliasing filter” and add high-frequency noises (as described in section 3.2), while relative phases (section 2.11) may not be corrected. If “infinite impulse response” algorithms should ever become available, I would consider them; otherwise it’s one of the very few cases where it’s actually better to convert a digital signal back to analogue, re-equalise it, and then convert it back to digital again.

6.9 Practical test discs

In effect, the next two sections will comprise reviews of different frequency records which I have found useful in calibrating pickups and equalisers. From Section 6.20 onwards I shall be dealing with their other role for a restoration operator, documenting the performance of obsolete recording machinery. But first I must stress the importance of having these tools. Do not assume that, simply because you have modern equipment, it will outperform old recording systems. It should be your duty, as well as a matter of courtesy, to make sure your equipment does justice to what former engineers have recorded for you. Most operators get a shock when they play an old test disc with a modern pickup. Some find themselves accusing the test disc of being inaccurate, so widely can the performance deviate from the ideal. Rogue test discs do exist; but I shall mention all the faults I know below, and later when we look at the “industrial archaeology.”

The first thing to say is that any test disc can give the wrong results if it is worn. All other things being equal, wear is worst at high frequencies and high volumes. Thus I prefer test discs with low volumes. Responsible reviewers in hi-fi magazines always used a new disc to test each pickup undergoing review; but this is obviously very expensive. In practice you cannot afford to thrash a new test disc every time you change a stylus, but you may find it difficult to separate the effects of surface noise when measuring a used disc with a meter. The trick is to use an oscilloscope and measure the sinewave with a graticule. Another trick is to use a relatively sluggish meter, rather than a peak-reading instrument. If the high frequencies give consistent results with two or three playings with your standard pickup, they will give valid results for hundreds of such playings, even though background noise may accumulate.

6.10 International standard microgroove test discs

This is usually the most important type of test disc. Not only will the majority of work be to this standard, but it is the starting point from which one develops coarsegroove and non-standard microgroove methods. There are many such discs made commercially. However, many of the test discs in this section have been “kludged” by taking advantage of the 75-microsecond pre-emphasis of the International Standard curve. To cut the highest frequencies onto the master disc, a constant frequency would be fed into the cutter (at about 8kHz), and then the cutting-turntable would be deliberately slowed. The recorded amplitude remained unchanged, of course; and the limitations I expressed in Section 6.8 (plus others such as stereo crosstalk) were circumvented. The discs below are all 33rpm 12-inch LPs unless stated otherwise.

Mono (lateral):

  • B.B.C. (U.K) FOM.2
  • Decca (U.K) LXT.5346
  • Decca (U.K) 45-71123 (7-inch 45rpm)
  • EMI (U.K) TCS.104
  • HMV (U.K) ALP.1599
  • Urania (USA) UPS.1


  • B & K (Denmark) QR.2009 (45rpm)
  • B & K (Denmark) QR.2010 * (both published by Brüel & Kjaer)
  • C.B.S (USA): STR.100, STR.120, STR.130, BTR.150
  • Decca (U.K) SXL.2057
  • DIN (Germany) 1099 111 and 1099 114 (published by Beuth-Vertrieb GmbH, Berlin)
  • DGG (Germany) DIN 45543
  • Electronics World Test Record #1 (USA) (this is a 7-inch 33rpm disc, useful for measuring inner-groove attenuation; the frequency tests are mono)
  • EMI (U.K) TCS.101 (constant frequency bands)
  • EMI (U.K) TCS.102 (gliding tone)
  • High Fidelity (Germany) 12PAL 3720
  • London (USA) PS121
  • Shure (USA) TTR.102 *
  • VEB (East Germany) LB.209, LB.210
  • Victor Company of Japan JIS.21 (spot frequencies)
  • Victor Company of Japan JIS.22 (sweep frequencies)

(*Items marked with asterisk comprise only “sweeps” for use with automatic plotters; the frequency range is swept too fast for manual logging of measurements).

Consumer demonstration records with frequency tests - mono:

  • Acos (U.K) un-numbered (7-inch 45rpm; one side is a re-pressing from
  • Decca 45-71123 above)
  • Urania (USA) 7084, also on Nixa (U.K) ULP.9084. (Five frequencies only)
  • Vox (USA) DL.130

Consumer demonstration records with frequency tests - stereo:

  • Audix (U.K) ADX.301
  • BBC Records (U.K) REC.355 (the frequency test is mono only)
  • C.B.S (USA) STR.101

Sound Canada Magazine (Canada) un-numbered: mono “interrupted sweep”

6.11 Coarsegroove (78rpm) test discs

In my opinion the most important coarsegroove test disc is Decca K.1803, recorded in 1947 (which was also available in North America as London T.4997). This has one side with constant-velocity sweep from 14kHz to 3kHz with an accurately modulated post-war style V-bottomed groove pressed in shellac. It was in the general catalogue for many years, and when copies turn up on the second-hand market they should be pursued. It gives the constant-velocity section of the so-called “Blumlein” shape used by EMI, the world’s largest recording company, between 1931 and 1953; and a similar characteristic was used by many other companies, including Decca itself until 1944.

Unfortunately, nothing’s perfect. The other side covers the range from 3kHz downwards, and suffers from two faults. One is that the level is about 1.5dB higher than side 1. The other is that Decca made a “kludge” at the turnover frequency. Officially, there should be a -3dB point at 300Hz (531 microseconds); but the actual discs show this as a “zero point”, not a “-3dB point.” In other words, someone has adjusted the signals fed into the cutter so they follow the asymptotes of the curve, rather than the curve itself (Fig. 2). So you should expect a +3dB hump at 300Hz if you have the correct equaliser.

The EMI equivalent of this disc is HMV DB4037, cut in 1936. Its main advantage is that it is cut with a typical U-bottomed groove of the period, so it will show the effectiveness (or otherwise) of truncated elliptical styli. Being made of shellac and recorded at rather a low level, wear-and-tear do not seem to affect the measurements, so the fact that the record may be second-hand is not usually important.

Unfortunately there are many criticisms of DB4037. First, examination of the groove-walls by the Buchmann-Meyer method and by direct measurement by a stereo pickup shows that they each carry the correct level of modulation. But there are phase differences between the two groove walls, which make the monophonic output wrong at high frequencies; this was because the cutting stylus which cut the master wax for matrix 2EA3181-1 was inserted askew. The effect appears on quite a few published discs mastered with a Blumlein cutter, and the symptoms can be ameliorated by twisting the pickup cartridge in its headshell by up to thirty degrees.

DB4037 is also useless for exploring the extremes of the frequency range. Its upper limit is only 8500Hz, the low frequencies are swept rather fast, and Sean Davies has recently shown (pers. comm.) that there is a loss below about 100Hz. DB4037 is one record from a set of five which includes fixed-frequency tones, and the lowest tones happen to be on the back of DB4037; so one would think one could use these. Alas, no. The accompanying leaflet is vague and misleading, and it took me some time to work out what was going on. To sum up briefly, the fixed tones are cut to a “Blumlein 150Hz” characteristic (1061 microseconds), the sweep tone is cut to “Blumlein 500Hz” (318 microseconds), while the leaflet implies (but doesn’t actually say) that the turnover frequency is 250Hz (636 microseconds). I hate to think how much confusion has been caused by this set of discs.

EMI deleted DB4037 after the war and substituted a more exotic disc - EMI JG449, which was also issued in Australia on two discs as His Master’s Voice ED1189 and ED1190. It was made to the “Blumlein 250Hz” (636 microsecond) curve in the summer of 1948. It never appeared in any catalogue and is much rarer. Furthermore, it consists only of fixed frequencies, with (maddeningly) even-numbered kiloHertz on one side and odd-numbered kiloHertz on the other. So exploring resonances is very difficult; but as it extends all the way up to 20kHz, and can be found pressed either in shellac or vinyl, it has a niche of its own.

Finally, I should mention Folkways (USA) FX-6100, issued in 1954 before international standards were established. It has a 78rpm side which seems to be “Blumlein 500Hz” (318 microseconds), although the label claims “0dB” at all frequencies. This is likely to be more accessible to American readers.

These coarsegroove discs are very important. Not only do we have a large number of records made with a “Blumlein” shaped characteristic or something similar, but it was often used by default for reasons we shall see in section 6.13. There is only one turnover, so it is easy to reverse-engineer it if subsequent research uncovers more information. And there is minimal effect upon the waveforms of clicks and pops, so a copy may subsequently be declicked using electronic techniques.

I consider the remaining discs in my review as “luxuries” rather than “necessities.”

Decca K.1802 (or London T.4996 in the USA) is similar to K.1803, but recorded to the 78rpm “ffrr” characteristic - the world’s first full-range recording characteristic, which included publicly-defined high-frequency pre-emphasis. This comprised a constant-amplitude section with a +3dB point at 6360Hz (25 microseconds). There are sufficient Decca-group 78s around for it to be well worthwhile getting this right; from 1944 to 1955 Decca’s chief engineer Arthur Haddy was responsible for getting the full frequency range accurately onto discs using this system, and it’s clearly incumbent upon us to get it off again.

Ideally, you should also have a test disc for the International Standard Coarsegroove Characteristic, but such test discs are very rare, because the standard was introduced in 1955 when the coarsegroove format was nearly dead anyway. The only ones I have found are BBC DOM86, EMI JGS81, and an American one, Cook Laboratories No. 10. The latter is made of clear red vinyl, which could be another variable in the equation. As far as I know, only a few private coarsegroove discs will not have equivalent microgroove versions or master-tapes with better power-bandwidth product.

One sometimes finds differences because a test disc is made of vinyl (which is compliant) instead of shellac (which isn’t). This effect will vary with the pickup and stylus being used, and I would encourage serious students to quantify the differences attributable to the particular pickup, and if necessary make adjustments when playing records made of different materials. In my experience, shellac always gives consistent results, because it is many orders of magnitude stiffer than any modern pickup stylus; but to show the size of the problem, I shall mention one case where I had a vinyl and a shellac version of the same coarsegroove frequency test record (it was EMI JG.449).

Using a Shure N44C cantilever retipped with a 3.5 thou x 1.2 thou truncated elliptical diamond, the discs were in agreement at all frequencies below 6kHz. But above this, the responses increasingly differed; at 8kHz, the vinyl was -3dB, and at 13kHz it was -7.5dB. These figures were 1dB worse when the playing-weight was increased to the optimum for shellac (about 8 grams). By repeating the comparisons at 33rpm, it was possible to show that this was a purely playback phenomenon, and I was not damaging the disc by pushing the vinyl beyond its elastic limits; but one wonders what would have happened with nitrate!

These three sections by no means exhaust the list of useful frequency test discs, but many others suffer from defects. They help to identify the performance of old recording machinery with some degree of objectivity, but are not recommended for routine alignment of modern replay gear. I shall therefore defer talking about them until we get to the industrial archaeology of obsolete recording equipment.

6.12 Generalised study of electromagnetic cutters

We now consider the characteristics of cutterheads. A cutterhead converts electrical waveforms into modulations in a groove, and if it isn’t perfect it will affect the wanted sound. “Simple” cutterheads were used for all electrically recorded lateral-cut discs between about 1925 and 1949. They were still being used by amateurs and semi-professionals until the mid-1960s; but after these approximate dates, more complicated cutters with “motional feedback” took over. The specific performance of a cutterhead can largely be neutralised by motional feedback.

Early cutterheads performed the subliminal function of determining recording characteristics. Microphones and amplifiers were intended to have a response which was uniform with frequency. This was rarely achieved to modern standards, of course, but it was an explicit aim. However most cutterheads had a non-uniform response which was found to be an advantage. Different makes of cutterhead would give different sections with constant-velocity and constant-amplitude features, so studying the cutterheads enables us to assess the objective performance of a recording machine before predetermined characteristics were adopted. In the early 1940s American broadcasters decided to use predetermined characteristics for their syndicated programmes, so cutterheads had to be modified or electronically compensated to bring them into line with the theoretical ideal. These theoretical characteristics will be left until section 6.62 onwards; but some organisations (and most amateurs) did not bring their cutting techniques into line with any particular standard until many years later.

There are very few cutterheads which do not conform to the simple “Blumlein shape” outline. The principal exceptions besides the motional-feedback types are piezo-electric cutters (confined to US amateur recording equipment of the mid-1940s), the Blumlein system (which involved “tuning” its resonance, described in sections 6.29 to 6.34below), and the BBC feedback cutterhead (in which the feedback was “non-motional” - the electromagnetic distortions were cancelled, but not the armature motion). So far as I know, all the others follow the same performance pattern, whether they work on moving-iron or moving-coil principles.

There is another reason for mentioning the general characteristics of cutterheads. When we do not know the apparatus actually used for a record, we can get a general outline of the frequency characteristic which resulted, although we may not know precise turnover frequencies. This at least enables us to avoid inappropriate action, such as variable-slope equalisation when we should be using variable-turnover. But we may not be able to guarantee high fidelity unless future research brings new information.

6.13 Characteristics of “simple” cutterheads

All cutterheads have to be capable of holding a cutting tool firmly enough to withstand the stresses of cutting a groove, while the tool is vibrated with as much fidelity to the electronic waveform coming out of the amplifier as possible. To achieve the first aim, the cutter must be held in a stiff mounting if it is not to be deflected by the stresses. In practice this means the cutter (and the armature to which it is attached) has a mechanical resonance in the upper part of the audio frequency range. The fundamental resonant frequency always lies between 3 and 10kHz for the “simple” cutterheads in this section.

It isn’t usually possible to define the resonant frequency precisely, because it will vary slightly depending on the type of cutting stylus. The early steel cutters for nitrate discs, for example, were both longer and more massive than later sapphires in duralumin shanks; this effect may de-tune a resonance by several hundred Hertz. Fortunately it is rarely necessary for us to equalise a resonance with great precision. Various ingenious techniques were used to damp a cutter’s fundamental resonance, and the effects upon nearby frequencies were not great.

To deflect the cutter, electric current from the amplifier had to pass through a coil of wire. In moving-iron cutters, the current magnetised an armature of “soft iron” which was attracted towards, or repelled from, magnetic pole-pieces. (In this context, “soft iron” means magnetically soft - that is, its magnetism could be reversed easily, and when the current was switched off it died away to zero). In moving-coil cutters, the current caused forces to develop in the wire itself in the presence of a steady magnetic field from the pole-pieces. The resulting motion was therefore more “linear”, because there was nothing which could saturate; but the moving-iron system was more efficient, so less power was needed in the first place, all other things being equal.

The pole pieces were usually energised from permanent magnets; but electromagnets were sometimes used when maximum magnetic strength was needed. This was particularly true in the 1920s and early 1930s, before modern permanent magnet materials were developed.

The efficiency of the cutter depended on the inductance of the coil, not its resistance. To put this concept in words, it was the interaction between the magnetic field of the flowing current and the steady magnetic field which deflected the cutter. The electrical resistance of the coil, which also dissipated energy, only made the coil get hot (like an electric fire, but on a smaller scale). Inductive impedance always increases with frequency, while resistance is substantially constant with frequency.

If the coil were made from comparatively coarse wire, it would have lower resistance in relation to its inductance. But however the coil was designed, there would inevitably be a frequency at which the resistance became dominant - usually at the lower end of the frequency range. Sounds of lower pitch would be recorded less efficiently, because most of the power heated the wire instead of propelling the cutter. The slope of the frequency response of the cutterhead would change on either side of the frequency at which the resistance equalled the inductive impedance. This difference was asymptotic to 6dBs per octave.

You should be aware that there were also two second-order effects which affected the turnover frequency. First, the output impedance of the amplifier: the lower this was, the less power was wasted in the output stages. Modern amplifiers always have a low output impedance, because they are designed for driving loudspeakers which work best this way. But in the 1920s and 1930s low output impedances were less common, and this could affect the turnover frequency by as much as thirty percent compared with a modern amplifier. The consistency of archival recordings isn’t affected, but you should be aware of the difficulty if you try measuring an old cutterhead connected to a modern amplifier. Some meddling recordists working with pirated cutterheads went so far as to wire a variable series resistance between the amplifier and the cutterhead to control the shape of the bass response (Ref. 4). The other effect was the strength of the field electromagnet before permanent magnets were used. A weak field would, in effect, cause more power to be wasted in the coil resistance. Most of the time engineers sought maximum efficiency; but written notes were sometimes made of the field voltage. Both methods formed practical ways of reducing volume and cutting bass at the same time.

Meanwhile, Newton’s laws of motion determined the performance of the armature/stylus mechanism. It so happened that when the natural resonance was at a high frequency, the response was constant-velocity in the middle of the frequency range, just what was wanted for an acoustic gramophone. As the frequency went up, the elasticity of the armature “pulled the stylus ahead of the magnetism,” and the velocity tended to increase; but meanwhile the magnetism was falling because of the coil inductance. These effects neutralised each other, and the stylus moved with the same velocity when a constant voltage was applied. Thus, simple cutterheads gave constant-velocity between the effects of the coil resistance at low frequencies and the effects of the resonance.

All this can be made clearer by an actual example. The graph below (Fig. 3) documents a test I carried out many decades ago, to check the response of a “Type A” moving-iron cutterhead. (These were mounted on BBC Type C transportable disc cutters, which were carried round the country in the back of saloon cars between about 1938 and 1960). At low frequencies the bass is cut by the coil resistance; in this case, the coil has the somewhat unusual nominal impedance of 68 ohms so the bass-cut occurs at the desired frequency, 300Hz, which I allowed for on reproduction. The fundamental resonance of the armature is at 4kHz. Between these two frequencies, the output is essentially constant-velocity.

As you can see from the above graph, cutterheads may have a large peak in their output at their resonant frequency. It was this difficulty which was responsible for the relatively late arrival of electrical recording. Western Electric’s breakthrough happened when they showed how the effect could be controlled mechanically. It was necessary to use a “resistive” element. I use this word to describe a mechanical part, as opposed to the electrical property of wire I mentioned earlier. A “resistive element” has the property of absorbing energy equally well at all frequencies.

With mechanical parts, a massive element tends to be easier to move at low frequencies, and a compliant element tends to be easier to move at high frequencies. At low frequencies where the mass is dominant, it tends to delay the motion, and there is a “phase lag.” At high frequencies, where the compliance is dominant, the springiness tends to pull the armature ahead of the signal, and there is a “phase lead.” Where the mass and compliance have equal effects, the phase lag and the phase lead cancel, and resonance occurs, resulting in more motion, apparently contradicting the laws of conservation of energy. Only mechanical resistance prevents infinitely fast motion.

The Western Electric people hung a long rubber tube on the armature, the idea being that the energy would be conducted away and dissipated in the rubber. The difficulty lay in supplying rubber with consistent properties. When the rubber line was new, it worked according to specification, and we do not need to take any special action to cancel a resonance today. But as the rubber aged, it gained compliance and lost resistance - it “perished.” Top professional recording companies were able to afford new spare parts, but by about 1929 there was rather a lot of ill-understood tweaking going on in some places (Ref. 4). When this happened, the resonant frequency and the amplitude cannot be quantified, so current practice is simply to ignore the problem.

In the next couple of decades, many other types of resistive element were used. Fluid materials, such as oils and greases, had only resistive properties, not compliant ones. But grease could dry out, or refuse to remain packed against the armature where it should have been; Fig. 3 demonstrates the result. Oil could be used, soaked in paper inserted between armature and pole-pieces, kept there by surface-tension. Or the cutterhead might have a sump, with the cutting-tool poking out of the bottom through an oil-proof but flexible seal. Thixatropic greases such as “Viscaloid” became available after the war. Many amateur machines used something like conventional bicycle valve-rubber round the armature. Although this required renewal every few years, there was a sufficiently high ratio of resistance to compliance to make the resonance inaudible, although it could still be shown up by measurements.

Above the frequency of resonance, the stylus velocity would have fallen at a rate asymptotic to twelve decibels per octave, unless the armature was shaped so as to permit other resonances at higher frequencies. Such a resonance can just be discerned in Fig. 3, but it isn’t audible. If the surface-noise permits (a very big “if”!), one could in principle equalise this fall, and extend the frequency range. I have tried experiments on these lines; but apart from being deafened by background noise like escaping steam, this often reveals lots of “harmonic distortion,” showing that something was overloading somewhere (usually, I suspect, a moving-iron armature). Clearly, the original recording engineer was relying on the restricted frequency-range of his cutterhead to filter off overload-distortion. Unless and until someone invents a computer process to detect and cancel it, I think it’s better to leave things the way they are.

Personally, I ignore fundamental resonances unless they are so clearly audible that they can be “tuned out” by ear. This is usually only possible when they occur at a relatively low frequency, within the normal musical range (say below 4kHz), which tended to happen with the cutterheads in the next section. Even so, I believe it is important to study many different records before quantifying and cancelling any resonance, so I can use the principle of “majority voting” to distinguish between the effects of the cutterhead, and specific features of musical instruments or voices.

6.14 High-resistance cutterheads

I cannot give industrial archaeology evidence for these cutterheads, because as far as I know there isn’t much in the way of written history or surviving artefacts. As I do not have the quantitative information I shall be using in subsequent sections, I shall explain instead their qualitative principle of operation. They were confined to inexpensive discs; in Britain, the first electrical Edison-Bells, Sternos, Imperials, and Dominions.

The simple electromagnetic cutterhead I have just outlined had a transition from constant-amplitude to constant-velocity at the low frequency end of the scale. Such cutters had a relatively low electrical impedance (tens of ohms). Before transistors, this meant a matching transformer between the output valves and the coil. It was a bulky and expensive component, and until the mid-1960s it is no exaggeration to say that the output transformer was the weakest link in any power amplifier. By winding the cutterhead with a high-impedance coil (thousands of ohms), it could be coupled directly to the amplifier’s output stage. But this brought its own complement of problems.

The most important was that the electrical resistance of the wire was dominant. Consequently, the cutter recorded constant-amplitude for much more of its frequency range. (Fig. 4)

Because more of the energy went into heating the coil rather than vibrating the cutter, such systems were less efficient. To compensate for this, the resonant frequency of the mechanism was lower so there was more output at mid-frequencies where the ear is most sensitive, and this resonance was less well damped.

In every case known to the author, the change from constant-amplitude to constant-velocity was above the frequency of resonance. Thus we have constant-amplitude through most of the musical range ending at about 1.5 or 2kHz with a massive resonance, then a falloff at 6dBs per octave until the resistive impedance equalled the inductive impedance, followed by a steeper fall at 12dBs per octave at the highest frequencies.

This sounds truly horrible today, and I mention it in case you need to deal with it. But I must also remind you that musicians would modify their orchestrations, their layouts, and their performing techniques to adapt to the strange circumstances (as they had done with acoustic recordings), because test records were judged on acoustic gramophones which gave better results with constant-velocity. All four makes cited above, for example, issued electrically-recorded dance-bands with brass bass lines. Thus it may not be fair to “restore the original sound”, except perhaps with solo speakers, or solo musical instruments with which it was impracticable to alter the volumes of separate notes sounded together.

You may wonder how such a dreadful system came to be adopted. I suggest six

  • (1) It was no worse than acoustic recording, and when you add the extra advantage of electronic amplification, it was better.
  • (2) It was much cheaper than Western Electric’s system (most things would have been!).
  • (3) It gave records which did not have large groove-swing amplitudes at low frequencies (section 6.4 above), and which therefore played on cheap gramophones without mistracking.
  • (4) Since most of the musical range was constant-amplitude, the advantages mentioned in section 6.4 were partly realised.
  • (5) The smaller companies did not have large sales among the affluent. The results matched the performance of down-market wireless sets.
  • (6) It enabled manufacturers to claim their records were electrically recorded, which was the great selling-point of the late 1920s!


It must also be said that at least two companies in Britain (British Homophone and Edison-Bell) seem to have altered their techniques after a year or so, to give something closer to a “Blumlein shape.” Paul Voigt (of Edison Bell) later revealed that he had started with constant-amplitude (without explaining why, Ref. 5), and had to change to a compromise slope of 3dBs per octave, achieved by electronic compensation. He claimed he had been “the first to perceive the advantages of constant-amplitude”; but I believe this to have been an accidental discovery, since it was abandoned very quickly! Instead, he can take justified credit for being the first to use electronics to obtain a particular recording characteristic.

In another case (Crystalate, makers of Imperial and Victory logos), Arthur Haddy was recruited as a recording engineer in 1929 because he said “I thought I could build a better lot on the kitchen table” (Ref. 6); but, so far as the London-recorded Imperials are concerned, the difference isn’t apparent until late 1931 (a gradual changeover occurring between matrixes 5833 and 5913). The third case seems to be the Dominion Record Company, which seems never to have changed its practices before its bankruptcy in 1930.

6.15 Western Electric, and similar line-damped recording systems

For the next few sections, I shall consider the properties of the earliest successful disc recording systems using electronic amplifiers. They included cutterheads whose main resonance was damped by a “rubber line.” This comprised a long rubber rod designed to conduct vibrations away and dissipate them; but if the rubber went hard, vibrations might be reflected back into the armature instead of being totally absorbed.

The first successful cutterheads were actually of this type. They were based on research by the Bell Telephone Labs in the USA in the early 1920s, and were originally offered to the sound film market. Although some experimental “shorts” were made, film producers did not take to the idea immediately, although it was adopted by Vitaphone from 1926 onwards. The first commercial success was with the record industry, and the cutterhead formed part of a complete “system,” comprising microphone, amplifier, cutterhead, and a scientifically-designed acoustical reproducer. Other record companies had similar cutterheads, and we will consider those as well.

6.16 Western Electric revolutionises sound recording

The technique was launched in 1925 by the manufacturing division of A. T & T, the Western Electric Company. It was almost immediately taken up by leading record makers, including the Victor/Gramophone Companies and the Columbia Companies on both sides of the Atlantic. The first published recordings date from mid-February 1925 (Ref. 1), and later developments continued in use until about 1949. All three components of the early equipment (the microphone, the amplifier, and the cutterhead) provide challenges to workers aiming to recover the original sound. First, we shall consider the earliest high-quality Western Electric microphone.

6.17 The Western Electric microphone

Fig. 5 shows the axial response of the Western Electric Type 361 condenser microphone (or “transmitter” as it was called in those days) (Refs. 7 and 8). The principal feature is a +7dB resonance at 2.9kHz due to the air in a cavity in front of the diaphragm. When the microphone was first invented, tests were done in an atmosphere of pure hydrogen; but the velocity of sound is much higher in hydrogen, and the cavity resonance was above the microphone’s official limit. In air the defect is quite unambiguous. Such microphones were used as prime standards in acoustic laboratories for many years, and the defects had to be taken into account by the scientific community.

There was an increase at high frequencies for a second reason, which I have separated out to make Fig. 6. The microphone reflected some of the sound back at the performers, and as a consequence the diaphragm was subjected to increased sound pressures. When the performer was smack on axis, this action doubled the output at high frequencies - an increase of 6dBs - although other effects conspired to bring the extreme high-frequency response down again. But the dotted line in Fig. 6 shows that the microphone had less high-frequency output at the sides - ninety degrees off-axis.

Since the performers might be anywhere in a semicircle around the microphone, the frequency response for any one individual might be anywhere between the solid and the dotted lines. It is normal practice to ignore the pressure-doubling effect altogether. Although we could compensate if we knew precisely where the artist was situated, we don’t usually know this, and we certainly cannot treat performers differently if there were two or more at once. But personally, I do compensate for the main cavity resonance, because it reduces “mechanical quality” and surface noise at the same time.

The Model 394 seems to have been substituted for the Model 361 for gramophone work in 1929. Its capsule was the same, but the original preamplifier (in a wooden box) was replaced by a new design in a metal tube, like a modern condenser microphone. (Ref. 9). Although there may have been slight differences in the performance of the preamplifiers, and of the capsules (because they were re-housed), our current knowledge does not enable us to distinguish between them.

6.18 The Western Electric amplifier

The amplifier (which went between the microphone and cutterhead) had a substantially flat frequency response between 50Hz and 5kHz. Unfortunately, all examples seem to have been returned to the manufacturers in America, and although microphone capsules and cutterheads survive, there seems to be no electronic equipment. But volume controlling, in the manner known to recording operators today, was clearly possible from the outset. Many details are preserved in contemporary documentation, and may be useful to restoration operators. It seems the settings were logged because HMV submitted test pressings to a “wear test.” Ideally, they had to survive 100 plays without wearing out; but if one failed, it had to be re-recorded using the same equipment at a lower volume.

Fred Gaisberg, the Gramophone Company’s chief recording expert, has left an account which mentions another way of avoiding trouble (he was speaking of sudden forte timpani attacks): “The way to deal with these ‘whacks’ was to cut off the lower frequencies.” (Ref. 10). And Moyer (Ref. 11) says “The actual effective low end of these curves is subject to some question, however, since it was common practice to use a rather elaborate “bass filter” to reduce the low-frequency response in order to obtain the best sound on average reproducers.”

6.19 Documentation of HMV amplifier settings, 1925-1931

Settings of Western Electric equipment made by HMV may be found on the “Artists’ Sheets”, which may be consulted at the British Library Sound Archive. Microfilms 360 to 362 include British recordings and many International Red-Label artists. Recordings made in Vienna and points east are on reels 385-6, Italy on 386-7, Spain on 387-8, Germany on 388, France on 388-9, and other places on 390-1. However, they do not show material recorded for the Zonophone logo.

It is my duty to point out that we have no independent description of the meanings of the settings. We must therefore “break the code.” There is a stage between the breaking of a code and the acceptance of the decoded messages as historically accurate. The latter only comes when new material is decoded by new workers with consistent results. I am still at the intermediate stage.

The settings are in three or four columns at the right-hand side of the sheets. Because the columns are not labelled or numbered, I am forced to list them in the following manner.

  • EXTREME RIGHT-HAND COLUMN. Volume settings, going from L.1 to L.10 and H.1 to H.10. The letters are thought to refer to a switch on the microphone preamplifier offering “low” and “high” gain, while the numbers are thought to refer to ten taps on an auto-transformer after the cutting amplifier. (An eleventh was “Off.”) The taps were equally-spaced, each step accounting for a uniform number of watts. These taps were originally provided for public-address applications, to control volume to several loudspeakers without wasting power. Often a range of taps is shown. (This information is deduced from a description of Western Electric public address amplifiers in Ref. 12). Occasionally there is a third parameter, generally a plain number covering a range from 4 to 16; this is thought to be a wire-wound volume control which could provide gradual changes in volume, in contrast to the switches.
  • SECOND COLUMN FROM THE RIGHT. Serial number of the microphone. When I say “serial number,” I do not mean one allocated by Western Electric, but the Gramophone Company’s number. Microphones in Continental studios often had a letter prefix (this is not always given); thus German recordings may be made with microphones numbered G.1 upwards; Milan, M.1 upwards.
  • THIRD COLUMN FROM THE RIGHT. “Serial number” of the amplifier or cutterhead (I do not know which).
  • FOURTH COLUMN FROM THE RIGHT. From May 1927 this column, which was originally designed for the copyright date of the music, was used for another purpose. It generally contains “2.L.” or “3.L,” but I have seen “1.L.” on Italian artists’ sheets. It is highly likely this documents what we now call a “brick-wall bass cut,” and (judging from listening tests only) it seems “2.L.” meant a cut below about 60Hz, and “3.L.” below about 100Hz. These codes also appear on some Blumlein recordings (which we will consider in sections 6.29 to 6.34).


6.20 The Western Electric cutterhead

The 1925 version of Western Electric’s cutterhead was upgraded in 1927, and the Gramophone Company of Great Britain distinguished the two by the terms “Western 1” and “Western 2”, although these were not used by the manufacturer. Indeed, a Western Electric Recorder Type 2A using motional feedback was introduced in 1949. (By this time pre-determined recording characteristics were used. The cutterhead followed these fairly faithfully, so I shall leave the results until section 6.62 onwards). In order to circumvent the ambiguities of nomenclature, I have therefore decided to call the first manifestation “Western Electric 1A”, the second “Western Electric 1B”, and so on; although please note these names weren’t used officially either!

The world’s first frequency records were cut using “Western Electric 1A” equipment by the Victor Company in the United States. To judge by the matrix numbers which appear on the British pressings, they were done in two sessions in November 1925 and late January 1926. They were issued in the US on single-sided discs at rather high prices, and in Britain as a set of fifteen double-sided twelve-inch 78s in March 1929 (His Master’s Voice DB1231 to DB1245). Unfortunately they comprise only fixed tones, there being no “sweep” which would enable us to assess frequency-response. They were used for many years by the magazine Wireless World for testing pickups. They are also quite rare (I have only ever found two), so I can’t even use modern methods to measure what there is. But I am afraid my judgement is that we cannot get any useful information about the recorded frequency-response from these discs.

Fig. 7 reproduces two frequency curves, documenting the performance of the 1A and 1B electromagnetic cutters with their rubber line damping. They are redrawn from Figure 3 of the paper by Moyer (Ref. 11). The 1A version had a primary resonance at about 4.5kHz, above which its response would have fallen asymptotically to 12dB per octave. The model 1B extended the response slightly to about 5.5kHz. This seems to have been achieved by redesigning the means of pivoting the armature. The 1A had quite a lot of compliance, and an electronic click could “throw” the armature and cause it to stick to one of the pole pieces; the 1B tightened up the compliance so this happened less often and the fundamental resonant frequency was raised.

From section 6.13, you would have expected a 6dB/octave rolloff in the bass due to the resistance of the coil, and indeed there was; and when the rubber line was new, that is all there was. (Ref. 13). I have shown it by the dotted line in Fig. 7.

The reason this doesn’t seem to have been noticed by anyone apart from the original designers was that another effect partially cancelled it. The rubber line did not always behave like an ideal “resistive element” at lower frequencies (Ref. 14). Different accounts say the same thing in different ways, but the normal bass cut due to the coil resistance was partly neutralised by a resonance at about 160Hz. Sound waves travelled along the rubber line torsionally at about 3000 centimetres per second (Refs. 13 and 15). The rubber line was about nine inches long, and when it wasn’t new, it seems vibrations might be reflected back into the armature a few milliseconds later, giving a bump at 160Hz. Although it neatly filled in the bass-cut due to the coil, to reverse-engineer it today we must mix-in antiphase signals delayed by about five milliseconds with a top-cut at 12dBs per octave (this would be how they propagated through a rubber line with mass and resistance). This gets rid of the “lumpiness” audible in many such recordings.

There is an unpublished British HMV frequency test record with the matrix number (no prefix) 5729 I ∆. It comprises only fixed frequencies from 8kHz downwards, and unfortunately there isn’t one between 200Hz and 130Hz. But the others depict an accurate Blumlein shape (Fig. 2) equivalent to “Blumlein 250Hz.”

Continuous development kept the “Western Electric 1-something” in the forefront in America, but British record companies found the royalties burdensome, and great efforts were made to find alternatives. By 1932 alternatives were in place for all the original British record makers with one exception. This was the EMI Mobile Recording Van, because the alternative required high voltage supplies which could not easily be obtained from accumulators; but Blumlein equipment was finally installed in 1934.

6.21 How to recognise recordings made with Western Electric 1A and 1B systems

Ariel, Gramophone Co. (including HMV and foreign equivalents), Hispanophone, Regal-Zonophone, Zonophone, and Victor and Victrola repressings from the aforementioned: A triangle after the matrix number.

Ariel, Columbia, Daily Mail “Brush Up Your French”, Hugophone, Regal, Regal-Zonophone: A capital W in a circle somewhere in the label surround (often in front of the matrix number), or on pressings from post-1945 metalwork, a matrix number beginning with W not in a circle.

Odeon, Parlophone, Parlophone-Odeon: A W in a circle as above, unless there is also a £ in a circle, in which case it was made by the Lindström system (section 6.27 below). Fortunately the same equalisation techniques apply to both, so I shall not consider the matter further; but listen out for the bass cut due to Lindström’s microphone or amplifier! Bluebird, Victor, Victrola: If repressed from a matrix “Recorded in Europe” with a triangle, the same as Gramophone Co. above. Otherwise sometimes the letters VE in an oval somewhere in the label surround, and/or the words “Orthophonic Recording” (not “New Orthophonic Recording”) on the label.

Homochord: Some records were made by the British Gramophone Company for the Homochord logo using this system in the years 1926 to 1928. They can be recognised by matrix numbers beginning with the prefixes HH, JJ, HR, or JR. Homo Baby, Sterno Baby, Conquest, Dixy, Jolly Boys. These are all logos of six-inch records made by the British Gramophone Company for Homochord in the year 1926; they all have matrix numbers prefixed by Ee.

6.22 Summary of equalisation techniques for the above

The above recordings should be equalised constant-amplitude below 250Hz (636 microseconds) and constant-velocity above that. When you are certain a Western Electric microphone was used (which was the case for all the big record companies pre-1931), a dip of 7dB at 2.9kHz may also be applied. You might also reduce any “tubbiness” by mixing in antiphase a portion of the signal with a 12dB/octave HF slope (above about 50Hz) and about five milliseconds delay.

It might also be worth researching the settings shown on HMV Artists’ Sheets, which will also give some idea of the amount of dynamic compression performed by the engineer; but currently, we do not know the exact characteristics of the bass-cut filters used after May 1927, and if we ever do, it may prove impossible to restore the full bass without contributing “rumble.”

6.23 Western Electric developments after 1931

In the early 1930s, RCA Victor made several improvements to the Western Electric system. The curve in Figure 8 is taken from a sweep frequency record Victor 84522 (also known as 12-5-5). It cannot be dated exactly, but the label design suggests 1931 or 1932. This disc shows that RCA Victor had managed to extend the performance considerably, which is obvious from listening to contemporary RCA Victor records. The treble response is flat (constant-velocity) to 5kHz and droops only very slightly after that, reaching an amazing figure of only -4.5dB at 10.5kHz, the highest frequency on the disc. This suggests high frequencies were being compensated electronically, albeit with the limitations we saw in section 6.8. The bass response is smoothed out, although Moyer says this did not happen until 1938. It comprises a constant-amplitude section with a -3dB point between 500 and 600Hz (318 to 250 microseconds). I am confident this is a real performance, although it may have been cut on a souped-up machine.

Between December 1931 and February 1933, RCA recorded some “long-playing” records running at 33 1/3rpm. Although I cannot date it, a disc from RCA’s “Technical Record Series” appears to document the characteristics of these long-playing records (it is pressed in similar translucent red material). Its catalogue number is 12-5-25V and its matrix number is 460625-6. Although it carries a range of fixed frequencies rather than the continuously varying tone of Victor 84522, the characteristic seems very similar. But it is clearly a very coarse groove. (It won’t give a satisfactory Buchmann-Meyer image unless the groove wall is illuminated by a light less than thirty degrees above the disc surface). In the circumstances, it is difficult even to focus on the image! But it is clearly a Blumlein shape (section 6.7). It is substantially constant-velocity above 1kHz, drooping by something like three decibels at 12kHz (the highest frequency on the disc). The -3dB point between the constant-velocity and constant-amplitude sections is between the bands of 700Hz and 500Hz.

About 1931, RCA invented the “ribbon microphone,” with a much more smooth and extended frequency response. However, it was found that the peak of the Western Electric microphone at 2.9kHz had been beneficial in overcoming surface noise, so it seems an electronic treble lift starting at 2.5kHz (63 microseconds) was added to emulate this, although it isn’t on Victor 84522. However, Moyer (Ref. 11) is not very clear about the chronology; according to him, it wasn’t until 1938 that the entire chain from microphone to pickup was changed. (He makes other mistakes about chronology). Whenever it may have been introduced, I call the new system “Western Electric 1C.” It comprised a cutterhead modified by RCA Victor, with a smoothed-out bass response and deliberate equalisation to compensate. It was used with “improved recording channels”, which I assume is Mr. Moyer’s expression for new electronic equipment. He says the pre-emphasis found to be advantageous with ribbon-mikes was moved into the “recording bus” (so it was permanently in circuit, giving constant-amplitude above about 2.5kHz or 63 microseconds), the adjustable bass filter was discarded, and a low-pass brick-wall filter was imposed at 8500Hz “primarily to reduce noise and distortion effects resulting from playback turntable flutter, pickup tracking, and manufacturing methods.” (sic)

We know that by 1943 RCA were selling their own wide-range cutterhead; but Moyer is adamant that Western Electric equipment, still working on the same principles, was upgraded yet again “about 1947”; I have called this the Western Electric 1D. He suggests that the 1D was essentially the same as the 1C without the brick-wall filter.

Moyer points out that when standardisation of all RCA Victor records was attempted in 1938, a 6dB/octave slope below 500Hz (318 microseconds) was usually found to be satisfactory for most older records. This writer agrees for post-1932 RCA Victor records, and the assertion is almost identical to the evidence of Victor 84522; but it is probably an oversimplification for earlier Victors.

Because I do not live in America, I cannot establish whether similar curves were used by other manufacturers. My only such (aural) experiment compared an early Columbia LP, assumed to have the characteristic defined in section 6.68 below, with its 78rpm equivalent. The 78 proved to have a bass-cut at 500Hz (318 microseconds). And purely aural evidence (not side-by-side comparisons) suggests a similar curve was used by the ARC/Brunswick companies which evolved into US Decca.

6.24 Recognising recordings made on RCA-modified and Western Electric 1C and 1D systems

Bluebird, RCA Victor, Victrola: all records mastered in the New World from 1932 until 1949; Gramophone Co. (including HMV and foreign equivalents, and a few Regal-Zonophones): A diamond after the matrix number (◊); matrixes with the prefixes A or 0A are European reissues of American material. There wasn’t a consistent policy of dubbing them to new matrixes. Sometimes the pre-emphasis was removed to bring the records to European standards (because clockwork gramophones were still popular), and sometimes the dubbings were made without the pre-emphasis being removed, possibly to maintain consistency in multi-record sets. It is not easy to describe the different appearance of such a record, since British records from both original matrixes and from dubbings carry the same matrix number. It is a case where experience counts; but one indication is the eccentric run-out groove. From 1933 it is “single groove” on all British dubbings, but the “double groove” of the original American metalwork continued until March 1940 (at least as far as matrix number 0A 048176◊).

Columbia, Parlophone, and Regal-Zonophone reissues from US Columbia and Okeh sources: A capital W in a circle somewhere in the label surround (often in front of the matrix number), or a matrix number beginning with W not in a circle. Western Electric 1C and 1D systems were used for many other electrical records on the American continent without any identification.

6.25 Summary of equalisation techniques for the above

The above recordings should be equalised constant-amplitude below about 500Hz (318 microseconds) or perhaps higher, and constant-velocity above that. I have not had enough experience to say definitely when 2.5kHz de-emphasis (63 microseconds) may be found. As the technique seems not to have been publicised at the time, I haven’t called this an “official” Recording Characteristic; but the story will continue when we consider such characteristics in section 6.70.

6.26 Other systems using line-damped cutterheads: British Brunswick, Decca, DGG 1925-1939

Victor’s smaller competitors didn’t wait long to get their hands on the new technology. By May 1925 the Brunswick-Balke-Callender company had electrical recording in its Chicago studio, and aural comparisons show suspicious similarities to the sound of Victor’s recordings. Because both parties were deliberately being secretive, it is impossible to know exactly what went on today; but I suspect the secrets went with W. Sinkler Darby, a Victor expert who defected to Brunswick about this time.

Brunswick may have erected a smoke-screen to conceal what they were up to. They claimed to use the General Electric Company’s “light ray” system of recording. G.E.C. took out three patents on the subject. One deals with what we now call a “microphone”, which comprised a tiny mirror vibrated by sound waves reflecting light into a photo-electric cell. (Ref. 16). This was similar to the principle of the “optical lever” used in scientific laboratories for measuring tiny physical phenomena. This microphone could well have been used, although it was subsequently shown that any extended high-frequency response would be compromised by lack of sensitivity. I am very grateful to Sean Davies for the suggestion this may have been the microphone used by the Deutsche Grammophon Gesellschaft until about 1930. Certainly DGG recorded items which became issued outside Germany on the Brunswick label; they are characterised by an unequalised diaphragm resonance, like the sound made by an undamped telephone earpiece of the time. We do not yet have technology for compensating this.

General Electric’s other two patents, for a moving-iron cutterhead built round a strip of steel under high tension so its resonance would be ultrasonic (Ref. 17), and an active mixer for combining “light ray” microphones for greater output (Ref. 18), simply would never have worked as the patents showed. (Indeed, the latter was disallowed by the Patent Comptroller in Britain). What, then, did Brunswick actually use?

History is silent until 1932. In the meantime, Brunswick set up a recording-studio in London (with audibly better microphones), the company went bankrupt, and the studio fell into the hands of the newly-formed Decca Record Company. On 1st July 1932 Decca patented a new kind of swarf-pipe for removing the shavings from the wax (Ref. 19). The patent drawing depicts something looking identical to a Western Electric cutter (although significantly the damping wasn’t shown)! And some of Decca’s engineers later recalled that, although they were concealed in home-made cases, they were “Chinese copies” of Western Electric rubber line recorders, manufactured by Jenks & Ader in the USA.

6.27 Other systems using line-damped cutterheads: The Lindström system

To identify the records upon which royalties were payable to Western Electric, we saw earlier that legitimate manufacturers marked their stampers. The Victor/Gramophone companies used a triangle, while Columbia and the German Lindström organisation used a W in a circle. In 1928 Lindström started marking their matrixes with a £ sign in a circle instead of a W in a circle. It appeared on many Odeon records (Parlophone in the British Empire); and when British pressing work was taken over by the Columbia company in 1929 the pressings acquired a W as well. (This was because Columbia’s Wandsworth factory was adding the W in blissful ignorance of what it meant!)

Clearly, Lindström had made their own version of the equipment. I am very grateful to Sean Davies for drawing my attention to a memo in the EMI Archives dated 28th December 1931. This shows that the Parlophone studio in Carlton Hill London had both Western Electric and Lindström systems, although the former was hardly ever used at that stage.

The earliest known recording showing the £ sign was made on 24th October 1928 (matrix number 2-21025, published under catalogue number P8512 (Germany) and E10839 (Britain)). The £ sign continued until about 1936 when the Blumlein system (which I shall consider in section 6.29) was adopted throughout the EMI organisation. Since not every country had access to pound-signs on their typewriters and matrix-punches, it is thought that the letters L and P serve the same function.

Some frequency test discs were made in Berlin in about 1929 which enable us to assess the frequency response of the Lindström cutter electronics and cutterhead; the matrix number of the sweep-tone side is XXRP2. They were issued in a commercially-issued set (Parlophone P9794 to P9798) with £ signs. Unfortunately it has not been possible to date them, because they are in a special matrix series; but the first three were published in Britain in August 1929. The labels attribute them to “Dr. Meyer of the Hertz Institute”. Dr. Erwin Meyer was a co-author of the paper on the Buchmann-Meyer image (Ref. 20).

The original paper includes a photograph of this record. It is uncanny how similar the performances are to a poor Western Electric cutterhead. Both resonances on XXRP2 are conspicuous to the naked eye. I speculate that it was these discs which inspired Meyer to write his paper. The resonances amount to +5dB at 5kHz for the treble resonance and +6dB at 150Hz for the bass resonance.

The other side of P9794 has a sweep “howling tone.” (Nowadays we use the term “warble-tone”). I speculate that Lindström’s system was modified to improve the response in view of Dr. Meyer’s discoveries. It has the matrix number XXRP3 Take 2, and measurement shows the resonances are better damped (especially the low-frequency one).

Incidentally, both records claim to have tones down to 100Hz. This is not correct. The lowest frequency on both sides is 130Hz.

The series also includes widely modulated warble tones on P9796 (matrixes XXRP6 take 2 and XXRP7). Analysis of these gives frequency responses in agreement with matrix XXRP3-2. The other records all comprise fixed tones, from which we can gain no useful evidence.

Listening suggests a £ sign is sometimes associated with a very considerable lack of bass, which may be due to the microphone or its microphone amplifier. I have been listening to Rosenthal’s recording of the Chopin First Piano Concerto, the first movement of which is “normal” and the other two have the extra bass cut, in an attempt to match them and estimate what was going on. The two sessions did not have the microphones in the same position, which makes rigorous comparisons impossible; but my conclusion is that one needs an extra 500Hz lift (318 microseconds) in addition to the usual 250Hz (636 microseconds), and even then the bottom octave doesn’t come right.

Through the good offices of David Mason, I was able to purchase a second-hand single-sided Odeon “Frequenz Platte” (catalogue number 1528b, matrix number FQ 1528b). It is a centre-start disc – the frequency sweeps up rather than down - and if the 1kHz tone is accurate, it needs to be played at a little over 80rpm. This too carries a Blumlein-shaped frequency-sweep, except that the very extreme bass has been boosted (that is, constant-velocity) in the manner common to later microgroove discs.

This is rather difficult to measure on my copy because it has become worn; but the overall flattest response comes with 398 microseconds and 1591 microseconds. There is also a droop in the extreme treble, which may be an effect of the cutting stylus rather then the actual modulation, and begins to take effect above 8kHz. If anyone can help, I’d like a date for this disc. I estimate it is about 1936, but I could be considerably in error.

6.28 Conclusion to line-damped systems

So, quite apart from the official Western Electric licensees, there is now evidence that several other recording companies were using something very similar. The final evidence is Courtney Bryson’s book, published as late as 1935, which gives a lot of space to instructions on how to fiddle such cutterheads with files, screwdrivers, and rheostats (Ref. 21). Despite careful patent protection, Western Electric’s rubber line system was being used very widely indeed, and I don’t believe we have yet reached the end of the matter.

Because I do not live in America, I cannot establish whether similar curves were used by other manufacturers. My only such (aural) experiment used an early Columbia LP, assumed to have the characteristic defined in section 6.68 below, compared with its 78rpm equivalent. The 78 proved to have a bass-cut at 500Hz (318 microseconds). And purely aural evidence (not side-by-side comparisons) suggests a similar curve was used for 78s made by the ARC/Brunswick companies which evolved into US Decca, while side-by side comparisons have shown that the earliest Capitol 78s depict a bass cut at 1kHz (159 microseconds).

6.29 The Blumlein system

Alan Blumlein is now remembered for his pioneer stereo experiments, but I shall be talking about his monophonic system and its derivatives. The recording characteristics he espoused were emulated by many other European manufacturers, there is a fair amount of objective evidence, and we saw in section 6.13 that “simple” cutterheads gave this curve by default. In section 6.7 I defined the meaning of a “Blumlein-shaped” frequency response; the sections until 6.46 will be describing these. I shall start by describing Blumlein’s system (used for commercial recording throughout most of the “Old World” by labels of the EMI Organisation), and then I shall consider other systems known to give similar characteristics.

Blumlein was recruited by English Columbia in 1929 to circumvent Western Electric royalties. His system seems first to have been used commercially on 22nd January 1931. But eight weeks later Columbia amalgamated with HMV to form EMI Ltd. HMV was in the process of building a new studio complex at Abbey Road where Western Electric gear was to be installed, and recording started there the following September. Between December 1931 and July 1932 the systems were compared. Blumlein’s won, and gradually all EMI’s recording facilities in Britain and overseas were converted.

The novel feature, which enabled Blumlein to dodge the Western Electric patents, was that resonances were not damped mechanically. Instead they were neutralised by a combination of eddy-current damping and electronic equalisation. This applies both to the microphones and the cutterheads, so the patents are interlinked.

Equalising resonances with antiresonant circuitry is rather like fighting fire with fire. If things are not exactly right, matters easily get out of control. It is reliably reported that Blumlein (and his mechanical engineer Herbert Holman) pressed their assistants to calculate the moments of inertia of complicated metal shapes theoretically, and chastised them if the resonant frequencies of the manufactured prototypes were not correct within one percent. But this sort of precision was vital if peaks and troughs in the frequency response were to be avoided. After the circuitry was tuned, the results should have stayed consistent because there was no rubber or other perishable materials; but peaks and troughs are exactly what make Blumlein recordings so distinctive - to my ears, anyway!

6.30 The Blumlein microphone

The Holman-Blumlein microphone (known as the HB1) was a moving-coil type with a diaphragm made of balsa-wood coated with aluminium foil. British patent 350998 refers to a primary resonance “at or below 500 cycles per second.” The original design had a field magnet, but by 1934 a version with a permanent magnet (known as the HB1E) was also used. Examples of each are preserved at the EMI Archive.

Both mikes had the same front-dimensions, but the HB1E was less deep. They had chamfered edges and slightly smaller diameters than Western Electric mikes, so the axial pressure-doubling was less pronounced. But their bulk still meant high frequencies from the sides and rear were attenuated.

Frequency curves are given in Refs. 22 and 23. Ref. 22 does not name the microphone, but shows a picture of an HB1E. Ref. 23 actually says it is an HB1E and adds a polar diagram. The on-axis curves show a 5dB rise from 2 to 5kHz, presumably the pressure-doubling effect, and the latter reference shows the output remaining above the 1kHz level up to the limits of the graph at 15kHz, albeit with some wobbles. It was claimed “the rigid structure of the diaphragm causes the first nodal resonance to occur at 17 kc/s, below which it moves as a single resonant system”; but neither the graph nor this writer’s listening experiences support this, so maybe further research is needed.

It could be argued that the shelf in the high-frequency response outweighed the mike’s insensitivity to HF from the sides and back. This would be reasonable for musical sources of finite size (rather than point sources), especially in a reverberant environment; and the mike was renowned for its performance on pianos (Ref. 24) so the graphs actually support this theory. I do not think we need to address the frequency response aberrations - on piano recordings, anyway! As it lacked the cavity resonance of its Western Electric predecessors, and its primary resonance was equalised by a circuit included within the microphone cable (shown in the patent), current practice is to consider it “flat.”

By the end of the second world war the HB1E microphone had given way to other types, including an EMI ribbon mike (also shown in Ref. 23), and early Neumann condenser mikes. Again, current practice is to consider these “flat.”

6.31 The Blumlein moving-coil cutterhead

A passive equaliser (that is, one having no amplification) was placed between the power amplifier and the cutterhead to correct resonances and give the required characteristic. The cutterhead used a novel principle (described in British Patent No. 350954) which made it virtually overload-proof; but the equaliser was described in Patent No. 350998. It had no less than 12 elements, and presumably shows the state of the art just before the patent applications were lodged.

Each individual cutterhead had its own equaliser, and there were strict instructions to prevent cutterheads and equalisers being swapped. But there were at least four subsequent versions of the cutterhead, and the equaliser components were designed to be easily changed by maintenance staff (not everyday operators). Since we do not know which equaliser and cutterhead was used for which record, further refinements do not seem possible today; but EMI engineering documentation always shows “Blumlein shape” was the target.

We saw another snag in section 6.8. To pull some curves straight by electronic equalisation implies infinite amplification, difficult with limited power to spare! In practice, both microphone and cutterhead systems “drooped” at the extremes of the frequency range.

6.32 Test discs made by Blumlein’s system

I shall now describe three frequency records made by Blumlein cutters. The earliest is TC17, a record made in the spring of 1933 with matrix number (no prefix) 5403 □ I. It was manufactured in large quantities for testing HMV radiograms. It starts with a brief frequency sweep, and the same groove continues with dubbings from records which were known empirically to give reproduction problems, such as cabinet rattles and motor-drag. Thus it probably shows the actual performance of a Blumlein recorder in 1933, rather than an idealised laboratory performance. The sweep runs from 8kHz to 30Hz at “normal recording characteristic.” Although it generally follows a “Blumlein 200Hz” shape (795 microseconds), it is by no means perfect. Above 2kHz there is a gentle roll-off in the treble, amounting to -1dB at 5kHz and -4dB at 8kHz, with slight ripples along the way. Below the turnover frequency at 200Hz there are signs of a massive but ill-tuned resonance; the response swings from +3.5dB at 180Hz to 0dB at 170Hz and +1.5dB at 160Hz in a manner reminiscent of FM demodulators. Its presence so near the turnover frequency makes it difficult to determine the intended characteristic accurately.

The next disc is numbered TC20, “Test Record for Electrical Pickups” (matrix number 2B4977 Take 5, mastered in No. 4 Studio, 19th April 1934). There had evidently been trouble with pickup armatures sticking to polepieces, because the record includes a band at 150Hz “for Freeze-over Test” which is about ten decibels louder than the rest of the record, which has eleven other fixed frequencies. The label doesn’t claim any particular characteristic, but the treble droops in a similar manner to TC17. The bass is considerably lifted; but I should explain that in those days the bass lift to equalise a “Blumlein shape” was hardly ever achieved electronically, but by the pickup arm resonating on the armature compliance at about 150Hz. I therefore suspect this disc was primarily used for checking such pickups weren’t thrown out of the groove, and it may not be meant as an accurate “Blumlein shape” at all.

In section 6.11 I mentioned the published disc HMV DB4037, cut in 1936. This shows no significant treble droop at the highest frequency (8.5kHz). It changes to constant-amplitude at 500Hz (318 microseconds), while the fixed-frequency tones on other discs of the set (DB4034-7) suggest a turnover at 150Hz (1061 microseconds), and the leaflet implies 250Hz (636 microseconds). It is too early to say whether these differences were consistent in any way. The result is that all Blumlein recordings made between 1931 and about 1944 are currently equalised constant-amplitude below 300Hz and constant-velocity above that. Occasionally there is strong subjective evidence that these transitions aren’t correct, but when I’m not sure I use 300Hz, because it’s roughly the geometrical average.

All these test discs have frequencies up to at least 8kHz (sometimes drooping slightly), but Blumlein’s system was not allowed to go as high as this in practice. There is an EMI memo written by B. Mittell on 27th October 1943 (when the idea of extending the range was going ahead), entitled “Experiments in Semi-High Quality Recording.” He lists experimental recordings which had been made between 28th March and 12th April 1935 “with equipment which cut off between 7,000 and 8,000 cycles.” His memo continues: “the results were not good. . . because of unsatisfactory reproduction at the higher frequencies. The present light-weight pickup was not then in existence, nor were the record surfaces considered good enough at that time to justify recording in that region.” Although the three test discs show 8kHz was quite feasible, the range was restricted to about 6kHz in practice.

6.33 How to recognise Blumlein cutters on commercial records, 1931-1944

HMV, Zonophone, other logos of The Gramophone Company, and repressings by RCA Victor: A square following the matrix number. A post-1945 matrix with no geometrical symbol is sometimes a dubbing of a 1931-1944 matrix.

Chappell, Columbia, Hugophone, Odeon, Parlophone, Pathé, Regal-Zonophone, Voice of the Stars: A C in a circle before the matrix number in the years 1931 to 1944. A post-1945 matrix with a plain C before the matrix number occasionally means a dubbing of a 1931-1944 matrix.

NOTE. A few Lindström recordings (issued on the Odeon and Parlophone labels) recorded in Strasbourg Cathedral in 1928-9, with matrix numbers prefixed XXP, have a C in a circle in the label-surround. These are not Blumlein recordings.

6.34 Summary of how to equalise the above

Equalise to a “Blumlein shape.” Start with Blumlein 300Hz (531 microseconds), and only vary it if you feel a compelling need to. For serious work, log the turnover you have used. Before 1936, a gentle treble lift amounting to +4dB at 8kHz may be added if the surface-noise permits.

6.35 The Gramophone Company system

A Gramophone Company report dated 12th June 1930 describes how HMV engineers hoped to evade Western Electric patents. Their Dr. Dutton made a trivial modification to a Western Electric microphone capsule (cutting “slots” instead of “grooves” in the backplate) in order to evade one patent; and a very conventional valve amplifier was built to replace the Western Electric push-pull design. A cutterhead invented by Angier, Gauss and Pratt (British Patent 372870) uses ideas voiced in that report.

When trials began in Abbey Road Studio 3 on 16th December 1931, the matrix numbers were followed by a “swastika,” actually the mirror-image of the swastika later used by the German Nazi party. When EMI was asked for an explanation of this in 1964, the enquirer was told that it meant “recorded by the Gramophone Company system,” which seems quite unambiguous; but I must also report an alternative explanation (Ref. 26). This says the mark identified a moving-coil cutter (presumably Blumlein’s), and that the swastika was changed after complaints from various continental export markets.

I have not been able to distinguish between these two explanations, and it is even possible both are true. If the first is correct, then only the cutterhead was novel. It was in fact a classical moving-iron cutter, and one of the features (in Claims 1 to 4 of the patent) was that the recording characteristic was defined by the electrical features of the coil, as we saw in section 6.13. Fortunately EMI did not pursue these claims, or a great deal of sound recording would have been strangled at birth! However, previous systems (like Western Electric) had used the same principle but not claimed it in a patent.

The resonant frequency of the armature was given as 5400Hz in the patent, with a peak of +4dB or +6dB, depending which graph you look at. Spectral analyses of surviving discs suggest a lower frequency and a lower peak than that, although the patent shows five designs of armature (which could have had different resonant frequencies), and the effect of the cutting stylus was not quantified.

The constant-amplitude turnover was described with the words “for example, 250 cycles per second.”

The performance of the microphone and amplifier do not seem to be documented, and we only have the historic photograph of Elgar and Menuhin at their recording of Elgar’s Violin Concerto in July 1932. This shows a Blumlein HB1 microphone, although the records all have “swastikas.” It is not generally possible to hear any difference between “swastika” recordings and others (possibly because they used the same microphones). Current practice is therefore to treat the former as if they were recorded to a “Blumlein 250Hz” curve.

6.36 How to recognise the Gramophone Company system on commercial records

HMV, Zonophone, RCA Victor re-pressings: A mirror-image swastika after the matrix number.

6.37 Summary of how to equalise “Gramophone system” recordings

Use “Blumlein 250Hz” (636 microseconds).

6.38 Extended-Range Blumlein recordings (1943-5) and later systems

In 1943 Blumlein’s cutterhead seems to have been given an extended frequency range. According to Ref. 27, 12kHz was reached, partly by replacing the original armature pivots with one steel torsion wire and one conical rubber pivot. Listening tests suggest that a primary resonance at about 500Hz is not always perfectly tuned - there is sometimes audible colouration in this region - but there is no objective evidence. Perhaps future spectral analysis methods will confirm this; in the meantime, I ignore the problem.

The “ER System” was first used towards the end of October 1943, but it was kept secret until March 1945, when the decision was made to modify the symbols on the matrixes so extended range records could be used for public demonstrations. For HMV the square had an additional diagonal line; for Columbia and Parlophone the C-in-a-circle had an additional diagonal line.

In the meantime another system was adopted, although it seems EMI engineers didn’t admit it, because it came from RCA in America! Since there were no patent implications, there was no geometrical symbol, although Blumlein ER was retained in Studio No. 1 until Sir Thomas Beecham had completed some multi-sided sets. After this the technical standard of EMI’s work is such that I cannot distinguish between different recording systems. The 78s all have an extended and apparently resonant-free frequency range of “Blumlein shape” until the introduction of International Standards in 1953. Test disc EMI JG.449 (mastered 5th-6th July 1948) documents this, showing a perfect response to 20kHz with the low-frequency turnover at 250Hz.

6.39 Summary of how to equalise “Extended-Range” and subsequent systems

Use “Blumlein 250Hz” (636 microseconds) for EMI coarsegroove discs mastered between November 1943 and (as we shall see) July 1953.

6.40 Early EMI long-playing and 45 r.p.m. records

Tape was often used for mastering purposes from 1948 onwards, but a great many commercial 78rpm releases were made from direct-cut discs recorded in parallel. From about 1951 tape-to-disc transfers became normal for 78s (they can usually be identified by a letter after the take-number).

Longer-playing records such as LPs and EPs became possible at the same time, and short items might appear on EPs and long-playing albums besides 78 and 45rpm “singles.” Historians of the gramophone have criticised EMI for being sluggish with the new media. Yet microgroove masters were cut as early as July 1949, evidently for export; British readers will appreciate the joke when I say LP masters were for US Columbia and 45rpm masters for RCA Victor. They had Blumlein characteristics (in this case, “Blumlein 500Hz” - 318 microseconds), even though they were not intended for clockwork gramophones! I have checked this by comparing 78s with early LPs and 45s of the same performances, assuming the former were “Blumlein 250Hz.” Some early microgroove discs had properties reminiscent of 1930s 78s, with colouration in the mid-HF and treble above about 6kHz attenuated; but these difficulties were solved within a year or so.

It is always easier to say when something started than when it stopped, and the same goes for this characteristic. I had to compare dozens of 78s with LPs and 45s of the same performances, after which I could say with some confidence that all three changed at about the same time. The critical date was 17th July 1953 or a few days later (Ref. 28), after which comparisons are consistent with what later became the two 1955 International Standards (section 6.7).

Unhappily matrix numbers cannot give foolproof boundaries, because EMI sometimes had twenty takes before cutting a perfect LP master (although the situation wasn’t quite so bad for 45s), and HMV’s 0XEA and 2XEA numbers seem to have been allocated well in advance (not in chronological order). In July 1953 Abbey Road was mastering microgroove in duplicate, and the take-numbers mean master-discs as opposed to artists’ performances. If a pair of masters failed, it would take three to eight weeks to get them cut again. Most previously issued Blumlein versions were later remastered to conform to International Standards, sometimes getting new matrix numbers in the process, in which case information for older versions has now disappeared. And I have found at least one case where the new version was still Blumlein 500Hz, presumably because of what was on the other side of the record; evidently changes in sound quality were avoided.

I hope the following table will help you identify the last Blumlein versions by matrix number, but please remember most must be approximate, and can only apply to Take Ones and Twos.


  • LPs: (10-inch and 12-inch) Between 2XEA213 and 2XEA392, and at 0XAV145.
  • EPs: 7TEA 19, 7TAV 28
  • SPs: Between 7XBA14 and 7XBA21, and at 7XCS 23, 7XLA 2, 7XRA 30, 7XSB 6, 7XVH 70
  • SPs: 7XEA688, 7XAV227 (Both series then jump to 1000)
  • 78s: Between 2EA17501 and 0EA17576


  • LPs: (10-inch and 12-inch) Between XA561 and XAX817; XRX12
  • EPs: 7TCA 7, 7TCO 6
  • SPs: 7XCA185, 7XCO 87 (Both series then jump to 1000)
  • 78s: Between CA22600 and CA22610, and at CAX11932


  • LPs: XEX 60
  • EPs: Probably all International Standard
  • SPs: 7XCE135 (Series then jumps to 1000)
  • 78s: Between CE14643 and CE14689

My comparison method falls down for most MGM records mastered in America,
because I haven’t anything else to compare them with. The following are British.

  • SPs: 7XSM203 (Series then jumps to 1000)
  • 78s: 0SM420


  • 78s: CAR6800


6.41 Other systems giving a “Blumlein-shaped” curve - amateur and semi-pro machines

I shall now list other systems known to have a characteristic comprising a constant-amplitude section below roughly 300Hz and a constant-velocity section above that. Virtually all disc-cutting machines meant for amateur or semi-professional use followed the principles outlined in section 6.13 which gave these results by default, unless altered by electronic means. You may find such discs (both coarsegroove and microgroove) in the form of “acetates,” “gelatines,” or short-run pressings.

In Britain, M.S.S. sold disc-cutters which dominated this market, and its director Cecil Watts maintained close co-operation with the BBC. A couple of pre-war BBC frequency test pressings exist mastered on M.S.S machines, which document Blumlein 300Hz objectively. During the war the strategic importance of M.S.S was such that it was taken over by the British Post Office, and development of the cutter was placed on a slightly more scientific footing; details are given in Ref. 29.

M.S.S cutterheads were comparatively cheap, and appealed to the man who could build his own amplifier and mechanism. A “Blumlein shape” will nearly always be appropriate in these cases. But M.S.S also sold its own electronic equipment and mechanisms, the former including the “Type FC/1 Recording Characteristic Control Unit” (1954) giving: “British” (i.e. Blumlein 250Hz); the BBC’s characteristic; and the “N.A.B. Lateral” characteristic (which will feature in section 6.65, but by 1956 it had been superseded). There was also circuitry for extending the cutterhead’s frequency range and flattening its resonance, and for compensating the end-of-side losses at the middle of the disc. In a well managed studio, these could have given almost professional results.

I was brought up with post-1956 M.S.S cutterheads, and can confirm that the fundamental resonance was 7kHz and the bass roll-off due to the coil was at 500Hz (318 microseconds). When International Standards matured, this didn’t give the 75 microsecond pre-emphasis needed for RIAA; a graph supplied by the company showed the additional equalisation needed, but not how to achieve it. (Ref. 30).

I should like to give you the equivalent figures for cutterheads supplied on other semi-pro disc-lathes, such as B.S.R., Connoisseur, EMI, Presto, and Simon; but in the case of the EMI machines, and sometimes the others, the recording amplifier included “tone controls” for subjective use, so the exact characteristic cannot be defined anyway. Many M.S.S and Presto machines were subsequently fitted with a Grampian cutterhead, a commercial version of the BBC Type B which could be used with non-motional negative feedback. (This would neutralise some or all of the bass-cut due to the coil). The resonance was 10kHz, thereby reducing S-blasting in the presence of pre-emphasis; but it was not available until 1953.

6.42 British Decca-group recordings 1935-1944

In section 6.26, I described how the early days of Decca seem to have been founded on a clone of a Western Electric cutter. The company’s chairman Edward Lewis thereupon took over most of the bankrupt record companies in London, acquiring several original pieces of recording equipment in the process, and it would be extraordinary if the best of each were not adopted. In the summer of 1935 Decca published frequency test disc EXP55 (later issued in Australia on Decca Z718). Although it comprises only fixed frequencies, it documents “Blumlein 300Hz” up to 8kHz.

It was not until mid-1937 that Decca took over the Crystalate group, which itself had been collecting equipment from other sources. Crystalate had recruited Arthur Haddy in 1931 to improve the (American) Scranton Button Co. equipment (which had also been supplied to the UK Dominion Record Company, see section 6.14). In a 1983 interview (Ref. 31), Haddy mentioned several subsequent pieces of cutting equipment, starting with a “Blue Spot” loudspeaker unit modified by himself, a cutter from Eugene Beyer of the German Kristall company, the first Neumann cutters, and a clone of the Western Electric cutter supplied by Jenks & Ader of America (in use when Decca took over Crystalate). In that year (1937) Haddy’s own moving-coil cutters were adopted throughout the group, since Haddy considered this was the only way to avoid S-blasting when stretching the performance of moving-iron cutters. These evolved in secrecy for the next twenty years, because the resonance was damped by the same means that Blumlein had described in British Patent 350998. Meanwhile, test disc EXP55 remained in the catalogue for another ten years, which I regard as evidence that there was no change of equalisation policy throughout all this.

In 1944 Haddy’s famous “full frequency range” system was implemented. As this used a pre-determined “non-Blumlein” characteristic, I shall discuss it in Section 6.66; but I believe “Blumlein 300Hz” will be appropriate for matrixes until number DR8485-2.

6.43 Summary of how to equalise Decca-group recordings 1935-1944

Use “Blumlein 300Hz” (531 microseconds)

6.44 Synchrophone

This was a company formed in 1931 to make use of a record factory in Hertford, England. The factory had changed hands several times during the years, and was vacant again after the collapse of the Metropole Record Company (makers of Piccadilly and Melba records). It is thought (but it’s not absolutely certain) that the recording machinery passed from Metropole to Synchrophone; at any rate, the matrix numbers are contiguous.

Synchrophone specialised in making records which would not have the usual legal complications when they were publicly performed. They were especially targeted at cinemas for providing interval music, when they have the logo “Octacros.” Some of these were reissues of the “Piccadilly” label, but new recordings for the Octacros label followed.

There is no known written evidence about the recording machinery, but various frequency test discs survive which provide embarras de richesse. I shall cut a long story short and tell you about the set of frequency records which I personally consider you should ignore - a set of ten single-sided 78rpm engineering test records, presumably for checking cinema sound systems, numbered from Tech.90 to Tech.99. They are so appallingly bad that it is difficult to see why they were published. But there is one frequency record which seems to make more sense. Several test pressings with the matrix prefix ST survive. One, ST.29, carries two frequency runs in about two minutes of groove, one comprising a sweep, and one fixed tones. It looks to me like a straightforward measurement of the performance of the cutter, rather than a disc intended to be used in anger; but its performance is much better than the series mentioned above. Better still, it seems to document the right curve for reproducing contemporary recordings – “Blumlein 300Hz.”

6.45 Summary of how to equalise Synchrophone recordings

Use “Blumlein 300Hz” (531 microseconds).

6.46 Conclusion to “Blumlein-shaped” equalisation

I am quite certain other manufacturers used “Blumlein characteristics”; unfortunately, I have no objective evidence. For example, I am sure Deutsche Grammophon used “Blumlein shape”, extending their frequency range at about the same time as EMI in Britain. Coarsegroove Polydor test discs were marketed at the end of the second World War; does any reader have access to one?

6.47 The Marconi system

There is a definite demand for the information in this section; but it isn’t complete, and several problems remain. So I am writing this in the hope that further research may nail things down.

The British Marconi company was probably the best equipped of all British companies to develop a new electrical recording system, and it was advertised as being exclusive to Vocalion. The first published matrix seems to be dated “August 11, 1926” (Ref. 32); and the system continued to be used until Vocalion’s studio was closed down after its takeover by Crystalate in 1932, that is to say about the end of 1933.

Many of its records have an M in a circle printed somewhere on the label. The following logos are known to have used the system at some time in their lives. Sometimes only a handful were electric; but you can assume that if it’s an electric record of British origin on any of these labels, it must be the Marconi company’s process: Aco, Aeolian-Vocalion, Broadcast, Broadcast Four-Tune (I believe), Broadcast Junior, Broadcast Twelve, Broadcast Super-Twelve, Coliseum, Scala and Unison. Also some Beltona issues (between catalogue numbers 1194 and 1282 - there was a change to Edison-Bell after that); many Linguaphones; a few Vocalions up to catalogue-numbers X10029 A.0269 and K05312 (later British issues were made by Decca from US metalwork); and National Gramophonic Society (catalogue “numbers” HHH to TTT and NGS.65 to NGS.102). Finally, some of the Broadcast Super-Twelves were reissued from original metals on the Rex label. Most early Rex records reproduce the catalogue number in the label-surround, and if there is a second number in a 3xxx series that is the Broadcast Super-Twelve catalogue number, and the Marconi process can be assumed.

After 1934 it is thought the recording equipment was sold to Linguaphone, who continued to use it until at least the second world war for recording new language lessons. It is also known that the system was used by the equivalent Vocalion company in Australia until about 1934.

I have discovered no written information about the technical aspects, except that it was a moving-iron system (Ref. 31) Listening tests suggest that at least three kinds of microphone may have been used at different times, and the first couple of years’ recordings suggest that they were cut by a “low resistance” cutterhead, since constant-velocity seems to give the correct musical balance above about 500Hz. The Marconi Company developed the Round-Sykes “meatsafe” microphone, which was also used by the BBC. A frequency curve for this microphone is given in Ref. 34, and the somewhat tubby bass of the earliest discs is consistent with that curve; but I do not consider we can equalise it with rigour, because there was a great deal of empirical adjustment. Controversy raged over whether it was better to hold the coil in place with three pieces of cotton-wool smeared with Vaseline, or four! (Ref. 35)

However, aural evidence suggests that there were two further developments at least. In late 1928 some of the early “Broadcast Twelves” were sounding very shrill and metallic, which might be a different microphone - possibly the Marconi-Reisz described in Ref. 36. But despite the lack of written information, a quirk of fate enables us to get some objective measurements of the cutter in the 1930s.

Two records were issued on the “Broadcast” label which we can analyse to obtain parts of the frequency response. One is Tommy Handley’s sketch “Wireless Oscillations and Aggravations” recorded in about November 1931 and issued on Broadcast Super-Twelve 3133. The other is “Sandy At The North Pole,” a comedy sketch recorded by Sandy Powell in about December 1932 and published on Broadcast 926 (a nine-inch
diameter disc). The plot of the latter involves his sending a radio message home. (This was the origin of his catchphrase “Can you hear me, Mother?”)

On both these records, it seems the Vocalion engineers plugged a beat-frequency audio oscillator into the mixing desk, and twiddled the frequency arbitrarily to simulate the heterodyne whistles common on A.M radio reception in those days. (There’s no proof this happened, but the measured results admit no other explanation). Although there is speech on top of the oscillations, and there are places where the volume alters while the frequency stays the same, it is possible to take slices out of the recordings between the words and analyse them.

These show that (in the years 1931-2, anyway) the electronics and cutter gave a constant-velocity characteristic between 1490Hz and 4300Hz accurate to one decibel, and between 800Hz and 4600Hz within two decibels. The frequency goes as high as 5200Hz on “Sandy at the North Pole”, and isn’t more than 4.5dB down there; but I’m not quite certain the volume was stable at the start of this sweep. Also the nine-inch disc shows frequencies around 750Hz to be about +3dB higher. The ten-inch doesn’t have this effect, and I can’t explain it. However, the consistent results between 1500Hz to 3000Hz also support the theory it was a low-resistance cutter.

I have also been trying to find examples of the same material mastered at different times, so I can compare “like with like” using aural methods. So far, my best experiment uses John Gielgud’s performance from “Hamlet” (Act 2, Scene 2), which he recorded for Linguaphone twice (once by Marconi’s system, and once by Decca’s ffrr system on 78). I am very grateful to Eddie Shaw for telling me the former was published in February 1931, and the latter in August 1955. Gielgud also recorded it a third time on LP (HMV ALP1483). The three performances are almost wilfully different, so comparisons are not easy; but there are a couple of sentences delivered at similar pitches and rhythms which permit qualitative comparisons.

The 1931 Linguaphone is noticeably brighter in the upper harmonics of the vocal cords than either of the others when played with a Blumlein-shaped curve. There are two possible explanations. One is that Marconi’s system emulated Edison-Bell’s (section 6.14), by equalising the mike to give a 3db-per-octave slope (but this didn’t apply to the oscillator). The other is that 1931 mikes were even bulkier than Western Electric ones, so they reflected more sound back at the performers, doubling the electrical output above about 2kHz.

I find the three discs match best when I set the 1931 one to the BBC’s “2dB-per-octave” curve (section 6.57). It still isn’t quite right to my ear; but listening is now being overwhelmed by differences between the performances.

That’s the current state of the art so far as the Marconi system is concerned. Needless to say, I should be interested if readers can come up with anything more.

6.48 BBC Disc Record equalisation - Introduction

The British Broadcasting Corporation did not always follow international (or de facto) standards with its discs. The next fourteen sections summarise the electrical characteristics of such discs so they may be reproduced correctly. There may be some urgency for this, because the BBC sometimes gave performers a cellulose nitrate disc instead of a fee, and such “nitrates” are not only unique, but have a limited shelf-life.

As you might expect from a large and cumbersome organisation like the BBC, there were several changes of disc recording policy over the years, but hardly ever any clean breaks. There were long periods of stability, so for 90 percent of BBC discs I can refer you to the summary in section 6.61 below. But the other ten percent are in “grey areas,” sometimes with very unexpected characteristics. I must recommend you to become familiar with the BBC’s disc recording history (Ref. 38) to assess what technique was used for the particular disc you hold in your hand.

From about 1955 onwards, the BBC tended to copy its older formats onto new media. The most important examples concern Philips-Miller film recordings, copied to microgroove LPs; but as the original films have now been destroyed, we must use the LP discs. Other items were copied from 78rpm and 60rpm discs, but there is evidence that occasionally engineers of the time did not get the equalisation right, or else they were copied before International Standards were adopted. Ideally, you should have access to the “originals,” whatever they may be; but if you haven’t, I can only ask you to be aware of the evolution of BBC practices, so you may detect when an equalisation error has been committed, and reverse-engineer it.

I have been doing industrial archaeology research to establish what was going on. There are three basic sources of information:

  • (1) Surviving engineering test discs
  • (2) Analysis of circuitry and hardware in contemporary engineering manuals
  • (3) Spectral comparisons of the same sounds recorded by different equipment

I will try and indicate when one system gave way to another as accurately as I can; but the dates and matrix numbers will often be approximate. The numbering-systems for both the matrixes of pressed discs, and for “current library” nitrates, should be understood for this reason. They will be considered in sections 6.51 and 6.52 below. After this we shall study the various phases of disc recording history with consistent equalisation.

6.49 The subject matter of these sections

I cover all disc records mastered inside the BBC. The range includes:

  • 1. Pressings and nitrates for the “BBC Archives,” also known as the “BBC Permanent Library.”
  • 2. Pressings for the “BBC Transcription Service” for use by broadcasters overseas, additional copies of which were sometimes pressed for (1) above.
  • 3. Pressings for the “BBC Sound Effects Library”.
  • 4. Records whose logo, printed in green, consisted of the words “Incidental Music.” (Signature tunes and the like, mass-produced for internal consumption).
  • 5. Nitrates cut for immediate use, administered by the BBC Current Library or one of its branches. These have a lick-and-paste label with a space for a handwritten “R.P Ref. No.” This means “Recorded Programme Reference Number.” Such a number has a distinct structure incorporating the code for the branch library concerned, and enables them to be distinguished from the other libraries. The BBC also mastered records for other organisations with yet more logos. Among these I can name the “London Transcription Service” and “The Joint Broadcasting Committee” (both wartime precursors of the BBC Transcription Service), the “Forces Broadcasting Service,” and “A Commonwealth Feature Programme.”


6.50 Pre-history of BBC disc recording

The earliest problem is simply to recognise which discs were done upon BBC equipment. Before the BBC got its first disc cutting machines in April 1934, and for some years after that, much BBC recording was carried out by commercial record companies, mainly EMI and British Homophone (makers of “Sterno” records). The former can be recognised by matrix numbers prefixed 0BBC or 0BBCR (ten-inch) or 2BBC or 2BBCR (twelve-inch). These were mastered using equipment designed by Alan Blumlein, and the “Blumlein 300Hz” characteristic is appropriate here.

The British Homophone records have a much finer groove-pitch (actually 150 lines per inch). They usually have plain matrix numbers in the shellac, but the prefix “Homo” is added on the label. This appears to have been a “high-resistance” system (section 6.14).

Anything you find with a matrix prefix plain “BBC” and the words “Record manufactured for the B.B.C. by the Decca Record Co. Ltd.” is from a nitrate cut by the BBC, not a Decca master. We shall see later that “Blumlein 300Hz” (531 microseconds) is correct.

When the labels carry the words “Record manufactured for the B.B.C. by the Gramophone Co. Ltd,” a pure “BBC” prefix still means a BBC master-disc; but when they have matrix prefixes 0BBCD or 2BBCD, these may be either Gramophone Co or BBC masters. As both organizations used similar characteristics, again it will be sufficient if I say “Blumlein 300Hz” (531 microseconds).

6.51 BBC matrix numbers

Any disc which was “processed” (that is, made into a metal master so copies could be pressed) was given a “matrix number,” which appeared on stampers and on finished pressings because metal cannot have a written label. The first BBC-mastered matrixes had the prefix “BBC”, often hand-scribed onto a “current library” nitrate after it was finished with. Number BBC1 was a ten-inch pressing made for the BBC Sound Archive (their catalogue number 588).

When the BBC became responsible for the engineering work associated with overseas radio stations during the war, the matrixes had different prefixes to indicate the size and the pressing company. (These records were the foundation of the Transcription Service).

In September 1942 the two operations were amalgamated. With true British-style compromise, the prefixes were adopted from the Transcription Service, and the number suffixes from domestic radio, which had by then reached about 8000.

An extra letter was added to determine which was which, and the following prefix code evolved:


  • 16 = 16 inch diameter
  • 12 = 12 inches
  • 10 = 10 inches
  • 7 = 7 inches

Second, if there at all:

  • F = fine-groove.


  • P = Transcription Service Processed Disc
  • R = Recorded Services (i.e. domestic radio or TV).

Fourth, the company which did the galvanic and pressing work, thus:

  • D = Decca, Raynes Park
  • H = British Homophone Company
  • M = EMI Ltd., Hayes
  • O = Oriole Ltd. (later CBS)
  • P = PR Records, Wimbledon
  • R = The Transcription Manufacturing and Recording Co. (C. H. Rumble), Redhill.
  • RR = Rediffusion, Caerphilly
  • S = Statetune, Leicester
  • W = Nimbus Records, Wyastone Leys, Monmouthshire

Fifth, matrix number, and if not “Take 1”, a take number. The take number indicates the attempts at cutting a master-disc, not different performances. The numbers formed an essentially continuous sequence, the world’s largest run of matrix numbers, ending at 162695 in 1991.

Sixth, -S if stereo (confined to the early days of stereo only).

For example: 16PH meant 16-inch (implied coarsegroove) disc processed by British Homophone from a Transcription Service master, and 7FRO meant 7-inch finegroove processed by Oriole from a domestic master.

6.52 BBC “current library” numbers

Apart from Broadcasting House in London, a large number of regional centres and overseas studios made disc recordings. There were literally hundreds of these at various times; the following is only a selection. To save constant communication and delays, they allocated their own serial numbers to form current library recordings. The actual numbers were originally restricted to five digits, which were thought to be enough for a current library where recordings were not kept permanently; but by 1963, Bush House had been “round the clock” several times! Duplicate-numbered tapes were liable to appear unexpectedly, so six digits became the norm. The serial numbers were prefixed by three or four letters as follows.

First letter (if there at all):

  • C = a copy from another recording, whose identity was supposed to be indicated in the box marked “Source” on the label and the accompanying Recording Report.
  • P = Master nitrate disc intended for processing, or a backup for same.

Second letter: Format (I shall only cover disc media here; a similar system applied to tapes):

  • D = 78rpm coarsegroove nitrate disc, or a set of such discs
  • F = Fine-groove nitrate disc
  • M = Mobile recording (usually cut on a disc-cutter in a van or car)
  • S = Slow-speed (33rpm) coarsegroove nitrate disc, or a set of such discs

Third and fourth letters: Studio centre allocating the number.

  • AB = Aberdeen
  • AH = Aldenham House, Hertfordshire
  • AM = America (usually the New York studio)
  • AP = Alexandra Palace
  • BE = Belfast
  • BG = Bangor, North Wales
  • BM = Birmingham
  • BS = Bristol
  • BT = Beirut, Lebanon
  • BU = Bush House, London
  • CF = Cardiff
  • EH = Edinburgh
  • GW = Glasgow
  • GY = Germany
  • LG = Lime Grove
  • LN and LO = Broadcasting House, London, and buildings nearby
  • LS = Leeds
  • MR = Manchester
  • NC = Newcastle
  • OX = 200 Oxford Street, London
  • PY = Plymouth
  • RW = Radiophonic Workshop, Maida Vale, London
  • SM = Southampton
  • SW = Swansea
  • WN = Wood Norton, near Evesham


These location-codes would usually give the home of the disc-cutting machine, not of the performance; the latter would be indicated in the “Source” box on the label. Thus, DBU123456 would be an original 78rpm coarsegroove nitrate cut at Bush House, and PSOX12345 would (nowadays) be a surviving backup disc for a master nitrate “slow speed disc” (33rpm coarsegroove, usually 44cm diameter), the original of which was sent off many years previously to be made into shellac pressings, cut at 200 Oxford Street London.

In the following sections, I will list the equalisation histories in approximately chronological order. For processed recordings (as opposed to nitrates), I have attempted to give the matrix numbers relevant to each recording system and characteristic. I shall not bother with matrix prefixes, because there were always several in use at once.

6.53 BBC M.S.S. recordings

This section comprises recordings mastered upon the early “Marguerite Sound Studios” (later MSS) equipment between 1935 and 1951. (Ref. 38). The cutting heads were hired to the BBC by inventor Cecil Watts, because he was not happy with their performance and wished to update them immediately. Two frequency test discs of the period survive, enabling us to measure the actual performance today. Number XTR.22 comprises a gliding-tone interspersed with fixed tones. The result is pure “Blumlein 300Hz” from 30Hz to 2kHz, but above that there is a broad peak averaging +3dB from 3kHz to 7kHz, falling to zero at 8kHz, the highest recorded frequency. This disc does not carry any matrix number, so it is difficult to date. But it is a centre-start disc (the tone glides
upwards). Centre-start was abandoned on 27th April 1937, so it is earlier than that, and definitely within Watts’ experimental period.

The other is a British Homophone ten-inch test-pressing, almost certainly made to see how British Homophone coped with electroplating nitrate lacquer rather than wax. It has the matrix number BBC210, which would date it to the end of 1935; it carries fixed-frequency tones only. It is pure “Blumlein 300Hz” all the way from 4kHz downwards, although 5kHz is -4dB and 6kHz is -5.5dB. This may partly be due to the low recorded diameter.

Although individual cutters and cutterheads may not have given results quite like these specimens, the basic equalisation characteristic is quite unambiguous – “Blumlein 300Hz.”

6.54 BBC American equipment

From 1942 to 1945 the BBC imported large numbers of Presto disc-cutting machines. These would have had a classical “Blumlein shape” until the BBC replaced the original cutterheads in about 1948. Test disc DOM46-2 is thought to date from this period; it carries professional announcements, so was certainly intended for everyday calibration of reproducing equipment, and it therefore shows the intended curve precisely. It is absolutely correct “Blumlein 300Hz” up to 10kHz. This assertion is also confirmed by some early coarsegroove 33s, which we shall consider in section 6.56.

6.55 BBC transportable equipment

The BBC’s Type A Cutterhead was used on its Type C transportable disc recorders from 1945 to 1961. My graph in section 6.13 shows it recorded Blumlein 300Hz characteristics. Nitrates for immediate transmission cut on these machines generally have an R.P. Ref. No. beginning with M; they are nominally 78rpm, batteries permitting!

The BBC also had portable disc recorders based on clockwork gramophones for its reporters on location during the war. They had piezo-electric cutterheads whose performance has not yet been analysed, although at least one machine survives. However, it seems that most of the discs cut on such equipment were dubbed. (Or worse! Sent by a short-wave radio link, for example). The clockwork motor could only provide enough torque for a ten-inch disc, and that with a relatively shallow groove. As far as I know, very few such recordings were made into pressings for these reasons, although I believe some appeared in the Sound-Effects catalogue.

6.56 BBC coarsegroove 33rpm discs

We have no definite evidence whether the same characteristic was used for long-playing coarsegroove 33rpm records, which became practicable when the BBC started making its own sapphire cutters in 1941. The documentation for the various recording machines does not mention the subject, so one would expect “Blumlein 300Hz” to be valid for these as well. There are a few scraps of evidence to support this claim, but the evidence is not complete.

A rather curious frequency-run appears on the back of some single-sided 16-inch 33rpm BBC Archives pressings (for example X.19910 or X.20865). Presumably it was just a suitable stamper which could be used for the “blank” side. It was definitely recorded on a machine with a manual hand-wound scrolling mechanism (the Presto would qualify). It starts at a diameter of thirteen inches. It too is “Blumlein 300Hz” in shape, but it has a well-damped resonance at 5kHz, and the higher frequencies are much inferior.

They peak -4dB at 6kHz and -13dB at 8kHz. I say “peak,” because the actual recorded level varies with the rotation of the disc - twice per rotation, in fact, following variations in groove depth. This shows that the mechanical impedance of the lacquer was comparable to the mechanical impedance of the cutter. Neither the MSS nor the BBC Type B cutterheads had such a low mechanical impedance, so I am sure this run was cut by a Presto cutterhead, despite the pressings being made almost a decade later. There are no announcements, so I do not think the run was intended to be used seriously. Instead, I consider it reflects the real performance of the original Presto gear.

Other single-sided sixteen-inch BBC Archives discs have a different frequency run on the back (for example, X18991 or X20755). This is recorded at the full sixteen-inch diameter and includes a professional announcer to speak the frequencies. So it is probably the stamper for a routine engineering test record which hasn’t survived elsewhere. But as it has no label, I cannot assume this. It is the weak link in my reasoning, which means it might not document the intended characteristic. But it is a very accurate “Blumlein 300Hz” at all frequencies up to 10kHz.

The curve for coarsegroove 33s definitely changed at a later date (about 1949), but as 78s are equally affected, I shall present the evidence in the next section.

6.57 Later BBC coarsegroove systems

I am obliged to give this section a rather woolly title, because what I mean is the equipment designed to cut discs to be reproduced by EMI Type 12 pickups. This model of pickup was developed during the last days of the war. Such was the demand from professionals and researchers that it wasn’t available domestically for some years. (Refs. 39, 40). With a minor BBC modification, its “open circuit response” (that is, its output when connected to a high electrical impedance) was flat to 10kHz. But if it was terminated with an impedance of the order of ten ohms, a slope approaching 2dBs per octave resulted. This was a reasonable compromise between the constant-velocity of British commercial records and the ideal constant-amplitude curve. I have examined the circuit diagrams of many BBC reproducing systems (including those on cutting lathes) to check that the pickup was always terminated in this manner. It was often used with a matching transformer whose characteristics were described in a different manual, so the circuit diagrams on their own aren’t sufficient; but when I took this into account, I found that wherever there was an EMI Type 12 cartridge, there was also a load between ten and twelve ohms, without exception. The 2dBs-per-octave characteristic would be engineered into the cutting amplifiers when the pickups changed.

A new cutterhead, the BBC Type B, was developed which also had a response to 10kHz. It was less sensitive than the Type A, so it could not be used with the Type C battery portable equipment. Its lack of sensitivity (and therefore liability to overload) was compensated by non-motional feedback, which also neutralised a low-frequency turnover due to the resistance of the coil (which now had to be engineered into the cutting amplifiers). The new cutterheads were installed on the new BBC-designed Type D lathes.

Prototype Type Ds were used from the spring of 1945, and aural evidence suggests they were recording Blumlein 300Hz. But there were evidently some modifications before July 1947, when a paper describing the equipment also described the “2dBs per octave” equalisation (Ref. 41), and the production run dated from January 1949. (You can tell a Type D recording because it has a motor-driven scrolling mechanism, giving a runout groove of absolutely constant pitch). The Type D was used for both 78rpm and 33rpm coarsegroove discs, although the radius compensation was different of course. An identical cutterhead mounted in a different shaped case was retro-fitted to the Presto machines about the same time. The electronics of all these machines were designed to give the inverse characteristic to the EMI 12 pickup.

This is documented by two surviving 78rpm test discs, XTR311 and DOM85, both with professional announcements, so giving the intended performance.
(= The curve is rather difficult to synthesise using conventional networks. Adrian Tuddenham has developed a circuit which gives the required characteristic within 0.5 dB.)

No 33rpm frequency-response test discs for use with EMI 12 pickups survive, but all the circuit diagrams for both recording and reproducing gear show there was no equalisation change between the two speeds, so it seems certain the “2dBs-per-octave curve” applies to 33rpm coarsegroove discs from this time as well.

An exception may be found in Wales, where semi-portable Presto recording kits were fitted with Type B cutters fed from non-feedback amplifiers; presumably these behaved like classical electromagnetic cutterheads under these conditions (section 6.13). This would only apply to mobile recordings made by Cardiff, Swansea or Bangor after 1945 (the R.P. Ref. No. prefix would be MCF, MSW or MBG). Also, the wartime Type A cutters continued on Type C lathes elsewhere until at least 1961. To recognise these I use the lack of scrolling facilities, the general instability in groove-depth, the striations caused by the swarf-brush, and (on Current Library nitrates) the M before the R.P. Reference Number. Irrespective of date, these should all be reproduced “Blumlein 300Hz”, as we saw in section 6.13 above.

From about 1949 to 1952 the BBC Permanent Library and Sound-Effects sections acquired, and in most cases re-mastered, a collection of wildlife and other sounds recorded by Ludwig Koch. Many of these had first been recorded in EMI’s mobile recording van in 1936-1937 using Blumlein equipment, while others were done upon Koch’s own portable MSS recorder after the war. Both would have given “Blumlein 300Hz” equalisation. I have had the privilege of hearing test-pressings of some of Koch’s originals, and can confirm that they were reproduced to the wrong characteristic when they were dubbed. Fortunately, the situation can be reversed by reproducing the dubbings to the “Blumlein 300Hz” characteristic instead of the “correct” characteristic. Ludwig Koch’s name always appears on the labels.

Now for the $64000 question. How can you tell whether a recording is done to the “Blumlein 300Hz” characteristic or the “2dBs-per-octave” characteristic? I am afraid you can’t. Before May 1945 it is bound to be “Blumlein 300Hz,” and after 1949 it is bound to be “2dBs-per-octave,” but the changeover is ill-focussed. I can only make the following suggestions:

  • (a) It would have been logical to change the recording equalisation at the same time as the new pickup cartridges were installed. This was probably done in one BBC building at a time, starting at Broadcasting House and working through the other London premises, through the major regions, to the minor regions.
  • (b) Material recorded especially for BBC Archives, BBC Sound Effects, etc. would have been given higher priority in view of the likely usage at a later date. (c) Listening tests done with a number of BBC Current Library nitrates suggest that the vast majority did not change to “2dBs-per-octave” until the spring of 1949, but the prototype Type Ds may have antedated this.

Some types of disc do not carry any dates - Sound Effects and Transcription pressings - so I will repeat the above paragraph in matrix number terms. Coarsegroove matrixes below 50000 are bound to be “Blumlein 300Hz,” and those above 70000 are bound to be “2dBs-per-octave.”

6.58 Early BBC microgroove discs

The first BBC microgroove discs of the 1953 Coronation were cut by a commercial firm. A year or two later the BBC commenced its own microgroove mastering on modified Type D equipment, but with new recording characteristics. First we will consider the “domestic” discs (i.e. those not mastered by the Transcription Service), which have an R in the matrix prefix. No details of the circuit modifications for the domestic microgroove machine seem to have survived, but there is little doubt that a curve equivalent to “Blumlein 1000Hz” was adopted. (That is, constant-amplitude below 1000Hz - 159 microseconds - and constant-velocity above that).

I obtained this information by three different methods: comparisons between BBC Sound Archive discs and surviving master-tapes (e.g. LP25682 and TBS17227), comparisons with commercial discs of the same material (e.g. LP24626 and Decca LF1330), and with double-sided discs of similar material (an example will be mentioned in the next paragraph). I have no doubts myself, but I must stress that this is my subjective judgement. Objective truth cannot be established unless someone discovers a microgroove BBC frequency test disc of the appropriate provenance, or an internal engineering memo on the subject.

This aural evidence also suggests that the “domestic” Type D microgroove machine retained this circuitry long after British and International standards were adopted by everyone else - at least until the end of 1960 - the change being between matrixes 105402 and 105506. Even this is not quite clear-cut, because Take 2s and Take 3s of lower numbers exist, which are also RIAA. One notorious example is BBC Sound Archives LP25926, comprising one long poem read by the author W. S. Graham, which has the matrix numbers 12FRM104536-3 and 12FRM104537. Side 1 is RIAA (the International Standard); but when you change to side 2, there is no readily-available technology which will equalise the sudden loss of bass and treble.

So, if there is no take number (implying “Take 1”), if it is a BBC microgroove disc with an R in the matrix prefix, and if the matrix number is less than 105403, I personally consider the item should be equalised Blumlein 1000Hz.

6.59 BBC “CCIR characteristics”

The 33rpm coarsegroove discs made by the BBC Transcription Service in 1954 and 1955 (with a P in the matrix prefix) specifically state “CCIR Characteristics” on the label. The earliest and latest examples known to the writer have the matrix numbers 79342 and 88969. The former was in fact recorded on 23rd December 1953, so it is possible the Transcription Service intended the new characteristic to take effect from January 1954. The last one cannot be dated, but its subject matter is the Mau Mau Disturbances of February-April 1956. Thus it seems the Transcription Service did not change to either of the new International Curves for their coarsegroove 33s in 1955 or 1956. The turnovers for the CCIR curve were 3180Hz and 400Hz (50 microseconds and 450 microseconds, see section 6.71 below).

The same curve is presumed to have been used for microgroove Transcription discs as well, but it is only occasionally mentioned on the labels. Quite a few domestic BBC Archive microgroove discs were made from Transcription matrixes during these years; these would also be CCIR, unlike the domestic ones. It is easy to tell; the P in the matrix prefix gives it away. Besides, they usually carry a conspicuous sticker over the Transcription label to “convert” them into Permanent Library discs. Or they have words such as “With T.S Announcements” or “As edited for T.S” on the label.

6.60 RIAA and subsequently

From 1956 the BBC Transcription Service used the new international RIAA standard for microgroove discs (75, 318 and 3180 microseconds). This is certainly true for all matrixes with a P in the prefix and numbers greater than 10FPH 97650.

Domestically the BBC did not change to RIAA for its microgroove discs for another four years. The earliest for which RIAA is certain has the matrix number 12FRD105972, but subjective evidence suggests that 10FRM105506 is also RIAA.

There is absolutely no doubt that the BBC continued its own “2dB-per-octave” characteristic (section 6.57) for all coarsegroove discs, whether processed or not. They continued to be compatible with EMI 12 pickups until the last Current Library nitrates were made in 1966.

There are numerous examples of early 78s being dubbed to microgroove in the 1960s and 1970s, and I must warn you that many show signs of having been reproduced to the “2dBs-per-octave” characteristic instead of “Blumlein 300Hz.” You may find it necessary to reverse-engineer this mistake, which will only occur on pre-1948 subject matter or location recordings made on Type C gear. If the microgroove dubbing has a matrix prefix incorporating the R which determines that it was mastered domestically, and the matrix number is less than 105403, then the situation has been made worse by the use of the “wrong” LP equalisation as well.

6.61 Brief summary of BBC characteristics

For all pre-1945 coarsegroove discs, and post-1945 ones cut on mobile recording equipment, use “Blumlein 300Hz,” except for pressings made from Homo (sic) matrixes.

For all post-1949 78s (excluding mobile recordings), and all post-1949 coarsegroove 33rpm nitrates, use the BBC’s own “2dBs-per-octave” curve. For all microgroove records whose matrix prefix includes the R for “domestic,” matrix numbers less than 105403, and no take number, use Blumlein 1000Hz. (This applies until about the end of 1960).

For BBC Transcription discs made in 1954 and 1955 (with a P in the matrix prefix), use CCIR Characteristics (50 and 450 microseconds).

For all post-1956 microgroove discs with the P for “Transcription” in the matrix prefix, and post-1961 microgroove discs with the R for “Domestic,” use RIAA Characteristics (75, 318, and 3180 microseconds). For all other discs, please consult the text.

6.62 “Standard” equalisation curves

Instead of studying the industrial archaeology of early disc recording equipment, from now on I shall now be listing the various “standard” curves used in disc recording - in other words those defined in advance, as opposed to those dictated by the machinery.

Such “standards” (apparently clear-cut) are bedevilled with traps for the unwary. I consider the whole subject should be a warning to today’s audio industry; practically everything which could go wrong did go wrong, and it isn’t anybody’s fault. But much worse is everyone’s apparent attempts to hide what happened.

6.63 General history of “standards”

The first standards were planned before the advent of microgrooves or slower rotational speeds, and then advertised without clearly saying what they applied to. Immediately after microgroove was introduced, the “battle of the speeds” (33rpm vs. 45rpm) meant that different standards didn’t matter, because the discs had to be played on different turntables. But two-speed and three-speed turntables soon became normal, and the chaotic state for commercial recordings began to be realised (Refs. 42, 43 and 44).

At that time standardisation was sometimes opposed on the grounds that microphones, microphone positioning, cutterheads, etc. had a bigger effect (Ref. 45). With present-day knowledge we have an even stronger argument on these lines, although it was never voiced at the time. Record companies made masters on transcription discs or on 78 sides before magnetic tape became good enough and with sufficient playing time; and there weren’t any tape standards until 1953! Consistent reproduction was only possible from tapes with a full set of calibration tones (with the additional advantage that the performance of the tape was documented as well as the recording characteristic).

Other attempts to define disc characteristics suffered the basic ambiguities of definition I mentioned at the end of section 6.7. The Audio Engineering Society of America suggested the “playback half” of the problem, and encouraged engineers to make their recordings sound OK played this way, thus dodging the problem of making a disc-cutting system which actually worked correctly.

Many early LPs were packed in sleeves with equalisation details; but I know cases where sleeves were reissued without the details, or records were re-mastered but packed in old sleeves. I have also met cases where the sleeves were printed in one country and the records pressed in another, with consequent mismatches in the documentation; and second-hand record shops have proved unhelpful, because proprietors (or their customers) swap sleeves and discs to get a complete set in good condition. I can think of only one way to solve these difficulties - collect a number of records of the same make, and of the relevant age and country of origin, and study the sleeves as well as the discs to work out “the originals” (which is what I’ve tried to do here).

Some manufacturers consistently advocated the same standard curves, even when they pressed records from imported metalwork (or even their own old metalwork) made to other characteristics; and of course there were always “black sheep” who never said anything at all on the subject! There is considerable aural evidence that users of each of the standards converged to reduce the differences between them, although they never admitted it. (Ref. 46)

International disc standards began to emerge in 1955, although leading manufacturers had adopted them somewhat earlier. The microgroove one is commonly called “RIAA” after “The Recording Industry Association of America Inc.”, which promoted it. Nearly everyone simply shut up about what they’d been doing to save old stocks of pressings becoming redundant. Such a problem has always afflicted a transition from one characteristic to another of course, but even this begs the question of when a standard “came into force” in the political sense. The national standards in European countries all changed at different dates. Any “standard” is preceded by practical tests and experiments, so today we have RIAA LPs which antedated the official promulgation. And of course some time might elapse before records made to new standards actually appeared in the shops. However, for stuff mastered on disc rather than tape, this was better than putting it through two extra generations just to change the equalisation!

We can often say what standard a company used prior to RIAA, but it is almost impossible to say when the change took place. Straight listening tests aren’t reliable when makers were trying to make their records sound like everyone else’s.

There were yet more problems after international standards were adopted. Several organisations refused to use them, and at least one country attempted a Unilateral Declaration of Independence on the matter. But the worst trouble was when old recordings were reissued. Although they might be “re-mastered” from original tapes, empirical tweaks might be superimposed to bring them to “modern standards” subjectively. Some were re-mastered with pre-1955 matrix numbers but new “take numbers,” and much discographical work, together with spectral measurements and reverse-engineering, may be needed to find the correct “original sound.” Or a pre-1955 record would be pressed from original metalwork (or dubbed for reissue) without equalisation being taken into account at any stage, even though the reissue might even carry a post-1955 copyright date. And there seem to be several cases where a manufacturer did not follow an unwritten convention – namely the take number on the disc itself should document the attempts to cut a satisfactory disc master (and to process it) until a satisfactory metal negative was achieved.

But the biggest record companies had rigorous engineering procedures, and respected the wishes of their producers. Reissues from-and-by companies like EMI, British Decca, and Philips are usually correct. But for lesser companies, the craft of recognising who cut the master-disc and when (normal to collectors of 78s) needs to be extended to early 45s and LPs. I shall simply be trying to recommend the equalisation to get what the disc cutting engineers intended. This usually does not include variables like microphone performance, the performance of a specific cutterhead, or misaligned master tapes. It is assumed that manufacturers compensated these phenomena at the time the disc was mastered if they wished.

6.64 Defining standard curves

I described the RIAA curve in section 6.7. I shall define the others in microseconds as follows. First the high-frequency pre-emphasis turnover; then the turnover between constant-velocity and constant-amplitude (usually equivalent to -3dB points between 300 and 636Hz, or 531 to 200 microseconds); and finally the transition (if any) to constant-velocity again for very low frequencies.

I must also say a few words about where I obtained my information. First, I must explain that I have ignored constructional articles and the like which made presumptions about curves without giving any references to official information, or clues beyond “it sounds best this way.” It’s not my job to trace such rumours to their source, but as a warning I shall cite Paul W. St. George and Benjamin B. Drisko’s article in the US magazine Audio Engineering for March 1949. They explained they had made protracted efforts to discover the characteristics used by record companies, but had had only partial success, and were forced to publish a list which they admitted was not necessarily accurate.

For example, their list shows (British) Decca’s “ffrr” 78-rpm pre-emphasis (section 6.68 below) as 3dB/octave above 3kHz. What is interesting is that this was reproduced in various other articles in American journals until at least November 1954, although I have never found it in Europe, and it is contradicted by “ffrr” test discs and written sources from English Decca themselves. I strongly suspect the source of this evidence was an ordinary tone-control being set to give the nearest flat response and then subsequently measured. And as far as I know, the American company called Decca never used British Decca’s curve at all, except when they repressed discs from British Decca’s metals (and their catalogues tell you this). And when British Decca established the “London” logo in North America in late 1950, they used the same procedures as in Britain.

Next, other evidence contains inconsistencies and sometimes contradictions, but it usually turns out to be a matter of interpreting ill-expressed evidence correctly. Each characteristic might be documented in one or more ways:

  • 1. In actual time constants. This is unambiguous.
  • 2. Published frequency records, which have to be measured and the results converted to microseconds (often with an element of “curve-fitting.”) Since the extremes of the frequency-range might well differ for the reasons I mentioned in section 6.8, I have had to exercise “editorial judgement” to decide where an engineer made a decision to leave the characteristic. I shall use the word “plant” to describe where the master-discs were cut. Usually each disc-mastering plant had only one or two cutting-lathes - sufficiently few for them all to have the same characteristics, anyway. I know only two exceptions: American Columbia, which had at least one microgroove lathe for American broadcasters, and another for the first commercial LPs (which they invented in 1948); and the BBC (who had different procedures for their Transcription Service and their domestic archives - see sections 6.48-6.61above). After this, the problem resolves into determining (a) what characteristics were used by what plant at what date, and (b) which records (or matrixes) come from that plant. I have attempted to list what I know about British plants, because that is my speciality; but I have only made a very incomplete start upon plants overseas. If anyone has any definite knowledge, PLEASE will they contact me?
  • 3. Published frequency responses, either of the measured response of the actual hardware, or of its theoretical characteristic. Such responses may be in tabular or graphical form. They have to be converted as in (2) above.
  • 4. In the form of circuit diagrams. Theoretically, the stated values of reactors and resistors can be multiplied to obtain the time constants, but (as Ref. 47 points out), source and load impedances can affect the result.
  • 5. In words such as “+12dB at 10kHz.” I have not used this type of statement, because it is inherently ambiguous.

Next I must state my “tolerances.” Most frequency test discs quote a tolerance of plus or minus 0.5dB. Calculated tables are usually given to 0.1dB, but I've found several 0.2dB errors. Graphical representations have been particularly troublesome, because an artist has usually had to hand-draw a logarithmic abscissa and plot a table onto it, sketching intermediate points by hand. Because 6dB/octave slopes don’t then look like straight lines, we sometimes have to assume a 1dB error, although the principal abscissae (100, 1000, and 10000Hz) are usually more accurate than that. The values in circuit-diagrams are usually plus or minus 10%, but there are always two such components which have to be considered together. Please bear these tolerances in mind when I describe where I found each piece of information.

6.65 Discographical problems

Having defined the characteristics, the next problem is deciding which records of which logos were made to which characteristics. The correct solution depends upon understanding who cut the master-disc and when. This is not the same as the official logo, nor who applied the matrix number, nor where the record was pressed, nor what sleeve it was packed in. For this reason I must reject the noble work of Peter Walker of the Acoustical Manufacturing Co., who provided pushbuttons on his early Quad preamplifiers for correcting different logos. The final version of his work appeared in the Audio Annual for 1968 (Ref. 48). To underline my point, that article gives only one equalisation for “Nixa” (the NAB Curve). But the British Nixa Record Company used American matrixes from Polymusic and Urania (RCA Victor’s curve), Westminster (US Columbia’s curve), and Lyrichord (three different curves), as well as ones made by itself, all of which changed to RIAA at various dates. I can only recommend that you become familiar with the “look-and-feel” of disc-cutting styles, so you can recognise a master-disc from its “house style,” and (more importantly) when an apparent anomaly occurs. In the rare cases where I actually know relevant matrix numbers, I shall give them.

Oddly enough, there isn’t much doubt about older 78rpm formats. The difficulty is worst on early microgroove discs, especially those from America, since dozens of logos were being mastered in various plants in competition with each other. However, the average collector using microgroove needn’t bother most of the time. It can usually be assumed that if it is a commercial pressing first published outside Germany in 1956 or later, then it will be to the International Coarsegroove Characteristic (50, 450 and 3180 microseconds) if it is coarsegroove, or the “RIAA Characteristic” (75, 318 and 3180 microseconds) if it is microgroove.

Officially RIAA first announced the latter in May 1954, and it was provisionally agreed by the International Electrotechnical Commission in Philadelphia. Many American mastering plants went over to it immediately, I suspect largely because they wanted nothing to do with the broadcasters’ NAB curve, or with the ivory-tower AES curve.

However, there were at least seven names for the curve when used in Britain (Ref. 49), where the British Standards Institution published details in May 1955, to take effect from 1st January 1956.

Fortunately, the 1956 British Copyright Act came into effect in June 1957, forcing publishers of records issued in Britain to print the year of first publication on the label, and similar verbiage had to be added to imported records. When this date is 1957 or later and you’re handling commercial pressings, you’re quite safe; but some nitrates and pressings mastered by small firms had no pre-emphasis at all for some years, if ever. However, you may need to research the first publication date for earlier records, which is a bore; and then your work might be rendered useless because the issue might have been re-mastered to the new International Standards at a later stage. There is no unambiguous way I can denote this - you will just have to recognise the “look and feel” of a later matrix by that particular company.

6.66 General history of changeover procedures

I have very little evidence of the type which might satisfy an industrial archaeologist. I have been forced to write this section after a number of listening tests, comparing the same performances mastered in different countries and formats before RIAA, and/or re-mastered in the years after RIAA. Obviously I cannot do this for every case, so I cannot write with great precision. But this “general history” may help readers see there may be several possibilities before 1955, even without the element of subjective “restoration”.

Before magnetic tape mastering became normal, the only way of moving sound from one country to another was on “metal parts”. In general, the country originating a published recording would send a metal positive to another country. There it could be copied to whatever format the business situation in the destination country dictated. My comparisons show that such metalwork would usually be copied without re-equalisation. Yet I was shocked to discover that the same even applies to American Columbia who had actually invented the LP in 1949 (section 6.69). Their format was carefully defined from Day One, because they hoped to set a World Standard. But British Columbia material sent to America may have been joined-up from several metal parts to make a continuous performance; yet it is still Blumlein 250Hz! And I have found a similar situation with US Decca - the latter as late as mid-1954 – and for RCA Victor. None of this even gets a mention in anyone’s official specifications.

Shortly after the invention of the LP, American RCA Victor launched the seven inch 45rpm disc, using a much faster auto-changer to minimise the breaks, while allowing customers to choose single items in what later became the “pop” market. The first 45s had a virtually incompatible equalisation standard (“Blumlein 795Hz”, or 200 microseconds). It is fascinating to see how other companies tried to match this. Evidently metal-part copying was still in force; some long-playing discs made up from short songs (for example, Broadway shows) seem to be mastered at Blumlein 795Hz. Again, no published information mentions this possibility. Graphs later published by RCA clearly show the 200 microsecond option, but do not make it clear it should only apply to their 45s!

The next problem is that the commercial recording industry wished to take advantage of the potential advantage of mastering on magnetic tape (cutting and splicing to gain note-perfect performances). But in the absence of “tape standards”, no one piece of tape could be guaranteed to play correctly on another machine, unless a fair length of tape was used for carrying calibration tones to document how well the playback machine matched the recording machine. Two tape standards became available in 1953, one in Europe and one in America (see section 7.8); and because they differed, only the leading professional companies could adapt to the other’s tapes. But we can often hear that the quality of early tape was worse than that of metal. So some of the muddles of playing metalwork continued until at least 1954.

Now to the various “non-World standards” for mechanical disc records.

6.67 NAB (later “NARTB”) characteristics

The National Association of Broadcasters was an American organisation concerned with the exchange of pre-recorded radio material between various broadcasting stations. It therefore introduced standard equalisation for its members. I do not know when it took effect, but Ref. 50 (December 1941) is the earliest mention I have found, and by January 1943 the recommendations had been adopted (Ref. 51). There were two characteristics at first, one for lateral-cut discs, and one for vertical-cut discs. Obviously they were used by radio stations; but they always seem to have been confined to 33rpm. Initially these were coarsegroove, of course, often on sixteen-inch discs; but some commercial microgroove issues are also known (using the lateral curve). The recording facilities of the Metropolitan Opera House New York also used it, as did some early Lyrichord, Vox, and Haydn Society LPs.

The lateral time constants appear to be 100, 275, and 2250 microseconds. This information is derived from NAB’s published graph, which was redrawn for Ref. 51, and a new drawing dated April 1949 was issued by the NAB and reproduced in facsimile in several places. (These graphs are in agreement; but not every writer interprets them in the same way. Please note that the lateral graphs suffer from not having zero-level at 1kHz.)

The hill-and-dale curve is shown in the same places, but shows an extraordinary amount of pre-emphasis, presumably to overcome the even-harmonic distortion of hill-and-dale. The curves actually show the recording standard, not the reproducing standard; yet I conjecture the extreme slopes were obtained by inverting a reproducing circuit (probably with an antiresonant top-cut). Nevertheless, very close numerical matching is achieved by assuming conventional time constants at 45 and 550 microseconds, plus two further treble boosts combining to make 12dBs per octave at 9362Hz (17 microseconds) during the recording process. All this was inspired by the development of the first motional-feedback cutterhead (Ref. 52). This gave outstanding performance. It was a decade ahead of its time; but it happened to be a hill-and-dale cutter.

6.68 Decca Group (UK) “ffrr” characteristics

The notes in this section apply to discs mastered at the Kingsway Hall or the West Hampstead plant in London. Before the war, “Western Electric” or “Blumlein” shapes apply, as we saw in sections 6.27 and 6.43. During the second world war “Full Frequency Range Recording” was developed. Its full implementation consisted of a new cutterhead, a new microphone, and a new equalisation curve, and they were gradually adopted for 78rpm coarsegroove discs.

The new cutter was available in 1941, after being researched at the behest of the British Government for identifying enemy submarines detected by sonar buoys. Suddenly a large number of takes called “Take 2” appeared (at a time of acute materials shortages!). Thus I suggest “Take 2” implies the wideband cutterhead, in which case the earliest would probably be matrix DR6570-2 recorded 17th December 1941. But it is thought it ran alongside a non-ffrr cutter for some years, and in any case the full 14kHz bandwidth could not apply until the FR1 full-range microphone had been developed, so it is practically impossible to tell by ear.

However, what concerns us is not the bandwidth, but the point at which pre-emphasis was applied. After the war, Decca’s published catalogues started to identify “ffrr” recordings, and it is thought (and aural evidence supports) that these all included the new pre-emphasis. The lowest such catalogue-numbers in each series are F.8440, K.1032, M.569 and X.281, but there are a few non-ffrr items in subsequent numbers, and readers should study the catalogues to identify them. The earliest ffrr session identified this way is Tchaikovsky’s Fifth Symphony (the first side of which had the matrix AR8486-2), recorded on 3rd June 1944.

Even so, neither the catalogue numbers or the matrix numbers are foolproof. First we shall consider the catalogue numbers. Decca F.8442 and F.8461 were shown as being non-ffrr, because only one side was made to the new characteristic. And I must ask you to be careful if you work with several editions of the catalogues, because sometimes it’s the ffrr ones which are marked with a special symbol, and sometimes it’s the non-ffrr ones.

Next I shall describe matrix numbers for “singles”, which are in a common series irrespective of prefixes (AR, DR and DRX). There are several cases of material re-recorded after the changeover, with lower numbers than 8486, but higher take-numbers than “2.” All one can safely say is that all numbers between 8486 and 18000 are ffrr.

The time constants are 25 and 531 microseconds (-3dB points: 6.3kHz and 300Hz), and this is confirmed by a published frequency disc (Decca K.1802 or London T.4996), and several published graphs.

Other UK Decca logos were affected. UK Decca used NAB characteristics for its own tape recordings, as well as ones imported from America, so that should not be an issue. At first, original American matrixes were imported from US Decca and US Capitol and pressed for UK Decca’s logos “Brunswick” and “Capitol”, but sometimes these would be dubbed in Britain. (I do not know whether the equalisation was corrected, but of course originals will give better quality than dubbings. This point is currently the subject of listening comparisons). Or copy master tapes would be acquired from America (in which case the equalisation of British-remastered 78s is ffrr and the quality may be higher). When the latter happened with Capitol tapes, they have the matrix prefix DCAP instead of just CAP. Theoretically, British issues of the following Decca-group 78rpm logos are also affected: Beltona, Editions de L’Oiseau-Lyre, London, Telefunken, Vocalion (V1000 series), the very first Felsteds, and taped Decca West Africa series mastered at Hampstead (as opposed to discs mastered on location), until late 1954. Decca also did some mastering for other “independent” British logos: Melodisc, Technidisc, Tempo and Vogue.

When Decca introduced microgroove recording, they introduced a new characteristic also called “ffrr”, but which applied to microgroove only. (The coarsegroove characteristic remained as before until International Standards were adopted in January 1956). Historical evidence shows several versions; I shall list them for the sake of historical completeness, although subsequent research has shown this evidence to be very defective, so please ignore the rest of this paragraph! The earliest, in Ref. 53 (Autumn 1950), appears exactly identical to the subsequent RIAA.

The second version is dated “Jan. 1951” in Ref. 43; only a few fixed frequencies are stated, but they correspond to two time constants at about 80 and 680 microseconds.

The third version first appears as a “provisional standard” in Ref. 54, which must have gone to press no later than mid-April 1952. Here the author implies there had been too much pre-emphasis in the first version, not surprising when you remember only Decca had a full-range microphone at the time. The characteristic depicted suggests 50, 450, and 2250 microseconds. (But the component values given in his amplifier circuit suggest 66, “may require adjustment”, and 2250). A fourth version appears to be documented by frequency test disc Decca LXT2695, which was issued in September 1952. (Matrix number EDP 145-1B on both sides). But the sleeve does not state what the characteristic is meant to be called - it may not be “ffrr” at all – although some copies of the disc label show the ffrr trademark. If the levels quoted on the sleeve are supposed to represent the curve, then the closest fit comes from time constants of 48, 300, and 1500 microseconds, while actual measurements of the grooves give a best fit at 41, 275, and 2250 microseconds. In March 1953 a book was published containing yet another version (Ref. 55), but accurate curve-fitting is possible giving time constants of 40, 225 and 1500 microseconds.

My own research shows there were actually three different curves. I did listening tests, comparing ancient LP issues with more-modern versions (assumed to be RIAA), or with original 78s (assumed to be 25 and 531 microseconds). Before I can give the results, I must describe Decca’s LP matrix numbers. Prefixes were ARL for 12" and DRL for 10". Next came the matrix number, allocated in numerical order as far as I can tell; and then the take number (signifying the attempts to make a satisfactory metal negative). Then a letter to indicate the disc cutting engineer, and sometimes a W which (I believe) indicates a master-disc cut in wax rather than nitrate. On British versions, an R may follow. This means “Remastered,” after which the take numbers go back to 1; unfortunately these may be any of the three equalisation curves. However, if the matrix number is engraved rather than punched, this proves RIAA, because the engraving machine was purchased some months after RIAA was adopted.

The three curves are:

  • (1) “Blumlein 500Hz” (318 microseconds), used for (Take 1s of) matrix numbers up to ARL1170-2B (and of course many higher take-numbers of these).
  • (2) 50 microseconds and 318 microseconds, used for (Take 1s of) matrixes ARL1177-1B to ARL2520-2A (and of course many lower numbers with higher take-numbers than 1). It is only fair to say that comparisons sometimes sound better using 40 microseconds instead of 50, but there is no consistency about this. During this period, UK Decca and Telefunken introduced the “MP” format (meaning “Medium Play”, ten-inch discs roughly paralleling what had been issued on 45rpm EPs). These matrixes were numbered in the TRL series.
  • (3) RIAA (75, 318 and 3180 microseconds), used for ARL2539-2A onwards. Decca’s RIAA engineering test disc is LXT5346, with matrix number ARL3466-2. MP discs switched to RIAA at about matrix TRL392.

Unfortunately, by the time Decca switched to the new International Standards the company had changed to a new internal procedure for its “single” records. They called it a “Sub-Matrix Number”. Each track was given a sub-matrix number when the tape master was made, and this number would be transferred to the 78rpm or 45rpm matrix when the disc was cut, which might be anything up to a year later. Thus we cannot use such a matrix number to tell us when the master disc was cut.

The only possible way of breaking this hiatus is to assume that the matrix engraving-machine was introduced simultaneously for both LPs and singles (and EPs and other media), and thus that everything with an engraved matrix number must be RIAA.

Presumably in the case of coarsegroove, this generally implies 50, 450, and 3180 microseconds; but I am told that many late Decca-group 78s (after about 1958) are RIAA! Evidently there was no change of equalisation – or tip radius - for what were, by then, niche-market issues; but the microgroove versions will have better power-bandwidth product anyway. The 78rpm single with the highest punched matrix number seems to be Decca F.10630 (matrix number DRX21213-1), but many singles following this had lower sub-matrix numbers, and others became remastered. The transition to RIAA was certainly before DR21429-2, because that is the matrix number of a 45rpm disc with the catalogue-number 71123, which carries frequencies recorded to the RIAA characteristic. But because such singles lack any chronology, the “grey area” may extend from DR18439 to DR21213.

6.69 American Columbia LPs

First, an important point of nomenclature. I use the pedantic phrase “American Columbia” to distinguish it from British “Columbia”, a constituent of Electrical & Musical Industries Ltd., which owned the Columbia tradename in the British Empire and most of the Old World until 1984.

American Columbia was the first organization to make LPs as we currently understand them, and they defined their characteristic very carefully from Day One because they hoped to set a world standard. (They succeeded with the rotational speed and the groove dimensions, but did not quite get the equalisation right!). And they owned the abbreviation “LP” as a trademark, so some writers use this abbreviation to indicate the recording characteristic as well, for example where American Columbia mastered material for other labels.

Since the 1920s, American Columbia had done a great deal of mastering for syndicated broadcasts, so they used NAB characteristics for microgroove as well as their own. (It is possible the deficiencies of the former showed the need for the latter). When they carried out mastering for other logos, they were often NAB. Unfortunately I do not know a rigorous way of distinguishing them, although after mid-1953 (when variable-pitch grooving was used for Columbia’s own LPs), the NAB-equalised LPs seem always to be cut at a constant pitch.

American Columbia seems to have allocated consecutive numbers for most of their microgroove masters, starting from 200 in early 1948. By 1954 they had reached about 20000. The prefixes include ZLP and ZSP (seven-inch 33rpm and 45rpm discs), TV (ten-inch 33s), and XTV or XLP (twelve-inch), plus ZTV and XEM (“Epic” seven-inch and twelve-inch) and recordings on the ENTRE logo. However, judging from record reviews and casual listening, I would warn you that material of European origin (Philips and English Columbia) might have the wrong equalisation, resulting in American Columbia LPs with no apparent pre-emphasis. In many cases this can be explained by having been copied from metalwork (section 6.66); but this currently needs further research.

The characteristic was, in engineering terms, a good one, and it does not seem to have changed with time. It was so well-founded that by 1952 constructional articles, texts, and preamplifiers were calling it simply “The LP Curve” without mentioning American Columbia’s name at all, which is rather misleading. However, I have yet to see an official statement of the time constants involved. They seem to have been 100, 400, and 1590 microseconds. This information is based on circuits, curve-fitting, and a verbal description of the difference between 900Hz and 10kHz in Ref. 56. But it must also be said that the graph in Ref. 43 suggests that a slightly closer fit would occur with 100, 350 and 1590 microseconds.

There was a fixed-frequency test disc available as early as September 1949 (American Columbia RD130A), together with a gliding-tone one (American Columbia XERD281) (Ref. 57). The range covered was 50Hz to 10kHz. Another frequency-run to the same characteristic was included on Westminster’s LP “High Fidelity Demonstration Record,” available in Britain as Nixa WLP5002 (matrix number XTV19129-2AD).

I do not know precisely when the change to RIAA occurred; the differences between American Columbia LP and RIAA are not conspicuous to the ear. Unfortunately, American Columbia itself maintained a public silence about the change. In sections 6.40, 6.67 and 6.71 I gave results of listening tests to resolve such difficulties with other characteristics; but I apologise to American readers for being unable to focus closer with discs I have found on this side of the Atlantic. (Finding two copies of the same performance with different equalisation is practically impossible here). Another difficulty lies in understanding the matrix number suffixes, which do not follow the same system as Decca’s “ffrr” (above). In the case of American Columbia, I conjecture that when the subject matter remained the same, the “take number” stayed the same; but if re-mastering proved necessary, the letter following the take number incremented. Anyway, I very much hope someone else may be able to improve my results.

To reduce this difficulty, I researched LPs mastered by American Columbia and issued under the US logos “Vox”, “Westminster”, and “Haydn Society”. These logos did not maintain the aforesaid public silence about the change to RIAA. If the information on their sleeves is correct, and assuming: (i) the parent label changed at the same time, (ii) the masters were numbered consecutively, and (iii) I have not been misled by wrong sleeves, then the changeover seems to be between matrixes XTV19724-1A and XTV19938-1C.

Another theory may be constructed for the material issued under Columbia’s own name, whose matrixes have the prefixes LP (ten inch) and XLP (twelve inch). (I do not know if they have the same number-series which follows XTV prefixes, or not). This is that the XLP matrixes switched from (something like) 22000 directly to 30000, so that demonstrators “in the know” would not have to remember a five-digit number in order to apply the correct equalisation.

6.70 “AES” characteristics

This was a reproducing curve proposed by the Audio Engineering Society of America in the summer of 1950 (Ref. 58). Curve-fitting gives two time constants only, 63.6 and 398 microseconds, and this agrees with verbal descriptions of the turnover frequencies, and a re-drawn curve in fig. 17.15A of Ref. 43.

As to its use, Ref. 59 (October 1952) also includes a letter from the then-Secretary of the A.E.S, which states that “all Capitol records, and all material recorded by West Coast organisations, is made exactly to this characteristic” (but the statement is certainly wrong for 78s).

He also says the curve was used by Mercury Records; but it does not apply to them all. Early American Mercury LP sleeves carry the message “Changeover from constant velocity to constant amplitude is at 500 cycles and there is a rising response characteristic at the rate of 3 decibels-per-octave beginning at 2000 cycles-per-second.” But Mercury had changed to the AES curve by early 1953, and the “Olympian” series (pressed in Britain by Levy’s Sound Studios, often under the logo “Oriole”) are certainly stated to be AES, although I have not yet found a pair of examples for a suitable comparison test. To make matters worse, American Mercury metalwork tends to carry a catalogue number (rather than a matrix number), slavishly followed by their UK manufacturing agency, so it seems impossible to state rigorously where the pre-emphasis changed from 3dBs-per-octave to 6dBs-per-octave.

And like other organizations, the AES disguised the changeover to RIAA, by calling it “The New AES Curve” in 1954.

6.71 Various RCA characteristics

Ref. 60 (July 1941) is the earliest contemporary reference I have found which describes RCA Victor using pre-emphasis on its 78s, although the time constant was not given. Straight listening suggests the idea was tried somewhat earlier, and we saw in section 6.23 that Moyer wrote about RCA’s Western Electric systems with pre-emphasis at 2500Hz (corresponding to 63.6 microseconds); but I am deeply sceptical. It seems to me far more likely that, if something which had been mastered direct-to-disc was reissued on microgroove, the remastering engineer would simply have treated everything the same. And I consider it likely that judging by “pure sound” clues, Victor’s then-unique use of multiple limiters (essentially one on each mike), would itself have resulted in a “brighter” sound.

Between 1943 and 25th February 1955, RCA 33rpm and 78rpm masters can be recognised because the first two characters in the matrix number are the encoded year of mastering. For example, 1943 = D3 and 1952 = E2. (For full details of the prefix system, see Ref. 61). Some were dubbed in the UK for issue by EMI; but apart from that, no other plant has matrix prefixes like this. As with American Columbia, they can turn up in the unlikeliest places.

When RCA Victor adopted microgroove (first with their 45rpm discs, and subsequently their 33s in March 1950), the same pre-emphasis and numbering-system was used, but the bass was cut more brutally. (Ref. 43 shows the same curve being used for all RCA’s commercial media in 1954, but maybe this was a hangover from a previous edition of the book). There are only two time constants, 63.6 and 200 microseconds. (No wonder British reviewers complained about the lack of bass on such records). However, listening comparisons between the earliest 45s and 33s show that 200 microseconds only applies to the 45s; the 33s are 318 microseconds. The curve was used for other records mastered at RCA, e.g. Lyrichord and Urania. An “Orthophonic Transcription” curve is also shown, evidently used for sixteen-inch coarsegroove records, with extended treble to the same time constant and an additional time constant equivalent to 1590 microseconds.

It is also known that RCA established a new standard in 1952 which was the same as the subsequent RIAA. (Ref. 62, and see the table in Moyer’s paper). RCA called it the “New Orthophonic” characteristic, and these words seem to appear on many LP sleeves to mean that RIAA was intended. Moyer’s paper includes a box with the following words: “Use of the “New Orthophonic” curve is recommended for all RCA Victor records and records released by RCA Victor since August 1952. With a few exceptions in the early 6000, 7000 and 9000 series, this applies to all LM, WDM, and DM records above 1701, and LCT and WCT above 1112. It also includes all LHMV, WHMV, LBC, WBC, and Extended Play 45s. Records issued prior to that date should be played with the same crossover and high-frequency characteristic, but without the rolloff at low frequencies. A 4- to 5-db increase in response at 50 cps. ... is suggested for these records.” With the exception of the words I have italicised in the above sentence, I cannot disagree with it. A demonstration LP was issued from American Urania 7084, published in Britain in the first quarter of 1954 on Nixa ULP9084. The sleeve clearly shows a RIAA graph, and although there are only five short tones cut on the disc, they are consistent with RIAA. The matrix number is E2 KP 9243, so it dates from 1952. And I am very grateful to Mike Stosich (pers. comm) for informing me that his copy of RCA Victor LM1718 is the earliest he has found with “New Orthophonic” on the sleeve; the matrix number on the disc inside is E2 RP 4095.

6.72 “CCIR” characteristics

Roughly, these were the European equivalent of NAB Characteristics, which were drafted in Geneva in June 1951 and agreed in London in 1953 for the international exchange of radio programmes on disc. But they applied to international exchange only (not domestic recordings), and no-one seems to have told those responsible for international exchanges after the domestic recordings had been completed! The same equalisation was used for coarsegroove 78s, coarsegroove 33s, and microgroove records. The time constants were 50 microseconds and 450 microseconds, with no lower turnover.

The 1953 version of the standard added: “Within the USA a different characteristic will be used for the interchange of programmes between broadcasting organisations but the C.C.I.R characteristic will be used by the broadcasting organisations of the USA for international exchange.” And the British Standards Institution document of May 1955 (which introduced RIAA for commercial microgroove records) allowed the continued use of the CCIR curve for “transcriptions,” defined as “Recordings made for programme interchange between broadcasting organisations and for other specialised purposes and not normally on sale to the public.”

A microgroove test disc to the CCIR curve was issued by the British Sound Recording Association at their May 1955 exhibition (catalogue number PR301) (Refs. 62 and 63). But meanwhile the CCIR had agreed to change to the RIAA time constants, and it seems the application of the CCIR curve to “transcriptions” was abandoned very soon after. PR.301 was therefore sold with a conversion table.

I have done some aural comparisons, and found that early (British) Philips and Caedmon LPs used the CCIR curve. But Deutsche Grammophon used 50 microsecond pre-emphasis on its first 33rpm LPs from 1952, with an additional turnover at the bass end of 3180 microseconds. (Ref. 64). There are two ways to identify a 50 microsecond LP: (a) if there is a “mastering date” in the label-surround (figures in the form, and (b) if the label carries the catalogue number in a rectangle. Pressings with 75 microsecond pre-emphasis would carry the catalogue number in an inverted triangle. (This was a German standard).

6.73 “DIN” characteristics

The German Standards Authority DIN tried to introduce another standard, called DIN 45.533, in July 1957 (to take effect from 31st October 1957). It had the same equalisation for both coarsegroove and microgroove, although the rest of the world had settled down to having two equalisations. The time constants were 50, 318 and 3180 microseconds, and at least one stereo test disc was made (Deutsche Grammophon 99105). However, it seems DIN 45533 was not formally adopted, and Sean Davies tells me there was much heart-searching before Germany adopted 75 microseconds instead of 50, to bring them into line with the RIAA microgroove standard.

6.74 Concluding remarks

As early as June 1950, the editor of the respected American journal Audio Engineering was bewailing the lack of standardisation. He advocated (and continued to advocate) a flexible equaliser system in which the three time constants could be varied independently and judged by ear. Although all the known pre-determined characteristics are given above, the problem is usually to decide which standard has been used on a particular record, and in many cases empirical selection may be the only solution. A suitable system is only available off-the-shelf on the “Mousetrap” disc processor (£3500). The U.S-made “Re-Equalizer” is designed to modify the RIAA curve in a conventional preamplifier via a “tape monitor loop” until it matches one of the above standards; it includes “Blumlein shapes”. And, given a constant-velocity amplifier between the pickup cartridge and the main amplifier, it is relatively easy to build a suitable unit. All operators likely to deal with this subject matter will certainly need one of these possible solutions.

Personally, I agree the three time constants should be varied independently. It is then possible to approach the sound you expect gradually, rather than having one switch with perhaps a dozen “standard” curves on it, which must be wrenched around in order to compare the subtle differences between three parts of the frequency spectrum at once. With the three-knob method it is surprising how frequently different operators agree upon one setting, and how frequently the three selected time constants prove to make up one of the “standard” curves.


  • Ref. 1: Peter Copeland, “The First Electrical Recording” (article). The Historic Record, No. 14 (January 1990), page 26.
  • Ref. 2: Anonymous report of a B.S.R.A lecture by P. E. A. R. Terry (BBC), “What is a Recording Characteristic?”, Wireless World Vol. LVIII No. 5 (May 1952), p. 178.
  • Ref. 3: “Cathode Ray”, “Distortion - What do we mean by it?” Wireless World, April 1955, page 191.
  • Ref. 4: H. Courtney Bryson, “The Gramophone Record” (book), London: Ernest Benn Ltd., 1935, pp. 86-91.
  • Ref. 5: P. G. A. H. Voigt, “Getting the Best from Records” (article), London, Wireless World, February 1940, pages 142-3; and in the discussion to Davies: “The Design of a High-Fidelity Disc Recording Equipment” (lecture), Journal of the Institution of Electrical Engineers 94 Part III No. 30, July 1947, pages 297-8.
  • Ref. 6: Arthur Haddy, “Haddy Addresses the 39th” (transcript of speech), Journal of the Audio Engineering Society Vol. 19 No. 1 (January 1971), page 70. Ref. 7: A. J. Aldridge, “The Use of a Wente Condenser Transmitter” (article), Post Office Electrical Engineers’ Journal, Vol. 21 (1928), pp. 224-5.
  • Ref. 8: W. West, “Transmitter in a simple sound field” (article), J.I.E.E., 1930 Vol. 68 p. 443.
  • Ref. 9: Harry F. Olson and Frank Massa, “Applied Acoustics” (book), second edition, 1939. USA: P. Blakiston’s Son & Co. Inc; London: Constable & Co. Page 102.
  • Ref. 10: “Notes on Actual Performance Recording”: unpublished typescript by F. W. Gaisberg quoted in Jerrold Northrop Moore, “A Voice In Time” (book), p. 178.
  • Ref. 11: R. C. Moyer, “Evolution of a Recording Curve” (article), New York: Audio Engineering, Vol. 37 No. 7 (July 1953) pp. 19-22 and 53.
  • Ref. 12: C. M. R. Balbi, “Loud Speakers” (book), London: Sir Isaac Pitman & Sons Ltd., 1926; pages 68 to 73.
  • Ref. 13: Halsey A. Frederick, “Recent Advances in Wax Recording” (article). Originally a paper presented to the Society of Motion Picture Engineers in September 1928, and printed in Bell Laboratories Record for November 1928.
  • Ref. 14: John G. Frayne and Halley Wolfe, “Elements of Sound Recording” (book). New York: John Wiley & Sons Inc., 1949; London, Chapman & Hall Ltd., 1949. Pages 230-1 contain an ill-disguised reference to this disadvantage of the rubber line.
  • Ref. 15: H. Courtney Bryson, “The Gramophone Record” (book), London: Ernest Benn Ltd., 1935, page 70. (quoting the above)
  • Ref. 16: US Patent No. 1527649; British Patent No. 249287.
  • Ref. 17: US Patent No. 1637903; British Patent No. 242821.
  • Ref. 18: US Patent No. 1669128; British Patent No. 243757.
  • Ref. 19: British Patent No. 405037.
  • Ref. 20: G. Buchmann and E. Meyer: “Eine neue optische Messmethode für Grammophonplatten,” Electrische Nachrichten-Technik, 1930, 7, p. 147. (paper). This is the original citation, but the mathematics of the principle were not described until E. Meyer: “Electro-Acoustics” (book), London: G. Bell and Sons Ltd., 1939, pages 76-77.
  • Ref. 21: H. Courtney Bryson, “The Gramophone Record” (book), London: Ernest Benn Ltd., 1935, pp. 85-91.
  • Ref. 22: Anon (but based on an I.E.E paper by I. L. Turner and H. A. M. Clark, probably given in June 1939), “Microphones at Alexandra Palace” (article), Wireless World Vol. XLIV No. 26 (29th June 1939), pp. 613-4.
  • Ref. 23: H. A. M. Clark, “The Electroacoustics of Microphones,” Sound Recording and Reproduction (the official journal of the British Sound Recording Association), Vol. 4 No. 10 (August 1955), page 262.
  • Ref. 24: George Brock-Nannestad, “The EMI recording machines, in particular in the 1930s and 40s” (article), The Historic Record No. 43 (April 1997), note 14 on page 38.
  • Ref. 25: Anonymous report of BSRA Meeting presentation by W. S. Barrell of EMI, “Wireless World”, Vol. 62 No. 4 (April 1956), page 194.
  • Ref. 26: Bernard Wratten, in a letter to Jerrold Northrop Moore dated 24-01-74, quoted in the latter’s book “Elgar on Record” (Oxford University Press, 1974) in the third footnote to page 174 of the paperback edition.
  • Ref. 27: B. E. G. Mittell, M.I.E.E., “Commercial Disc Recording and Processing,” Informal
  • lecture delivered to the Radio Section of the Institution of Electrical Engineers on 9th December 1947. (Available as an EMI Reprint).
  • Ref. 28: Microfilms of index cards of EMI metalwork, available for public consultation at the British Library Sound Archive. The HMV metalwork is in two runs on Microfilms 30 to 44 and 138 to 145, and the other logos are on Microfilms 58 to 85 and 146 to 151. Each run is in numerical order irrespective of prefix! Where the cards say “Recorded”, this means the date the master disc was cut.
  • Ref. 29: F. E. Williams, “The Design of a Balanced-Armature Cutter-Head for Lateral-Cut Disc Recording,” (paper, received 22nd June and in revised form 24th November 1948).
  • Ref. 30: M.S.S Recording Co. Drawing No. CA 13020/A, dated 9th November 1959.
  • Ref. 31: Interview with Arthur Haddy, 5th December 1983. The British Library Sound Archive Tape C90/16.
  • Ref. 32: Brian Rust and Sandy Forbes, “British Dance Bands on Record 1911-1945” (book), page 732.
  • Ref. 33: Journal of the Institution of Electrical Engineers, Radio Section, Paper No. 805, pp. 145-158.
  • Ref. 34: BBC Engineering No. 92 (October 1972) (journal), page 18.
  • Ref. 35: Edward Pawley, “BBC Engineering 1922-1972,” (book) BBC Publications 1972, page 41.
  • Ref. 36: ibid., page 42.
  • Ref. 37: ibid., pages 178ff, 270ff, and 384ff.
  • Ref. 38: J. W. Godfrey and S. W. Amos, “Sound Recording and Reproduction” (book), London. First published for internal use by the BBC in 1950, then published for public sale for “Wireless World” (magazine) and as a BBC Engineering Training Manual by Messrs. Iliffe and Sons, Ltd., 1952; pp. 69 to 73.
  • Ref. 39: “EMI Pickup for Experimenters,” Wireless World, Vol. 52 No. 5 (May 1946), p. 145.
  • Ref. 40: “High-Quality Pickup,” Wireless World, Vol. 55 No.11 (November 1949), p. 465.
  • Ref. 41: Davies: “The Design of a High-Fidelity Disc Recording Equipment” (paper), Journal of the Institution of Electrical Engineers 94, Part III No. 30, July 1947, page 298.
  • Ref. 42: Ruth Jackson, “Letter To The Editor,” Wireless World Vol. LVIII No. 8 (August 1952), pp. 309-310.
  • Ref. 43: F. Langford-Smith, “Radio Designer's Handbook” (book), London: Iliffe & Sons Ltd., 4th edition (1953), Chapter 17 Section 5, pages 727-732 (and supplements in some later reprints).
  • Ref. 44: O. J. Russell, “Stylus in Wonderland,” Wireless World Vol. 60 No.10 (October 1954), pp. 505-8.
  • Ref. 45: Two “Letters To The Editor,” Wireless World Vol. LVIII No. 9 (September 1952), p. 355.
  • Ref. 46: Edward Tatnall Canby: The Saturday Review Home Book of Recorded Music and Sound Reproduction (book), New York: Prentice-Hall, Inc., 1952, page 113.
  • Ref. 47: P. J. Guy, “Disc Recording and Reproduction” (book), London & New York: Focal Press, 1964, p. 209ff.
  • Ref. 48: James Sugden, “Equalisation,” in the Audio Annual 1968, Table Two on page 37.
  • Ref. 49: John D. Collinson, “Letter To The Editor,” Wireless World Vol. 62 No. 4 (April 1956), p. 171. The names he gives are “RCA New Orthophonic”, “New AES”, “RIAA”, “NARTB”, “B.S. 1928:1955”, “CCIR” and “IEC”. Most of these are amendments to existing standards as everyone adopted RIAA, but of course this makes things worse if anything.
  • Ref. 50: Wireless World, Vol. XLVII No. 12 (December 1941), page 312.
  • Ref. 51: Wireless World, Vol. XLIX No. 2 (February 1943), p. 44.
  • Ref. 52: L. Veith and C. F. Weibusch, “Recent Developments in Hill and Dale Recorders,” Journal of the Society of Motion Picture Engineers, Vol. 30 p. 96 (January 1938).
  • Ref. 53: C. S. Neale, “The Decca Long-Playing Record” (article), Bristol: “Disc”, Number 15 (Autumn 1950), p. 121.
  • Ref. 54: D. T. N. Williamson, “High Quality Amplifier Modifications,” Wireless World, Vol. LVIII No. 5 (May 1952), pp. 173-5.
  • Ref. 55: G. A. Briggs, “Sound Reproduction” (book), 3rd edition; Bradford: Wharfedale Wireless Works, 1953, p. 285.
  • Ref. 56: Donald W. Aldous, “American Microgroove Records,” Wireless World (April 1949), pp. 146-8.
  • Ref. 57: anon., “Test Record List” (article), New York: Audio Engineering, Vol. 33 No. 9 (September 1949), p. 41.
  • Ref. 58: Editorial, Audio Engineering Vol. 34 No. 7 (July 1950), p. 4.
  • Ref. 59: Wireless World, Vol. LVIII No. 10 (October 1952), p. 419.
  • Ref. 60: Wireless World, Vol. XLVII No. 7 (July 1941), p. 175.
  • Ref. 61: (Reprints of two RCA internal memos as appendixes to a discography) The Record Collector, December 1980, page 131.
  • Ref. 62: Peter Ford, “Frequency Test Disk and Tape Records”, Sound Recording and Reproduction (Journal of the British Sound Recording Association), Vol. 5 No. 3 (November 1956), pages 60 and 71.
  • Ref. 63: Wireless World, Vol. 61 No. 7 (July 1955), p. 313.
  • Ref. 64: Peter Ford, Reply to Letter To The Editor, Sound Recording and Reproduction (Journal of the British Sound Recording Association), Vol. 5 No. 4 (February 1957), page 101.


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