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4 Grooves and styli
4.1 Introduction

This chapter will be about “mechanical” sound recordings, in which the sound waves were translated into varying waveshapes along the length of a spiral groove. These waves are nearly always “baseband” - that is, the sound frequencies were recorded directly, instead of being modulated onto a “carrier frequency” in some way. The principal exception will be quadraphonic recordings using the CD-4 system, which I shall leave until we explore spatial sounds in section 10.16.

A century of development lies behind sound recording with mechanical techniques. The technology is unlike any other, since a frequency range of some ten octaves might be involved, with signal-to-noise ratios in the upper-sixties of decibels. Thus the power-bandwidth products of the best mechanical recordings can rival anything else analogue technology has achieved.

In this chapter I shall be considering the best ways to extract the original sound from disc or cylinder grooves. It will deal with the groove/stylus interface and also electronic techniques for minimising surface noise and distortion. Thus we will have taken the story to the point where electrical waveforms are coming out of a socket in anticipation of adjustments to playing-speed or frequency-equalisation. At this socket we will have extracted all the frequencies from the groove while minimising distortion, and will have reduced the surface noise as far as we can; thus we will have recovered the maximum power-bandwidth product. This is the fundamental limitation. After that we may refine the sound if we want to.

It is an old adage that if the groove/stylus interface is wrong, it is pointless to rely on electronics to get you out of your difficulties. I think this is something of an oversimplification, although the basic idea is correct. My point is that we should really work upon the groove/stylus interface at the same time as electronic noise reduction. Some electronic treatments are very good at removing showers of loud clicks, for instance. When this is done, it is then possible to choose a stylus to reduce the remaining steady hiss. Had the stylus been chosen without the de-clicker in place, it might not have been the one which gave the least hiss. This would be wrong, because the current state-of-the-art is that it is much more difficult to approach the intended sound in the presence of steady hiss.

Although I shall leave electronic treatments until the sections following 4.15, please remember the two topics should actually go hand-in-hand. And once again, I remind you of the importance of having the best possible originals, as we saw in section 2.6.

I shall be making great use of the term “thou” in this chapter. This means “thousandths of an inch”, and has always been the traditional unit of measurement for styli and grooves. In North America, the term “mil” has the same meaning. Although it would be perfectly acceptable to use the metric term “microns”, where one micron is one-millionth of a metre, specialist stylus-makers still speak in “thou”, so I shall do the same. One thou equals 25.4 microns.

Two other terms I must define are “tracing” and “tracking.” I do so because many textbooks confuse them. “Tracing” concerns the way a stylus fits into, and is modulated by, the groove. “Tracking” concerns the alignment of the reproducer compared with the geometry of the original cutting lathe.

4.2 Basic turntable principles

I assume my readers know what a turntable is, but they may not be aware that the equivalent for cylinder records is called a “mandrel.” My first words must be to encourage you to use “a good turntable (or mandrel) and pickup”. It isn’t easy to define this; but to recover the best power-bandwidth product, the unit must have a lower background-noise and a wider frequency-range than any of the media being played on it. This is something we can measure, and does not need any “black magic.”

The unit should also have reasonably stable speed, although we shall come to a technique in section 4.16 where this need not necessarily be to the highest standards. In Chapter 4 we shall be studying the problem of establishing the correct playing-speed for rogue records. These are in the minority, but at least one machine must have variable playing-speed so this matter may be researched. (It may be necessary to have two or more machines with different features to get all the facilities).

The unit should be insulated so it is not significantly vibrated by sounds from the loudspeakers, loose floorboards, etc; this may mean fixing it to a massive mounting-plate, and suspending it in compliant supports. All this is good engineering practice, and needs no further comment from me.

Rather more difficult to define is that the unit should have “low colouration.” This is much more a black magic subject. Basically it means that the wanted sounds are reproduced without “hangover” - that is, the machinery should not contribute resonances or sounds of its own which continue after the wanted sounds have stopped.

The main point about hangover is that the stylus and record themselves generate it. As the stylus is vibrated by the groove, reaction-forces develop in the disc or cylinder itself. It is important that such vibrations are quelled instantly, especially in the presence of clicks and pops. When we remove these clicks and pops using electronic techniques, we may not be able to get a clean-sounding result if hangover exists in the record itself or the pickup arm.

To deal with the record first. We cannot prevent the vibrations being set up; we can only ensure they are eliminated as quickly as possible. One method is to use a heavy rubber turntable-mat in close contact with the back of the disc. The only suitable kind of mat is the kind with a recessed area about four inches across in the middle, so the disc makes good contact irrespective of whether the label-area is raised or not. As I say, it helps if the mat is thick and heavy; the kind with lightweight ribs is pretty ineffective. Some vibrations can also be attenuated by a clamp - a heavy weight placed over the record label.

The Pink Triangle Turntable is designed without a mat at all. It is made from a plastics material of the same specific gravity as vinyl. Any vibrations set up in the vinyl are efficiently conducted away without being reflected back towards the pickup. Basically this is a good idea, but it cannot give perfect reproduction unless the disc is in intimate contact all over. You should be prepared to flatten warped records to cut down wow and tracing errors, anyway; please see Appendix 1 for details. In my experience, Pink Triangle’s method is as good as the best turntable mats, but not better.

I can see no virtue in “anti-static” turntable mats, especially ones that aren’t thick and heavy. A more suitable method of neutralising electrostatic charges is to keep an open saucer of lukewarm water near the turntable, perhaps with a sponge in it for increased surface-area. Turntable lids are usually provided to keep dust off, and in my opinion they are potential sources of hangover; but in cold frosty weather (when the natural humidity is zero), they can define a microclimate where a high degree of humidity may be established. Probably the optimum solution is a removable turntable lid, perhaps with a motor-operated pickup lowering mechanism for use when the air is particularly dry and the lid must be shut.

The Pink Triangle company also make a mechanism which brings eccentric records on-centre. While it is important to play records on-centre, you may not need this if the records are your own property. Frankly, attacking the centre-hole with a round file is just as good; but after the disc has been positioned, you may need the clamp to hold it in place through the subsequent operations.

To extend the above ideas to cylinders: Edison invented the “tapered mandrel,” and the vast majority of cylinders have tapered orifices so the cylinder cannot be loaded backwards. But the system has the disadvantage that the cylinder is subjected to tensile stresses as it is pushed on, which may split it. After you have done this once, you know how hard not to push! Seriously though, practicing with some sacrificial cylinders is a good way of learning.

Another problem is that playing the cylinder may generate reaction stresses between cylinder and mandrel. After a minute or two, these have the effect that the cylinder will try to “walk off the mandrel,” by moving towards the narrow end in a series of very small steps. My answer is very low-tech: a rubber-band pressed up against the end of the cylinder! Eccentric or warped cylinders can often be helped with pieces of paper packed between the ends of the cylinder and the mandrel. All these disadvantages can be avoided with a machine which presses the cylinder between two contact points, holding the cylinder by its ends (Ref. 1, for example).

To reduce the overhang difficulty, I recommend a plain solid tapered mandrel made from anodised aluminium. I do not recommend anything compliant (analogous to a turntable-mat), although Reference 1 involves just that! Once again, the ideal might be two machines which provide all the features between them.

4.3 Pickups and other devices

“Pickup” is the term for a device which touches the groove as the record rotates, and translates the sound recorded in the groove into analogue electrical signals. But before we go any further, it is my duty to mention three other ways of doing the job, and why they aren’t satisfactory for a sound archive. It is quite possible one of these techniques may blossom into something suitable one day. In my judgement this hasn’t happened yet; but you should know the principles, so you may judge them if they improve.

The first is the original principle invented by Edison, which is to couple the stylus to something (usually a “diaphragm”) which vibrates, and radiates sound directly into the air. Having stated this principle, it would normally be followed by some hundreds of pages dealing with the design of diaphragms, horns, and other acoustico-mechanical devices for improved fidelity and “amplification” ! I have put that last word in quotation-marks deliberately. The laws of conservation of energy make it clear that you cannot get “amplification” without taking energy from the rotating record, and the more you attempt this, the more wear you cause to the original record. Obviously we don’t wish to wear out our records; so I propose to abandon this principle, although there will be other lessons for us further on. The remaining principles imply electronic amplification somewhere, which does not stress the record itself.

The next principle is to play the record by looking at it with beams of light. There is a fundamental difficulty here, because the sound waves recorded in the groove may be both larger, and smaller, than typical wavelengths of light. Thus it is necessary to invent ways of looking at the groove so both large and small waves are reproduced accurately. At the time of writing the most successful of these is an analogue machine using infra-red light modulated at an ultrasonic frequency, and demodulating it using radio-reception techniques. This has three practical disadvantages over mechanical pickups. First, dirt is reproduced as faithfully as the wanted sound. Second, the frequency response is limited at the high end in a way which makes it difficult to cure the first problem.

Third, the hardware can only play grooves with straight-sided walls (which we shall come to later), and only those made of something which will reflect infra-red light. The third principle is to use an optical sensor. Here the idea is to measure the entire recorded surface in three dimensions. This might be done by scanning it with a laser-light detector at intervals of less than one micron, measuring the third dimension (“depth”) by sensing when the laser light spot is in focus. This results in a vast file of some gigabytes of numerical data, which might be processed into a digital recording of the original sound.

4.4 Conventional electrical pickup considerations

We now come to the way a pickup is carried across the record to minimise geometrical sources of distortion. As this is not a textbook on conventional audio techniques, I shall not describe tracking distortion in detail, but only the specific problems encountered by present-day operators playing old discs.

It is generally assumed that discs were mastered by a cutter which moved across the disc in a straight line, whereas most pickups are mounted on an arm which carries the stylus in a curved path. In 1924 Percy Wilson did the basic geometrical study for minimising distortion from this cause (Ref. 2). This study remains valid today, but nowadays we use Wilson’s formulae slightly differently. Wilson originally sought to minimise tracking error as averaged over the whole disc. Nowadays we minimise the tracking error at the inner recorded radius, which is usually taken as two and a quarter inches (56mm). There are two reasons for this: (1) the effects of tracking error are much worse at the inner radius; (2) most music ends with a loud passage, and loud passages are more vulnerable to tracking distortion.

The result of all this is that a pivoted pickup-arm should, in effect, have a bend in it so that the cartridge is rotated clockwise with respect to an imaginary straight line joining the stylus to the pivot. The exact angle varies with the arm’s length, but is usually in the order of twenty degrees. In addition, the stylus should overhang the centre-pin of the turntable by an amount which also varies with arm length, but is of the order of about 15mm. When a pickup arm is installed in this manner, minimum tracking distortion is assured from conventional records. But operators should be aware that the “alignment protractor” supplied with many pickup arms will not give the correct alignment for unconventional records.

A pivoted arm is much more amenable to experimental work than a non-pivoted one such as a parallel tracking arm. This doesn’t mean that either type is inherently superior, only that one must use the right tool for the job. Many pivoted arms have an oval-shaped base for the actual pivot, and the whole arm can be moved bodily towards or away from the turntable centre. This enables you to neutralise tracking distortion when the disc ends at an unusual radius. Coarsegroove 33rpm discs, for example, may finish 100mm from the centre; but tracking-distortion can be quite noticeable in these circumstances because the sound waves are packed close together in a coarse groove. The arm can be moved towards the turntable slightly to make the cartridge perfectly tangential to the inner radius. On the other hand, many small 78rpm discs of the late 1920s and early 1930s were recorded much further in; the worst example I know ends only 20mm from the centre hole. The tracking distortion is terrible under these conditions, and may be reduced by moving the whole arm a centimetre or so away from the turntable centre.

However, tracking distortion can be totally eliminated from conventional records by means of a “parallel tracking arm” - a mechanism which carries the pickup across the disc in a straight line. In practice this is difficult to achieve without causing other problems, so parallel tracking arms are more expensive; but in this situation, the centre-line of the cartridge should be aligned perpendicular to the direction of motion, and the path of the stylus should pass through the centre-pin of the turntable.

However, I must report that, although a parallel tracking arm eliminates tracking distortion on conventional records, it is practically impossible to do anything about unconventional ones. In practice, these fall into two types. (1) Discs cut on a lathe where the cutterhead was correctly aligned, but the cutting stylus was inserted askew. In this case the tracking error is the same at all radii. (2) Discs cut on a lathe which carried the cutter in a straight line not passing through the centre of the disc, but along a line parallel to the radius; or, what comes to the same thing, discs cut on a lathe whose cutterhead was not mounted squarely. In these cases the tracking error varies with radius. These features result in various types of distortion which we shall examine in detail later.

Whether it is a pivoted arm or a parallel-tracker, the arm itself should not contribute any significant resonances after being shock-excited by cracks and bumps. You may need a long arm (such as the SME 3012) for playing outsized discs; but any long arm will have noticeable resonances. Since the original Model 3012 was made, SME have an upgrade-kit comprising a trough of silicone damping fluid. This greatly reduces the resonances, but a later design (such as their Series V) is preferable for conventional-sized discs. All other things being equal, a parallel-tracker has less metal which can vibrate; experience with massive de-clicking operations tends to show that this type is better for badly cracked and scratched records.

All cylinders were mastered with a straight-line movement parallel to the axis, so this geometry should be followed in principle. All other things being equal, it is of course irrelevant whether it is actually the pickup or the cylinder which moves; but other considerations (such as groove-jumping) may force one method or the other. A machine called “Ole Tobias” at the National Library of Norway combines the principle of having no mandrel as mentioned at the end of 4.2, with a pivoted tonearm whose pivot is driven in a straight line parallel to the axis. This would seem to combine all the advantages of both; but I have no knowledge of the time it takes to get each cylinder running concentrically.

4.5 Operational procedure for selecting a stylus

Because there are techniques for dealing with crackle and clicks, the maximum power-bandwidth product comes from choosing a record with the best balance between the basic hiss and the distortion due to wear. Although psychoacoustic tricks exist for reducing hiss, there are currently no cures for wear, so priority should be given to an unworn copy.

An experienced transfer operator will be able to choose the correct stylus simply by looking at the grooves. I am afraid this is practically impossible to teach; the operator has to look at both groove walls, the groove bottom, and the “horns” (if any; see section 4.6). A “point source” of light is needed (not fluorescent tubes), preferably in an “Anglepoise” so you may choose different ways of looking at the surface. For selecting a stylus, put the anglepoise behind your shoulder so you are looking down the beam of light; then turn a disc about a horizontal axis between the two hands, while watching how the overall amount of reflected light varies with the angle of the disc. I know that’s a totally inadequate explanation, but I simply can’t do any better. Until you have learnt the trick, you will be obliged to go through a procedure of consecutive feedback-loops to identify the best stylus.

Thus you may have to compare two or more styli to see which gives the greater power-bandwidth product. Unfortunately practical pickup cartridges cannot withstand frequent stylus-changes (which in many cases can only be done by the manufacturer anyway). So we must use exchangeable headshells, which will annoy the hi-fi buffs. Allow me to deal with this objection first. Exchangeable headshells are inherently heavier than fixed headshells. But the reduction of mass is only significant when dealing with warped or eccentric records at very low playing forces; various types of distortion can occur if there is any tendency for the pickup to move up and down or to and fro. Frankly, it is much better to have flat concentric discs to start with! For an archive copy this is essential anyway, as it is the best way to minimise speed inconsistencies.

So the professional transfer operator will have several cartridges mounted in headshells ready for comparison. Sometimes, however, we find ourselves at the point of diminishing returns. When we have got reasonably sensible noises out of the groove, it may require a lot of work to make a not-very-significant improvement to the power-bandwidth product. By the time I have unscrewed one head-shell and tried another, I find I have forgotten what the first one sounded like. There are two cures: (1) Transfer one version before changing the shell; (2) to have two pickup arms playing the same record and switching between them (this is a better way, as it reduces wear-and-tear on the headshell contacts).

To close the feedback loop, and expedite the choice of one stylus from dozens of possibilities, we must learn the mechanisms involved and their effects upon the reproduction. This chapter therefore continues with a look at the history of grooves and styli. Whenever we come across a technique which is still applicable today, I shall interrupt the history lesson and examine the technique in more detail. I am afraid this will mean a rather zig-zag course for my argument, but I hope that sub-headings will allow you to concentrate upon one strand or the other if you wish.

4.6 U-shaped and V-shaped grooves

I shall talk mainly about two kinds of groove – “U-shaped” and “V-shaped” - but I shall not formally define these terms. I use them to differentiate between two philosophies for playback purposes; you should not assume that all “V-shaped” grooves have straight walls and sharp bottoms, for example. And recordings made at the dawn of sound recording history do not fit either category.

Edison’s tinfoil phonograph did not cut grooves. It indented them in a sheet of metal commonly known as “tinfoil.” The noise-level of the groove was determined principally by the physical properties of the foil. It was virtually impossible to remove it and replace it correctly without either corrupting the indentations or crinkling the sheet; and there was inevitably a once-per-revolution clunk as the stylus crossed the seam where the foil was wrapped round the mandrel. These features were largely responsible for the eclipse of the phonograph as a practical recording machine during the years 1878 to 1887. They also explain why so few tinfoils survive today, and those in unplayable condition.

Bell and Tainter’s “Graphophone” circumvented these difficulties by using pre-shaped cardboard cylinders coated in a wax-like substance called “ozokerite.” Thus the problems of coiling up the tinfoil, aligning it on the mandrel, and arranging for an inoffensive seam, were avoided. But Bell & Tainter’s fundamental improvement was that the groove was cut instead of indented. The recording machine was fitted with a stylus which actually removed a continuous thread of ozokerite, leaving behind a fine clean groove with much lower noise. (In parentheses, I add that the Graphophone used an untapered mandrel. So there may be ambiguity about which is the start and which is the end of the recording).

Edison’s “Improved Phonograph” of 1888 adopted the cutting idea, but he favoured cylinders made of solid wax much thicker than the Graphophone’s layer of ozokerite. It was therefore possible to erase a recording by shaving it off. This was much better suited for dictation purposes, which is how both the Graphophone and the Improved Phonograph were first marketed. I do not know the details of Graphophone cutters, but I do know that Edison’s Improved Phonograph used a sapphire cutting-tool. Sapphire is a jewel with a hardness greater than any metal. Anything less hard was found to wear out quickly. This was the main reason behind the commercial failure of the Graphophone, because a blunt cutter would not make a quiet groove.

Artificial sapphires were made for the jewelled bearings of watches. They were cylindrical in form and smaller than a grain of rice, about one-hundredth of an inch in diameter. To make a phonograph cutter, one end was ground flat and mounted so it would dig end-on into the rotating wax. The sharp edge where the flat end met the curved rim would be where the swarf was separated from the cylinder, leaving behind a groove so smooth it would reflect light. In practice, the cutter would be tilted at a slight angle, and the front face ground to a complimentary angle. This left a groove bottom which wasn’t shaped like an arc of a circle, but an arc of an ellipse with a low degree of eccentricity. This is what I mean when I talk about “U-shaped” grooves.

In the case of Edison machines, recordings were reproduced by another jewel, this one deliberately “blunt” so it would not cut the wax again, but small enough to run along the bottom of the groove. The vertically-modulated sound waves would cause the reproducing stylus to be vibrated up and down as it pressed against the groove bottom, and thus the sound would be extracted. Later developments resulted in playback styli made to a specific diameter to fit the grooves, minimising noise and wear. Edison established standards for his two-minute cylinders, his four-minute cylinders, his “Voicewriter” dictation-machines, and his “Diamond” discs. Edison also showed that minimum distortion occurred with a button-shaped playback stylus (the correct geometrical term is an oblate spheroid). This was designed to sit across a plain groove whilst remaining in contact all round, while its minor radius was sufficiently small to follow the most intricate details of the recorded waveform.

Meanwhile, back in 1888, Emile Berliner was developing a quite different way of recording sound. There were three fundamental differences. (1) He preferred discs to cylinders, which gave him two advantages. His reproducing machines needed no mechanism to propel the reproducing stylus, the disc itself would do it; and he could mass-produce copies of his records like printing. (2) His styli vibrated side-to-side rather than up-and-down. The groove walls therefore pushed the playback styli to and fro rather than the unidirectional propulsion of the hill-and-dale (vertical cut) format. (3) He did not cut grooves, but used an acid-etching process.

“Acid-etched” disc recordings, made between 1888 and 1901, therefore have grooves of rather indeterminate cross-section. Partly because of this, and partly because Berliner was competing with cylinder manufacturers on cost grounds, Berliner used relatively soft steel reproducing needles and made his discs in relatively abrasive material. The first few seconds of groove would grind down the tip of the reproducing needle until it had its maximum area of contact, thereby ensuring the needle would be propelled by the groove walls, while his machines avoided the cost of jewelled playback styli. On the other hand, steel needles could only be used once; and this philosophy remained the norm until the 1950s.

In 1901 Eldridge Johnson (founder of the Victor Company) adapted the wax-cutting process to the mastering of disc pressings, so the groove now had consistent cross-section throughout the side of the disc. For several decades they were usually U-bottomed like hill-and-dale recordings. Although the abrasive nature of the pressings did much to hide the advantages, the wax masters and the stampers had smoother surfaces than acid-etched recordings, and today much of our restoration work consists of trying to get back to the low noise-level of the wax masters.

The vast majority of such pressed records were played with steel needles. The only exceptions were collections belonging to wealthier or more careful collectors, who used “fibres” (see section 4.8).

In 1911 a British inventor, P. J. Packman, patented a new type of cutting stylus in which a cylindrical sapphire had its axis perpendicular to the wax, rather than substantially parallel to it. (Ref. 2). His aim was to cut deeper grooves. He wanted to pack more sound into a given space, and reasoned that if one used hill-and-dale recording, one would not have to leave space between the grooves for lateral modulation. By combining hill-and-dale recording with a finer groove-pitch and the technique of an abrasive record to grind a steel needle, he hoped to make inexpensive long-playing disc records; a couple of hundred were actually published under the tradename “Marathon.” They were not a success; however, the principle of Packman’s cutter was gradually adopted by the rest of the sound recording industry.

There were several advantages to a relatively deep groove. The deeper it was, the less likely the sound would be corrupted by scratches and dirt. Also a reproducing stylus was less likely to “skid,” or to be thrown out of the groove by heavy modulation. These advantages meant it was easier to accommodate louder sounds. There could be a greater area of contact between stylus and groove, so there could also be less hiss as we shall see in section 4.8.

If one tries to cut a groove of U-shaped cross-section which is impracticably deep, the walls will become nearly vertical at the surface of the disc. A number of difficulties come to light if this happens. During the cutting process, the swarf does not separate cleanly, because material being forced up from the bottom of the groove causes shearing action (rather than cutting action) at the top. Even if this problem were overcome, it would be much more difficult to press or mould records from a negative with ridges of near-semicircular shape. The material would adhere to the stamper rather than separate cleanly, because of different coefficients of thermal contraction as stamper and material cooled. When the groove walls are less than 45 degrees, the thermal contraction results in the record being pushed away from the stamper; when it is greater than forty-five degrees, the record tends to be gripped by the stamper.

Therefore the deepest possible groove can only be a V-shape rather than a U-shape, with no part of the groove walls greater than forty-five degrees from the horizontal. This therefore represents the ultimate practicable groove-shape to make use of the advantages I have just described.

Nowadays, when most people have done applied mathematics at school, the idea of a force being resolved into two components is commonplace. But evidently this wasn’t the case in the first quarter of this century; it was thought grooves must have flat bottoms if the record was to take the playing-weight of acoustic soundboxes (over a hundred grams). Today we know that a V-shaped groove is equally capable of bearing such a weight, since the force exerted upon each inclined groove wall can be resolved into horizontal and vertical components, and the two horizontal components from each of the two walls cancel. Packman’s groove worked this way, although he did not claim it as part of his invention. During the first World War the English Columbia company adopted V-shaped grooves for its records, I suspect largely because they had much worse surface noise than their competitors at that time. But the mistaken idea of U-bottomed grooves being inherently better remained the dominant philosophy until the early 1930s.

What forced the change was the advent of the auto-changer for playing a stack of records without human intervention. Reproducing needles suddenly had to be capable of playing eight consecutive sides. Less wealthy customers still used relatively soft steel needles, so the records had to retain abrasive qualities to grind them until there was a perfect fit - an early case of “downwards compatibility.” Only the very hardest stylus materials would stand up to eight abrasive sides, and various forms of tungsten were tried, followed eventually by the renaissance of jewels. To ensure the grooves would always propel such styli backwards and forwards in the lateral plane, the walls had to be in control. This was impossible so long as different records had U-bottomed grooves of different sizes and the playback styli couldn’t adapt themselves. So the industry gradually changed to V-shaped grooves cut by Packman-type cutters, a process which was complete by 1945.

Although Packman’s patent shows a V-shaped cutter coming to a definite point, sapphires of this design are very fragile. Sapphire is very hard and it resists compression tolerably well, but it has little shear strength. Cutting a V-shaped groove with a sharp bottom is practically impossible. Instead, the tip of the cutter is deliberately rounded, and the resulting groove actually has a finite radius in its bottom. In 78rpm days this might be anything between 1 thou and 2.5 thou, even in nominally V-shaped grooves. With the introduction of microgroove, the bottom radius might be 0.3 to 0.7 thou. If we consider mass-produced pressings, the radius tended to increase as the stamper wore, and greater radii may be encountered in practice.

Before it became possible to copy a disc record electrically (in about 1930), a factory might “restore” a negative by polishing it, so the pressing would have a groove with a flat (or flattish) bottom, no matter what the original groove shape was. This was done to clear up background-noise due to irregularities in the bottom of the groove, which were reproduced loud and clear when played with a steel needle ground to fit. Background-noise was certainly ameliorated, but the process was not without side-effects. A steel needle would take longer to grind down, resulting in an extended period of wear; and before modern stylus-shapes became available, such blunter styli could not trace the high frequency detail.

To continue my history lesson: Cecil Watts invented the cellulose nitrate lacquer recording blank in 1934. This comprised a layer of lacquer upon a sheet aluminium base, and although there were many alterations in the detailed composition of the lacquer and in the material used for the base, as far as I know cellulose nitrate was always the principal constituent. His development accompanied rivals such as “gelatine”, “Simplat,” “Permarec,” and others, but these were all aimed at amateur markets. Only “nitrate” was adopted by professionals, because (when new) it had lower background-noise than any other analogue medium, either before or since. (For some reason it was called “acetate” for short, although as far as I know there was never a formulation relying on cellulose acetate as its principal component. I shall call it “nitrate.”)

Nitrate was gradually adopted by the whole disc-recording industry to replace wax, which was more expensive and too soft to be played back; the changeover was complete by 1950. Wax cutters had had a plain sharp cutting edge (known, for some reason, as a “feather edge.”) Packman-type sapphires with feather-edges could not withstand the extra shear stresses of nitrate, so steel cutters were widely used for such discs. However it was noticed they sometimes gave lower surface noise as they wore. There was a great deal of hocus-pocus pronounced about this subject, until the New York cutting stylus manufacturer Isabel Capps provided the correct explanation. The swarf was being separated by the front face of the cutter, as intended; but when it was slightly blunt, the following metal pushed the groove walls further apart and imparted a polishing action. When this was replicated by adding a “polishing bevel” to a sapphire cutter, there was a major improvement in background-noise and the sapphire was better able to withstand the shear stresses at the same time. From the early 1940s sapphire cutters with polishing-bevels became normal for cellulose nitrate mastering.

These polishing-bevels had the effect of increasing the minimum possible recorded wavelength. Although insignificant compared with the losses of playing contemporary grooves with contemporary styli, they caused a definite limit to the high-frequency reproduction possible today.

Because the polishing-bevel pushed some of the nitrate aside, the result was that miniature ridges were formed along the top edge of the groove walls, called “horns.” If not polished off the “positive,” they are reproduced upon the pressed records, and when you have learnt the trick of looking at them, the horns provide conclusive evidence that nitrate was used rather than wax. We may need to know this when we get to section 4.11.

With microgroove recording it was necessary to adopt another technique to allow small recorded wavelengths. The cutting stylus was heated by a red-hot coil of wire. The actual temperature at the cutting-edge proved impossible to measure in the presence of the swarf-removal suction, but it was literally like playing with fire. Cellulose nitrate is highly inflammable, and engineers have an endless supply of anecdotes about the resulting conflagrations. By almost melting the lacquer at the cutting-edge, it was found possible to make the polishing-bevels much smaller and improve the high frequencies, to reduce the background-noise of the master-lacquer, and to extend the cutter’s life at the same time. (Ref. 3).

The final development occurred in 1981-2, when “direct metal mastering” was invented by Telefunken. To oversimplify somewhat, this involved cutting a groove directly into a sheet of copper. A diamond cutter was needed for this, and for a number of reasons it was necessary to emulate the polishing action by an ultrasonic vibration of the cutter. A decade later, more than half the disc-cutting industry was using the process.

This has been a grossly oversimplified account of what became a very high-tech process; but I mention it because operators must often recognise techniques used for the master-disc to ensure correct geometry for playback.

4.7 The principle of minimising groove hiss

We now come to the problems of reproducing sound from grooves with fidelity. This is by no means a static science, and I anticipate there will be numerous developments in the next few years. The power-bandwidth principle, however, shows us the route, and quantitatively how far along the road we are. Most of what I shall say is applicable to all grooved media; but to save terminological circumlocutions, I shall assume we are trying to play a mono, lateral-cut, edge-start shellac disc, unless I say otherwise.

The irregularities in the walls of the groove cause hiss. These irregularities may be individual molecules of PVC in the case of the very best vinyl LPs, ranging up to much larger elements such as grains of slate-dust which formed a major constituent of early 78s. The hiss is always the ultimate limit beyond which we cannot go on a single copy, so we must make every effort to eliminate it at source. In fact, steady hiss is not usually the most noticeable problem, but rather crackles and pops; but we shall see later that there are ways of tackling those. It is the basic hiss that forms the boundary to what is possible.

The only way to reduce the basic hiss from a single disc is to collect the sound from as much of the modulated groove walls as we can. It is rather like two boats on a choppy sea; a dinghy will be tossed about by the waves, while a liner will barely respond to them. Playing as much of the groove as possible will imitate the action of a liner. We can quantify the effect. If our stylus touches the groove with a certain contact area, and we re-design the stylus or otherwise alter things so there is now twice the area of contact, the individual molecules or elements of slate dust will have half their original effect. In fact, the hiss will reduce by three decibels. So, if the basic hiss is the problem, we can reduce it by playing as much of the groove as we possibly can.

Please note that last caveat – “If the basic hiss is the problem.” That is an important point. If the noise we hear is not due to the basic structural grain of the record, this rule will not apply. Suppose instead that the record has been scratched at some time in the past, and this scratch has left relatively large protruding lumps of material in the walls of the groove. If we now attempt to double the contact area, the effects of the scratch will not be diluted; to the first approximation, the stylus will continue to be driven by the protruding lumps, and will not be in contact with the basic groove structure. Thus the effects of the scratch will be reproduced exactly as before. To use our boat analogy again, both the liner and the dinghy will be equally affected by a tidal wave.

So whatever we subsequently do about the scratches and clicks, our system must be capable of playing as much of the groove as possible, in order to reduce the basic hiss of an undamaged disc. I shall now consider some ways of achieving this ideal - which, I must repeat, is not necessarily an ideal we should always aim at, because of other sources of trouble.

4.8 “Soft” replay styli

I must start by making it quite clear that there are several quite different approaches we might take. Nowadays we tend instinctively to think in terms of playing a mono lateral-cut edge-start shellac disc with an electrical pickup fitted with a jewel stylus, but it is only right that I should first describe other ways of doing it. The Nimbus record company’s “Prima Voce” reissues of 78s on compact discs were transferred from an acoustic gramophone using bamboo needles, and whatever your opinion might have been about the technique, you could only admire the lack of surface noise.

For modern operators used to diamonds and sapphires, it is necessary for me to explain the thinking behind this idea. I use the unofficial term “soft” needle for any stylus which is softer than the record material. It forms a collective term for the needles better known as “bamboo”, “thorn”, or “fibre”, but please do not confuse my term with so-called “soft-toned” needles; I am referring to physical softness.

Most shellac lateral-cut discs were deliberately designed to be abrasive, because they had to be capable of grinding down a steel needle. Microscopic examination would then show that the grooves were populated with iron filings embedded in the walls; the result was additional surface noise. From about 1909, quality-conscious collectors used soft needles instead. (Ref. 4). They were obtained from various plants and treated in various ways, but all worked on the same principle. A slice from the outer skin of a bamboo, for example, was cut triangular in cross-section. Soundboxes with a triangular needle-socket were obtainable. The needle could be re-sharpened by a straight diagonal cut; you could do this with a razor, although a special hand-tool was easier to use. This action left a point sharp enough to fit any record groove. “Bamboos” were favoured for acoustic soundboxes. The smaller “thorn” needles for lightweight electrical pickups had to be sandpapered or otherwise ground to a conical point. “Fibre” needles seems to be a collective term for the two types. All such materials had much more hardness along the grain of the fibre, and it was possible to achieve a really sharp point - as little as 0.5 thou was often achieved. (Ref. 5). Sometimes specialist dealers provided needles impregnated with various chemicals, and much effort was expended in getting the optimum balance between lubricant (when the needle was relatively soft) and dryness (when the needle was much harder, transmitted more high frequencies, and lasted longer in the groove).

At the outside edge of a shellac disc, a “soft” needle wore very rapidly until it was a perfect fit for the groove - this happened typically within one revolution, so the parts of the groove containing the music were not affected by wear. After that, the close fit with the groove-shape minimised the wear and the hiss, as we saw in our boat analogy. By having a very fine point with an acute included angle (semi-included angles of only ten degrees were aimed for), the shape would fit the groove perfectly in the direction across the groove, but would be relatively short along the groove. This was found empirically to give less distortion. I am not aware that a printed explanation was ever given, although clearly users were emulating Edison’s oblate spheroid of four decades earlier, and the “elliptical” jewels of four decades later.

The hiss was also attenuated by the compliance of the needle, which however affected the reproduction of wanted high frequencies (Ref. 6). However, greater compliance meant less strain between needle and groove, so less wear at high frequencies - exactly where steel and jewel styli had problems. Modulation of high amplitude would sometimes cause the point to shear off a “soft” needle, but collectors considered this was a small price to pay for having a record which would not wear out. Only one playing with steel would damage the record, adding crackle, and would render further fibres useless by chipping at the ends of the bundle of fibrous material. Collectors therefore jealously guarded their collections, and did not allow records out of their hands in case the fibred records became corrupted.

“Soft” needles were in common use through the 1920s and 1930s, and the pages of "The Gramophone" magazine were filled with debates about the relative merits. However these debates always concentrated upon the subjective effect; there is nothing objective, and nowadays we find it difficult to tell which gives the optimum power-bandwidth product. Soft needles were even tried on microgroove records in the early 1950s, evidently with some success (Ref. 7).

The motivation was, of course, to make contact with all the groove so as to reduce the hiss, as we saw earlier. Reference 6 quite correctly described the disadvantages of thorn needles (the attenuation of high frequencies and the risk that the tip might shear off), but it perversely did not say what the advantages were. To archivists nowadays, those two disadvantages are less important. We can equalise frequency response aberrations (so long as they are consistent), and we can resharpen the stylus whenever needed (as Nimbus did in the middle of playing their records, editing the resulting sections together).

A number of experiments by myself and others may be summarised as follows. A “soft” needle undoubtedly gives lower surface noise than any other, although the differences are less conspicuous when the high-frequency losses due to the needle’s compliance are equalised. (The response starts to fall at 3 or 4 kiloHertz when a bamboo needle is used in an acoustic soundbox). This is apparently because, with “hard” styli, in most cases we are not playing the basic hiss of the record, but the damage; this does not mean reduced noise. But damage shears bits off a “soft” point, and is then reproduced more quietly - a sort of mechanical equivalent of the electronic “peak clipper.” The reproduction is particularly congested at the end of the disc, because the needle has a long area of contact along the groove. Scientific measurements of frequency response and distortion give inconsistent results, because the tip is constantly changing its shape; thus I must regretfully conclude that a soft needle is not worthy of a sound archive. Also the process requires a great deal of labour and attention. However, it shows that better results are possible, and we must try to emulate the success of the soft needle with modern technology.

There seems to be only one case where the soft needle is worth trying - when the groove is of indeterminate shape for some reason - perhaps an etched “Berliner” or a damaged cutter. Grooves like this sometimes turn up on nitrate discs, and are not unknown in the world of pressings; operators derisively describe them with the graphic expression “W-shaped grooves.” Obviously, if the groove is indeed W-shaped, or otherwise has a shape where a conventional stylus is useless, then a soft needle is worth trying.

I should like to conclude this topic by advising potential users to experiment. It is comparatively easy to construct soft needles to one’s own specification, and there seems to be little harm to be caused to shellac 78s. I should also like to see a stereo pickup which takes such needles; in section 0 we shall be examining another method of reducing noise which depends upon the record being played with a stereo pickup. But I should like to remind you that if the needle develops a “flat” (which means it has a long area of contact in the dimension along the groove), various forms of distortion become apparent on sounds which cause sharp curves in the groove. So if you are doing any serious experimenting, I recommend you make use of an Intermodulation-Distortion Test Disc. In America the obvious choice is RCA Victor 12-5-39, and in Britain EMI JH138. References 8 and 9 give some hints on how to use them and the results which may be expected.

4.9 “Hard” replay styli

I count steel needles as being neither “soft” nor “hard.” They were soft enough to fit the groove after a few turns when played with pickups whose downwards force was measured in ounces, but the knife-edges worn onto the tip during this process cut into the waveform and caused distortion. They are not used today for this reason, and I can say with confidence that sacrificial trials of steel needles upon shellac discs do not give a better power-bandwidth product. If anyone wants to confirm my experiment, I should mention that “soft-toned” steel needles (soft in volume, that is) have extra compliance. This gives high-frequency losses like fibres, which must be equalised for a fair trial.

On the contrary, from the end of the second World War onwards, it has been considered best practice to use hard styli, by which I mean styli significantly harder than the record material. Styli could be made of sapphire, ruby, or diamond; but I shall assume diamond from now on, because sapphires and rubies suffer appreciable amounts of wear when playing abrasive 78s, and do not last very long on vinyl. The cost of a stylus (especially a specialist shape) is now such that better value is obtained by going for diamond right from the start. In the author’s experience there are two other advantages. Diamonds are less likely to be shattered by the shocks imparted by a cracked disc. Also diamonds can play embossed aluminium discs; sapphire is a crystalline type of aluminium oxide, which can form an affinity with the sheet aluminium with mutual destruction.

At this point I will insert a paragraph to point out the difficulties of worn “hard” styli. The wear changes a rounded, and therefore inherently “blunt”, shape, into something with a cutting edge. In practice, U-bottomed grooves cause a worn patch whose shape approaches that of the curved surface of a cylinder; V-bottomed grooves cause two flats with plane surfaces. Where these geometrical features intersect the original curved surface of the stylus, we get an “edge” - a place where the stylus has a line separating two surfaces, rather than a curved (blunt) surface. This can (and does) cut into record grooves, particularly where the groove contains sharp deviations from the line of an unmodulated spiral. Thus we cause distortion and noise on loud notes. The damage is irreversible. Therefore it is better to use diamond tips, which develop such cutting edges less easily. The increased cost is greatly outweighed by the increased life. Inspection with a 100-diameter microscope and suitable illumination is sufficient to show the development of flats before they become destructive.

The “bluntest” shape, and therefore the one least likely to cause wear, is the sphere. “Spherical” tips were the norm from about 1945 to 1963. The spherical shape was ground onto the apex of a substantially-conical jewel mounted onto the armature or cantilever of an electrical pickup. Being spherical, there were relatively few problems with alignment to minimise tracing distortion (section 4.10), so long as the cantilever was pointing in about the right direction and there was no restriction in its movement. The spherical shape gave the maximum area of contact for a given playing-weight. So acceptable signal-to-noise ratio and reasonable wear was achieved, even upon the earliest vinyl LPs with pickups requiring a downward force of six grams or more.

From 1953 to 1959, the British Standards Institution even went so far as to recommend standardised sizes of 2.5 thou radius for coarsegroove records and 1 thou for microgroove records. This was supposed to ensure disc-cutting engineers kept their modulation within suitable limits for consumers with such styli; it did not directly influence the dimensions of grooves themselves. However, the idea had some difficulties, and neither recommendation lasted long. You may often find older records needing larger styli (particularly microgroove from the Soviet Union). Something larger than 1 thou will be absolutely vital for undistorted sound here.

But first I shall mention the difficulties for coarsegroove records. We saw in section 4.6 that manufacturers were forced to change to V-shaped grooves to enable harder needles to last in autochangers. 2.5-thou spherical styli could be relied upon to play such V-shaped grooves successfully, but they were often troublesome with older U-shaped grooves. When you think about it, a hard spherical tip is very likely to misbehave in a U-shaped groove. If it is fractionally too small, it will run along the bottom of the groove and not be propelled by the laterally-modulated groove walls at all; the result is noisy reproduction and distortion at low levels. If it is fractionally too large, it will not go properly into the groove at all, but sit across the top edges. The result, again, is noisy; but this time the distortion occurs on loud notes and high-frequency passages. As long as customers could only buy spherical tips, the only way forward was to buy tips of different diameters. Enthusiasts had a range of spherical-tipped styli such as 2.5-thou, 3-thou, 3.5-thou, and 4-thou, specifically for playing old U-shaped grooves. We saw in section 4.6 that U-shaped grooves had, in fact, the cross-section of a ellipse with a low degree of eccentricity. It was found that a range of styli with differences of 0.5 thou, could trace nearly all such grooves. It was also found that for modern coarsegroove discs with V-shaped grooves and high levels of modulation (or high frequencies), smaller tips were helpful, such as 2-thou and 1.5-thou.

For microgroove discs, the recommended 1-thou tip was found unsatisfactory for a different reason. Pickup development was rapid, aided by Professor Hunt’s papers about the effects of pickup design upon record-wear. These showed the advantages of high compliance, high areas of contact, and low effective tip-mass. Below certain limits in these parameters, Professor Hunt showed that pickups would work within the elastic limits of vinyl and cause no permanent wear (although instantaneous distortions could still occur). (Refs. 11 and 12). Great efforts were made to approach the ideals laid down by Professor Hunt, which the introduction of stereo and the high volumes of pop discs did little to hinder. By the end of the 1950s it was apparent that greater fidelity could be achieved with smaller spherical tips, 0.7 thou or 0.5 thou. Although such styli often “bottomed” on older records, they could trace the finer, high-frequency, details of newer discs with less distortion; thus more power-bandwidth product was recovered. There was increased disc wear due to the smaller area of contact, but this was soon reduced by improvements in compliance and effective tip-mass.

There was another consideration in the days of mono, which is of less importance now. Consider a spherical stylus playing a lateral-cut mono groove with loud modulation. Where the groove is slewed, its cross-section also narrows. To overcome this, the stylus must also rise and fall twice a cycle - in other words, it must possess vertical compliance. Even if we are only interested in the horizontal movement, the centre of the spherical stylus tip does not run exactly along the centre-line of the groove. The resulting distortion is called “pinch effect distortion,” and permanent deformation of a groove was caused if a stylus had low vertical compliance.

For years two sources of distortion tended to cancel each other. The harmonic distortion which resulted from the large tip being unable to trace the fine detail was almost exactly opposite to the harmonic distortion caused when a massive stylus modified its course by deforming the groove wall. (Ref. 13). The fact that two types of distortion neutralised each other was often used deliberately, but at the cost of permanent damage to the groove. This explains why so many “pop singles” could be recorded at such high volumes. If you don’t have a copy in unplayed condition, your only chances of getting undistorted sound are to use a large tip with low vertical compliance, or find another version of the same song.

4.10 Stereo techniques

To carry a stereo recording, a disc has to be capable of holding two channels of sound in accurate synchronism. Several different methods were tried at different dates. In the 1930s both Blumlein in Britain and the Bell Labs engineers in America tried cutting the left-hand sound as lateral modulation and the right-hand channel as vertical modulation. This worked, but the two systems have different distortion characteristics. So the reproduced sound had asymmetrical distortion, which was very noticeable because this cannot occur in nature. Arnold Sugden of Yorkshire England attempted the same thing using microgroove in the years 1955-6 (Ref. 14).

Meanwhile Cook Laboratories in America recorded stereo by using two lateral cutters at different radii, an idea taken up by Audio Collector and Atlantic. It was found that the inevitable tracking errors of pivoted pickup arms caused small time-shifts between the two channels, which were very noticeable on stereo images.

Back in the UK, Decca tried an ultrasonic carrier system. One channel was recorded using conventional lateral recording, while the other was modulated onto an ultrasonic 28kHz carrier-wave, also cut laterally.

But the solution ultimately adopted was one originally patented by Blumlein, although not actually used by him as far as I know: to record the sum of the two channels laterally, and their difference vertically. Not only do the two channels have symmetrical distortion characteristics, but the record has the advantage of “downwards compatibility,” so a mono record will reproduce in double-mono when played with a stereo cartridge.

This is geometrically equivalent to having one channel modulated at an angle of 45 degrees on one wall of a V-shaped groove, and the other at right-angles to it upon the other groove-wall. The convention adopted was that the wall facing the centre of the disc should carry the right hand channel, and the one facing away from the centre should have the left-hand channel. This standard was agreed internationally and very rapidly in April 1958, and I shall be assuming it from now on. The other (very rare) systems amount to “incunabula.”

V-shaped grooves were standard by now, as we have seen. There were no immediate consequences to the design of hard styli, except to accelerate the trend towards 0.5 thou sphericals and ellipticals (which form the topic of the next section) as part of the general upgrading process.

But a new source of distortion rapidly became noticeable – “vertical tracking distortion.” Cutters in the original Westrex 3A stereo cutterhead, and its successor the model 3B, were mounted on a cantilever whose pivot was above the surface of the master-disc. So when “vertical” modulation was intimated, it wasn’t actually vertical at all; it was at a distinct angle. In those Westrex cutterheads the angle of the cantilever was twenty-three degrees, while in another contemporary cutterhead (the Teldec) there was no cantilever at all, so when the cutter was meant to be moving vertically, it was moving vertically.

So besides the various tracing and tracking distortions known from lateral records, there was now a new source of trouble. When a perfect vertical sine wave is traced by a cantilever, the sine wave is traced askew, resulting in measurable and noticeable distortion. This is exactly analogous to the “tracking distortion” on lateral modulation when you play the groove with a pivoted arm (section 4.4), so the phenomenon is called “vertical tracking distortion.” The solution is to ensure that cutter cantilevers and pickup cantilevers operate at the same angle. It proved impossible to design a rugged stereo pickup without a cantilever, so the angle could not be vertical. Some years of research followed, and in the meantime non-standard stereo LPs continued to be issued; but the end of the story was that a vertical tracking angle of fifteen degrees was recommended. (Ref. 15).

The first difficulty was that the actual physical angle of the cantilever is not relevant. What is important is the angle between the tip of the stylus and the point (often an ill-defined point) where the other end of the cantilever was pivoted. Furthermore, variations in playing-weight and flexing of the cantilever at audio frequencies had an effect. All this took some time to work out, and it was only from about 1964 onwards that all the factors were understood, and a pickup could be guaranteed to have a fifteen-degree vertical tracking angle at high frequencies (which is where the worst of the trouble was). Unfortunately, by 1980 a gradual drift had become apparent among pickup makers, if not disc cutting engineers; the average angle was over twenty degrees.

Similar problems applied to cutting the master-disc. Some designs of cutter proved impossible to tilt at the required angle. And more research was needed because of a phenomenon known as “lacquer-springback.” We saw in section 4.6 that cellulose nitrate lacquer discs gradually took over for disc mastering, the changeover years being roughly 1936 to 1948.

It was found that elasticity of the lacquer also caused some vertical tracking error, because after the cutter removed a deep section of groove, the lacquer tended to creep back under its own elasticity. This effect had not been noticed before, because the springback was consistent for lateral cutting (with constant groove depth), and for wax (which was not elastic). But the vertical tracking error from lacquer alone might be twenty degrees or so. It varied with the make of lacquer, the size of the polishing bevels, and the temperature of the cutter. When this effect was added to the twenty-three degrees of the Westrex cutterheads, or the fifteen degrees of the proposed standard, major redesigns were needed in three dimensions.

The Westrex 3D, the Neumann SX68, and the Ortofon cutterheads were the result. It proved possible to modify the Teldec by chamfering off a bottom edge and tilting it (to oversimplify greatly); thus all discs mastered after late 1964 should be to the fifteen-degree standard. But we should be prepared to mess about when playing stereo discs mastered before 1964. The best technique is to mount the pickup rigidly in its headshell, and arrange for the turntable to swing in gimbal mountings beneath it while listening to the vertical component of the sound, choosing an angle for minimum distortion.

A new type of cutting stylus was introduced in 1964 called the “Cappscoop.” (Ref. 16). This was specifically intended to make the lacquer-springback more consistent, giving straighter groove-walls; but I have no experience of it or its results.

4.11 “Elliptical” and other styli

I have said several times that difficulties were caused when styli failed to trace the smaller wiggles in grooves, but I have not yet formally mentioned the solution. The smaller the zone of contact, the more accurately fine wiggles can be traced; but with small spherical tips the result is generally an increase in hiss and an increase in disc wear, because the contact takes place over a smaller area. Both problems can be ameliorated if we use a “bi-radial” stylus - that is, with a small size in one dimension and a large size in another. Edison’s “button-shaped” stylus was one solution, and a sharp-tipped fibre was another. The so-called “elliptical” stylus was a third.

This is really only a trivial modification of Edison’s idea. Edison made an oblate spheroid sit across the groove; the “elliptical stylus” comprises a conical tip rounded, not to a spherical shape, but to an ellipsoidal shape. When either Edison’s or an ellipsoidal stylus sits in a V-shaped groove, the effect is the same. If you draw a horizontal cross-section of the stylus at the level of the points of contact, both styli are shaped like an ellipse; hence the shorter but inaccurate term, “elliptical stylus.” Historically, the Ferranti ribbon pickup of 1948 was the first to be marketed with an elliptical sapphire stylus (Ref. 17), followed by a version of the Decca “ffrr” moving-iron pickup. Decca had been cutting full frequency-range coarsegroove shellac discs for four years, but towards the centre of the record the high frequencies were so crammed together that spherical styli could not separate them. An elliptical stylus not only extracted them better, but did so with less distortion.

In the case of Decca’s stylus, the “dimensions” were 2.5 thou by 1.0 thou. By convention, this means that if you look at the horizontal cross-section at the point where the jewel sits across a V-shaped groove with walls of slope 45 degrees, the ellipse has a major axis of 5 thou (twice 2.5 thou) across the groove, and 2 thou (twice 1.0 thou) along the groove. It is also understood that the third dimension, up and down in the groove, also has a radius of 2.5 thou, but this axis may be slightly off vertical. When we speak of the “dimensions of an elliptical stylus,” this is what we mean.

This turns out to be a useful compromise between a small spherical tip and a large spherical tip. In the example given, the stylus will follow the groove wiggles as satisfactorily as a 1 thou tip, while riding further up the groove walls (so there is less risk of the stylus running along the curved bottom of the groove, or hitting any noisy debris in the bottom). The disadvantage is that, although the area of contact is much the same, the ability to trace smaller wiggles can mean greater accelerations imparted to the stylus, and greater risk of disc wear on loud passages. Your organisation may like to consider the policy of using spherical tips for everyday playback purposes, and more sophisticated shapes for critical copying.

Reduced distortion can only be achieved if the major axis of the ellipse links two corresponding points of the opposite groove walls. It has been shown that the major axis has only to be in error by a few degrees for the reduction in distortion to be lost. Thus the pickup must be aligned for minimising both tracking and tracing distortions, particularly on inner grooves (section 4.4). The conventional alignment procedure assumes that the edges of the cutter which cut the two groove walls were oriented along the disc’s radius. This was nearly always the case on discs mastered on cellulose nitrate, or the swarf wouldn’t “throw” properly; but it is not unknown for wax-mastered discs to be substantially in error. A feather-edged cutter would cut a clean groove at almost any angle. It may be necessary to “misalign” the arm, or the cartridge in the headshell, to neutralise the recorded tracing distortion.

At the time elliptical tips became popular, hi-fi enthusiasts were encouraged to spend time aligning their record-playing decks for optimum performance. This generally meant balancing record-wear against quality, and if you didn’t want to damage any records in your collection, you needed to investigate the issues very thoroughly. But if you do not play vinyl or nitrate very much, most of the risk of wear can be circumvented by playing and transferring at half-speed.

Elliptical styli did not become commonplace until the mid-1960s. In the meantime, an attempt was made to reduce tracing distortion by pre-distorting the recorded groove. The RCA “Dynagroove” system was designed to neutralise the tracing distortion which occurred when a 0.7 thou spherical tip was used. (Ref. 18). So presumably that’s what we should use for playing “Dynagroove” records today. But the “Dynagroove” system was also combined with a dynamic equalizer supposed to compensate for the Fletcher-Munson curves (a psychoacoustic phenomenon). The rationale behind this was essentially faulty, but the characteristics were publicly defined, and can be reversed. (Ref. 19)

If an elliptical tip does not have its vertical axis at the same angle as the vertical tracking angle, a phenomenon known as “vertical tracing distortion” occurs. This doesn’t occur with spherical tips. I suspect the simultaneous existence of “vertical tracking” and “vertical tracing” distortion was responsible for the confusion between the words, but the terminology I have used is pedantically correct. Vertical tracing distortion can occur with mono lateral-cut discs, under extreme conditions of high frequencies and inner diameters. To put it in words, if the minor axis of the elliptical tip is tilted so that it cannot quite fit into the shorter modulations of the groove, results similar to conventional tracing distortion will occur. John R. T. Davies had some special styli made to play cellulose nitrate discs. These suffered from lacquer-springback even when the recording was made with a mono cutterhead, and for some reason the surface noise is improved by this technique as well. But a turntable in gimbals seems equally effective so long as the clearance beneath the cartridge is sufficient.

I said earlier that Edison’s oblate spheroid was equivalent to an elliptical in a V-shaped groove; but it’s not quite the same in a U-shaped groove. An elliptical will have the same difficulties fitting a U-shaped groove as a spherical, because looking in the direction along the groove, it appears spherical. The problem was solved by the “Truncated Elliptical” tip, a modern development only made by specialist manufacturers. It’s an “elliptical” shape with the tip rounded off, or truncated, so it will always be driven by the groove walls and never the bottom. This shape is preferred for the majority of lateral coarsegroove records. (It even gives acceptable, although not perfect, results on most hill-and-dale records).

Although a range of sizes is offered, it is usually only necessary to change to avoid damage on a particular part of a groove wall, or to play lateral U-shaped grooves which have such a large radius that even a truncated tip is too small. Truncation can reduce the contact area and increase disc wear. Fortunately it is hardly ever needed for vinyl or cellulose nitrate records, which nearly always have V-shaped grooves.

We now reintroduce the lessons of “soft” styli, which had a large area of contact giving less hiss from particles in the pressed disc. Electronic synchronisation techniques permit us to play grooves with several styli of different sizes and combine the results. Thus, given a family of truncated ellipticals of different sizes, we emulate fibre needles without their disadvantages. I shall say more about this in section 0.

In the microgroove domain, the success of the elliptical stylus stimulated more developments, which are known collectively as “line-contact” styli. There were several shapes with different names. The first was the “Shibata” stylus, introduced in 1972 for playing the ultrasonic carriers of CD-4 quadraphonic discs (Section 10.16). The idea was to pursue lower noise, better frequency response, or lower wear, (or all three), by making contact with more of the groove walls.

But all line-contact styli suffer the same disadvantage. If the line of contact is not exactly correct - parallel to the face of the cutting stylus in the horizontal plane and fifteen degrees in the vertical - tracing distortion becomes very obvious. When everything is right they work well; but when anything is slightly misaligned, the result is disappointing. In 1980 an article in Hi-Fi News listed some of the types of line-contact stylus, mentioning that fundamentally faulty manufacturing principles and bad finish were adding to the difficulties. The author advocated the new “Van den Hul” stylus as being the solution; but a review of the very first such cartridge in the very same issue revealed that it had more distortion than half-a-dozen others. That review seems to have killed the idea for widespread use.

The trouble is that variations in the lacquer-springback effect and the tracking distortions of pivoted pickup arms made the ideal impossible to achieve without much fiddling. Cartridges with line-contact styli were expensive and delicate, and hi-fi buffs preferred fixed headshells, so fiddling was not made easier. So perfect reproduction was hardly ever achieved. It is significant that professionals have never used them. From the archival point of view, there is little need; most master tapes of the period still exist, and the subject matter is often available on compact digital disc. But clearly there is an avenue for exploration here. The reproduction of some older full-range records might well be improved, so for a general article on line-contact styli I refer you to Reference 20.

4.12 Other considerations

The above history should enable you to choose suitable styli and playing-conditions for yourself, so I do not propose to ram the points home by saying it all again. Instead, I conclude with a few random observations on things which have been found to improve the power-bandwidth product.

Many coarsegroove discs with V-shaped grooves have bottom radii which are smaller than the stylus sizes laid down by the British Standards Institution. Try a drastically smaller tip-radius if you can, but learn the sound of a bottoming stylus and avoid this. Not only does a small radius minimise pinch-effect and tracing distortions, but the bottom of the groove often survives free from wear-and-tear. This particularly applies to cellulose nitrates and late 78s with high recorded volumes. Indeed, these is some evidence that record companies did not change the sapphire cutter between a microgroove master and a 78 master. Eventually you will train your eye to tell the bottom radius of a groove, which will cut down the trial-and-error.

On the other hand, it sometimes happens that an outsized stylus is better. This is less common, because (all other things being equal) you will get increased tracing distortion, and there will be greater vulnerability to noise from surface scratches. But just occasionally the wear further down in the groove sounds worse. For various reasons, it seems unlikely we shall ever be able to counteract the effects of wear, so evasive action is advised. You can then concentrate upon reducing the distortion with elliptical or line-contact styli, and the noise with an electronic process.

Although my next point is not capable of universal application, there is much to be said for playing records with downward pressures greater than the pickup manufacturer recommends. To reduce record-wear, an audio buff would set his playing-weight with a test disc such as the Shure “Audio Obstacle Course”, carrying loud sounds which might cause loss of groove contact. He would set his pickup to the minimum playing-weight to keep his stylus in contact with the groove walls at the sort of volumes he expected (different for classical music and disco singles!), thereby getting optimum balance between distortion and wear. But nowadays, little wear is caused by higher playing weights; most is caused when the grooves vibrate the stylus, not by the downward pressure.

There can be several advantages in increasing the downward pressure for an archival transfer. The fundamental resonant frequency of the cantilever is increased (according to a one-sixth power law - Ref. 21), thereby improving the high frequency response. Clicks and pops are dulled, partly because the stylus can push more dirt aside, and partly because the cantilever is less free to resonate. But most important of all, the stylus is forced into the surface of the disc, thereby increasing the contact area and reducing the basic hiss. Obviously the operator must not risk causing irreparable damage to a disc; but if he is sufficiently familiar with his equipment, he will soon learn how far to go whilst staying within the elastic limits of the medium.

Shellac discs seem practically indestructible at any playing-weight with modern stereo pickup cartridges. Modern pickup arms are not designed for high pressures, but a suitably-sliced section of pencil-eraser placed on top of the head-shell increases the down-force with no risk of hangover. Pressures of six to ten grams often give improved results with such discs; special low-compliance styli should be used if they are available. With ultra-large styli, like those for Pathé hill-and-dale discs, it may even be necessary to jam bits of pencil-eraser between cantilever and cartridge to decrease the compliance further; twenty to thirty grams may be needed to minimise the basic hiss here, because the area of contact is so large.

Records should, of course, be cleaned before playback whenever practicable (see Appendix 1). But there are sometimes advantages in playing a record while it is wet, particularly with vinyl discs. Water neutralises any electrostatic charges, of course; but the main advantages come with discs which have acquired “urban grime” in the form of essence-of-cigarette-smoke, condensed smog, and sweaty fingerprints. Also, if previous owners have tried certain types of anti-static spray or other cleaning agents relying upon unconventional chemicals, there may be a considerable deposit on the groove walls which causes characteristic low-volume sounds. Conventional cleaning does not always remove these, because the sludge gets deposited back in the grooves before the record can be dried. Unfortunately it is impossible to give a rule here, because sometimes cleaning makes matters worse (particularly with nitrates - it may be essential to transfer each disc twice, once dry and once wet, and compare the results of the two transfers).

Centrifugal force often makes it difficult to play 78rpm discs wet. But for slower-speed discs, distilled water may be spread over the surface while it plays, perhaps with a minute amount of photographic wetting agent. The liquid can be applied through the outer casing of a ballpoint pen with the works extracted; this can be used as a pipette to apply the liquid, and as a ruler to spread it. Some types of disc have a “vinyl roar” which is caused when the stylus runs across the surface and excites mechanical resonances within the plastic. Although a proper turntable-mat and centre-clamp should eliminate the effect on most records, the liquid also helps. However, some transfer engineers have reported that dry playing of discs previously played wet can reveal a subsequent increase in surface noise. The author accepts no responsibility for damage to record or pickup!

I deliberately concentrated upon laterally-modulated records from section 4.7 onwards, but I shall now deal with a specific problem for hill-and-dale records. It is essential to take vertical tracking and vertical tracing into account of course, and strike a compromise between tracing distortion (caused by a large area of contact) and hiss (caused by a small area of contact). Even so, much even-harmonic distortion may remain, and in many cases this will be found to be recorded in the groove. The reason for this will be dealt with in section 4.15, where we look at electronic techniques for improving the power-bandwidth product.

Finally, the archivist should be aware that the metal intermediate stages in the disc-record manufacturing process – “master”, “mother” and “stamper” - sometimes survive. Since these do not contain abrasives, the power-bandwidth product is usually better. I have no experience in playing metalwork myself, but a consensus emerged when I was researching this manual, which was that most people preferred to play fresh vinyl pressings rather than metal. There are a number of difficulties with metal - it is usually warped and lacks a proper-sized central hole, the nickel upsets the magnetic circuit of the pickup, you can have only “one bite of the cherry” whereas you may have several vinyl pressings, etc. However, as vinyl pressing plants are decommissioned, it will become increasingly difficult to get fresh vinyl pressings made, and the risk when a unique negative is clamped in a press by an inexperienced worker will increase. Until sound archives set up small pressing-plants, I think we are more likely to be playing metalwork in the future.

Pressing factories usually had the wherewithal to play a metal negative (with ridges instead of grooves), if only to be able to locate clicks or noise. The turntable must rotate backwards (see section 4.13), and the stylus must obviously have a notch so it can sit astride the ridge. Top quality isn’t essential for factory-work; it is only necessary to locate problems without having to examine every inch of ridge under a microscope. The Stanton company makes a suitable stylus for their cartridges. In effect, it comprises two ordinary diamond tips side-by-side on the same cantilever. I am not aware that there are any options over the dimensions, so this could conceivably give disappointing results; but I must say the few I’ve heard sounded no worse than vinyl, and often better.

4.13 Playing records backwards

I shall now continue with a couple of “hybrid” topics. They combine mechanical techniques with electronic techniques. After that, the remaining sections will deal with purely electronic signal-processing. It has often been suggested that playing a record backwards and then reversing the transfer has some advantages.

Among those cited are:

  • 1. The opposite side of any steep wavefront is played, so wear has less effect.
  • 2. Resonances and other effects which smear the signal in time are neutralised.
  • 3. It is easier to extract the first milliseconds of modulation if the cutter has been lowered with sound on it.
  • 4. It is easier to distinguish between clicks and music for electronic treatment.
  • 5. If you are using fibre needles, the problems which would be caused by the needle being most-worn at the middle of the disc are ameliorated.
  • 6. Needle-digs and other sources of repeating or jumping grooves are more easily dealt with.


Unfortunately the author simply does not agree with the first two reasons, although he has tried the idea several times. Worn records still sound worn (if the needle is tracing the groove correctly, of course). The theory of neutralising resonances is wrong. Even if electronic anti-resonance circuitry is proposed, the original waveform can only be recreated if the sound passes through the anti-resonant circuit forwards.

However, the other four arguments for playing a record backwards do have slightly more validity, but not much. In the case of argument (3), the writer finds that (on coarsegroove records, anyway) it is quicker to lower the pickup onto the correct place, repeating the exercise until it’s done correctly! For argument (4), analogue click detectors work more efficiently because the circuitry is less confused by naturally-occurring transients, such as the starts of piano notes. But since all current analogue click detectors remove the click without replacing the original sound, they are not suited to archival uses. Computer-based declicking systems do not care whether the record is playing backwards or not; in effect, they shuttle the sound to and fro in RAM anyway. The writer has no experience of argument (5), because there is not yet a satisfactory electrical pickup using fibre needles, so you cannot reverse an electronic transfer anyway.

This leaves only the groove-jumping argument. For some records the reverse process can be very helpful. It will, of course, be necessary to use a reverse-running turntable, with a pivoted arm with a negative offset angle or a parallel-tracking system. Seth Winner, of the Rogers and Hammerstein Archives of Recorded Sound, has a conventional headshell with the cartridge facing backwards. He made this for playing disc-stamper negatives rather than records liable to groove-jumping. If his cartridge were to be used for groove-jumping, one would have to risk the cantilever being bent, because it will be compressed when it was designed to work under tension.

Also there are distinct disadvantages to the reverse-playing process. To start with, we need another turntable, or one which can be modified. A practical difficulty is that if the operator cannot understand the music, he may well miss other faults, such as wow, or lack of radius compensation (section 4.19). When some defects of equipment (such as tone-arm resonances) are reproduced backwards, the result is particularly distracting, because backward resonances cannot occur in nature.

To get the recording the right way round again, an analogue tape copy has to be reversed. For stereo, the left and right have to be swapped when the tape is recorded, so they will come out correctly on replay. Although I’d count it a luxury, if you were thinking of buying a digital audio editor, I’d advise getting one with the additional feature of being able to play a digital recording backwards while you were at it.

Since I haven’t said much about groove-jumping, I shall now devote a paragraph to the subject, although I hesitate because any operator worth his salt should be able to invent ways round the difficulties much more quickly than I can advise him. The obvious way, adjusting the bias on the pickup-arm, causes the whole disc to be affected; so ideally you need a short-term aid. My method (which can also be applied to a parallel-tracking arm) is to apply some side-pressure through a small camel-hair paintbrush. With grossly-damaged records this isn’t enough, so you may simply have to grab the cartridge lifting-handle between finger and thumb and push. This latter idea works best when you are copying at half-speed, which is the topic of the next section. You can’t always get a transfer of archival quality under these conditions; so you may have to use your digital editor for its intended purpose, editing the results! For some notes on playing broken records, please see Appendix 1.

I shall now share an idea which I have not tried personally. We have seen that tracing distortions occur because a cutting-stylus does not have the same shape as a replay stylus. Obviously, if we play a groove with a cutting stylus, we shall cut into it. But this wouldn’t happen with a cutting stylus running backwards, and this could eliminate many kinds of tracing distortion. Extremely accurate matching between the shape and dimensions of the two styli would be needed, plus considerable reduction in the effective mass of the replay one to avoid groove deformation.

4.14 Half-speed copying

This is a technique which is useful for badly-warped or broken records which would otherwise throw the pickup out of the groove. It is particularly valuable for cylinders. It is almost impossible to get warped cylinders back to the original shape, and most of them rotate faster than discs anyway. The solution is to transfer the item at half the correct speed to a system running at half the desired sampling frequency.

The principal disadvantage is that the low-end responses of all the equipment have to be accurate beyond their normal designed limits. Another is that the natural momentum of all moving parts is lower, so speed variations in the copying equipment are always higher. It is true that, given good modern equipment, the errors are likely to be swamped by those of the original media; but you should remember the danger exists.

4.15 Distortion correction

You will note that this is the first significant area in which the word “maybe” occurs. I shall be talking about processes which have yet to be invented. I don’t intend to infuriate you, but rather to show where techniques are possible rather than impossible. In the archival world time should not be of the essence, so you could leave “possible but not yet practical” work until a later date.

At present, harmonic and intermodulation distortion are faults which never seem to be reverse-engineered electronically. In principle, some types of such distortion could easily be undone; it seems the necessary motivation, and therefore the research, hasn’t happened. I can only recall one piece of equipment which attempted the feat during playback - the Yamaha TC800 cassette-recorder of 1976. It certainly made the Dolby tone (Section 8.4) sound better; but personally I could hear no difference to the music!

In the circumstances, I can only advise readers to make sure as little distortion as possible comes off the medium at source, because (as we shall see later) there are electronic ways of dealing with noise. Until someone breaks the mould, we must assume that retrospective distortion-removal will never be possible, and therefore we must concentrate upon it at source.

Harmonic distortion is, in practice, always accompanied by intermodulation distortion. For a reasonably complete survey of this idea I refer you to Reference 22; but in the meantime I will explain it briefly in words. If two frequencies are present at the same time, say m and n, we not only get harmonics (2m, 3m, 4m . . . and 2n, 3n, 4n . . . , the conventional “harmonic distortion”), but we also get “sum-and-difference frequencies” (m+n, m-n, 2m-n, 2n-m, etc). The latter case is called “intermodulation distortion.” Subjectively, the worst case is usually (m-n), because this means extra frequencies appear which are lower in pitch than the original sounds, and very conspicuous. They are often called “blasting.” If they have come from this source (they could also come from transient effects in the power-supply of the recording amplifier), the only hope for removing them without filtering is to generate equal-and-opposite sum-and-difference frequencies by reverse-engineering the original situation.

Gazing into my crystal ball, I can see no reason why distortion-removal should always remain impossible. One can visualise a computer-program which could look at a musical signal in one octave, pick up the harmonic and intermodulation products in other octaves, and by trial-and-error synthesise a transfer-characteristic to minimise these. By working through all the frequency-bands and other subsequent sections of sound, it should be possible to refine the transfer characteristic to minimise the overall distortions at different volumes and frequencies.

It would be an objective process, because there would be only one transfer characteristic which would reduce all the distortion products in the recording to a minimum, and this would not affect naturally-occurring harmonics. If future research then finds a transfer characteristic which is consistent for several separate recordings done with similar equipment, we might then apply it to an “objective copy.” I admit that, unless there is a paradigm shift because of a completely new principle, it would mean billions of computation-intensive trials. But computer-power is doubling roughly each year, so ultimate success seems inevitable.

The conventional approach - reverse-engineering the original situation - would depend upon having access to the sound with the correct amplitudes and relative phases. I have already mentioned the importance of phase in section 2.13. When we come to frequency equalisation in later chapters, I shall be insisting on pedantically correct ways of doing equalisation for this reason.

The first progress is likely to be made in the area of even-harmonic distortion, which occurs on recorded media which do not have a “push-pull” action. These include hill-and-dale grooves, the individual channels of a stereo groove, and unilateral optical media. Sometimes these show horrendous distortion which cries out for attention. Sometimes they are essentially reproduction problems, but at other times the recording medium will cause varying load on a cutter, meaning distortion is actually recorded into the groove.

In the late 1950s even harmonic tracing distortion was heard (for the first time in many years) from stereo LP grooves. The two individual groove walls did not work together to give a “push-pull” action to a stylus; they acted independently, giving only a “push” action. It was suggested that record manufacturers should copy a master-nitrate with the phases reversed so as to exactly neutralise the tracing distortion when the second reproduction took place. Fortunately, this was not necessary; as we saw in section 4.11, new types of playback styli were developed to circumvent the difficulty. And there was very little recorded distortion, because by that time the cutterheads were being controlled by motional negative feedback, which virtually eliminated distortion due to the load of the nitrate.

Many decades before, some manufacturers of hill-and-dale records did actually copy their masters, incidentally cancelling much of this sort of distortion. Pathé, for instance, recorded on master-cylinders and dubbed them to hill-and-dale discs (Ref. 23), and at least some of Edison’s products worked the opposite way, with discs being dubbed to cylinders. And, of course, “pantographed” cylinders were in effect dubbed with phase-reversal. So there are comparatively few cases where hill-and-dale records have gross even-harmonic distortion. It is only likely to occur with original wax cylinders, or moulded cylinders made directly from such a wax master.

The fact that it was possible to “correct” the even harmonic distortions shows that it should be easy with electronics today; but archivists must be certain such processes do not corrupt the odd harmonics, and this means we need more experience first.

The CEDAR Noise reduction System includes an option which reduces distortion. This uses a computerised “music model” to distinguish between music and other noises. Details have not yet been made public, so it is impossible to assess how objective the process is, so I cannot yet recommend it for archive copies.

4.16 Radius compensation

Edison doggedly kept to the cylinder format long after everyone else, for a very good engineering reason. With a disc rotating at a constant speed, the inner grooves run under the stylus more slowly than the outer grooves, and there is less room for the higher frequencies. Thus, all things being equal, the quality will be worse at the inner grooves. Cylinders do not have this inconsistency. Earlier we saw some of the difficulties, and some of the solutions, for playing disc records. But I shall now be dealing with the recording side of the problem, and how we might compensate it.

A “feather-edged” cutter was not affected by the groove speed. Such cutters were used for wax recording until the mid-1940s. With spherical or “soft” styli, there would be problems in reproduction; but today we merely use a bi-radial or line-contact stylus to restore the undistorted waveform. We do not need to compensate for the lack of high frequencies electrically.

The problem only occurred when the cutter did not have a sharp edge, e.g. because it had a polishing bevel. Here the medium resisted the motion of the cutter in a manner directly proportional to its hardness. For geometrical reasons it was also inversely proportional to the groove speed, and inversely proportional to the mechanical impedance of the moving parts. (A stiff armature/cutter will be less affected than a floppy one. A cutter with motional feedback has a high mechanical impedance). Finally, the effect was also dependent upon the size of the polishing bevel and the temperature of the wax or lacquer at the point of contact. All these factors affected the high-frequency response which was cut into the disc.

Thus, even with perfect groove contact, we may notice a high-frequency loss today. The effect will be worst on a recording “cut cold” in lacquer, using a duralumin-and-sapphire coarsegroove cutting-tool in a wide-range cutterhead with low mechanical impedance. In practice, the effect seems worst on semi-pro nitrate 78s recorded live in the 1950s.

Because of the complexity of the problem, and because no systematic analysis was done at the time, the effect cannot be reversed objectively. On any one record, it’s usually proportional to the groove speed; but human ears work “logarithmically” (in octaves rather than wavelength). The subjective effect is usually imperceptible at the outside edge of the disc. It is often inaudible half-way through; but the nearer the middle, the worse it starts to sound.

We do not know precisely when recording engineers started compensating for the effect as they cut the master-disc. It is thought Western Electric’s Type 628 radius-compensator circuit was in use by 1939. Before this date, the official upper-frequency limits of electrical recording systems prevented the effect from demanding much attention. After 1939, it can be assumed that commercial master-disc cutting incorporated radius compensation in some form. We may have to play the pressings with line-contact or elliptical styli to minimise the pinch-effect distortion, but this should not affect the intended frequency-range; compensation for the recorded losses will have been performed by the mastering engineer.

For other discs, the present-day transfer operator should compare the inner and outer radii. The usual procedure is to assume that the outside edge suffers no radius loss, and compensate for the high-frequencies at other radii by ear on the service-copy only. The operator will certainly have to do this if the subject matter requires the sides to be seamlessly joined!

Because the effect is wavelength-dependent, the compensation circuit should ideally vary the frequency of the slope continuously, not the slope itself. There is a limit to the compensation possible without making drastic increases in hiss and harmonic distortion. When we consider this, we observe that objective compensation is impossible for another reason. The transfer operator must use subjective judgement to balance the effects and minimise them for the listener.

The author knows of only one organisation which treated radius-compensation scientifically, and unfortunately its research was based on a different foundation. During the second world war, the BBC was attempting to stretch the performance of its nitrate lacquer disc-cutting operation to 10kHz, and the engineers considered the whole system (recording and reproduction) together. So far as reproduction was concerned, they settled on a standard stylus (2.5 thou spherical sapphire) and a standard pickup (the EMI Type 12 modified so its fundamental resonance was 10kHz), and they devised radius-compensation which gave minimum distortion when nitrates were played with this equipment.

And higher frequencies were ignored, because the landline distribution system and the characteristics of double-sideband amplitude-modulation transmission usually eliminated frequencies above 10kHz anyway. The compensation was designed for cold cutting of V-shaped grooves into cellulose nitrate blanks. The result was a family of resonant circuits in the recording electronics, each with a different resonant frequency and peak level. An electrical stud system (like a stud fader) switched between these circuits about five times during every inch of recorded radius. (Ref. 24). This continued until the BBC abandoned coarsegroove nitrate discs in about 1965.

From today’s viewpoint, this puts us in a dilemma. It would seem that we should play such discs with a 2.5 thou spherical sapphire in an EMI Type 12 cartridge; but this is a destructive instrument by today’s standards, and it will damage the disc. Furthermore the BBC assumed the nitrate had consistent hardness and elasticity. Several decades later the material has altered considerably, so accurate reconstruction of the intended situation is impossible anyway. Finally it may be impossible for academics running a sound-archive to recover the original intended sound, because of the tradeoffs made to minimise side-changes after the sound was broadcast with limited bandwidth.

The current policy at the British Library Sound Archive is to compensate only for the steady-state recording characteristics (which we shall be considering in chapter 5). We generally play the discs with the usual truncated elliptical styli to recover the maximum power-bandwidth product, but we do not attempt to neutralise the resonant artefacts at high frequencies, which are audible (but not severe) under these conditions. It is possible that some form of adaptive filtering may analyse the high-frequency spectrum and compensate it in future; in the meantime we have preserved the power-bandwidth product, which is the fundamental limitation.

The remainder of this chapter is concerned with the state-of-the-art in audio restoration technology, but can only be considered to be so at the time of writing. While much of the information will inevitably become outdated, it may still remain instructive and of some future use.

BOX 4.17
METHODS OF REPLACING CLICKS

1. The first is to cut both the click and the sound which was drowned by it, and to pull the wanted sounds on either side together in time. This is how tape editors have worked for the past forty years. Not only does it destroy part of the original waveform, but in extreme cases it can destroy tempo as well.

2. Another answer is to replace the click with nothing. Certainly, it is true that leaving a hole in the music is less perceptible than leaving the click; but we can hardly call it “restoring the original sound” - at least, if we mean the objective sound-wave rather than the sensation.

3. Another answer is to synthesise something to fill the gap. A very popular method is the “two-band method” (where there are two processors, one dealing with high frequencies, which leaves little holes as before, and one dealing with low frequencies, which holds the instantaneous voltage throughout the gap). This is subjectively less noticeable, but again you cannot call it “restoring the original sound.”

4. John R. T. Davis was the inventor of the “Decerealization” technique, which emulates this process. It involves a quarter-inch analogue magnetic tape of the clicky disc. A special jig which incorporates a tape-head and an extremely accurate marking-device holds the tape. Its dimensions are such as to permit a “stick-slip action” as the tape is pulled by hand. The operator listens on a selected loudspeaker, and as the individual short segments of sound are reproduced, the click stands out conspicuously. After the position is marked, the surface layer of the tape is scraped off where the click is. Although very labour-intensive, this remains the best way to deal with some types of material, because the operator can scrape off different degrees of oxide, thus creating the effect of the previous method with variable crossover frequencies for each click. In addition, when you can’t hear the click, the waveform isn’t attacked.

5. Another technique is to take a piece of sound from somewhere else in the recording and patch it into the gap. This technique was first described by D. T. N. Williamson, and although automatic devices using the idea have been proposed, they have never appeared. (It was envisaged that sound delayed by a few milliseconds could be patched into place, but it was found difficult to change to the delay-line without a glitch). Manual equivalents of the principle have been used successfully by tape editors. It has the “moral” advantage that you can say nothing has been synthesised. All the sounds were made by the artist!

6. More elaborate synthesis is used by the digital computer-based noise reduction methods “No-Noise” and “CEDAR.” They analyse the sound either side of the click, and synthesise a sound of the same spectral content to bridge the gap.

7. The final solution to the question “what do we replace the click with” only works if you have two copies of a sound recording and each of them suffers from clicks in different places. Then we can take the “best of both” without interpolating the waveform.

4.17 Electronic click reduction

The elimination of clicks has been a tantalising goal for more than half a century, because it is a relatively simple matter to detect a click with simple electronic techniques. The problem has always been: What do we replace the click with? (see Box 4.17)

All but the last method disqualify themselves because they do not pretend to restore the original sound waves; but if you need more details, please see Ref. 25. Unfortunately, there are relatively few pieces of equipment which can reduce noise without affecting the wanted waveform. In fact there are so few that I must mention actual trade-names in order to make my points; but I should remind readers these pieces of apparatus will be displaced in time. Some of them may be needed only occasionally, and may be hired instead of purchased; or recordings might be taken to a bureau service to cut down the capital expenses. You will need to know the various options when formulating your strategy.

The most important objective technique is Idea (7) in the Box, which is employed as the “first stage” of the Packburn Noise Reduction System (Packburn Electronics Inc, USA; Ref. 26). This is an analogue processor widely used in sound archives, and it has three stages. The first is used when a mono disc is being played with a stereo pickup, and the machine chooses the quieter of the two groove walls. It cannot therefore be used on stereo records.

Analysis of the actual circuit shows that it only attenuates the noisier groove wall by 16dB, so the description I have just given is something of an oversimplification; but it is certainly effective. The result is a little difficult to quantify, because it varies with the nature of the disc-noise and how one measures the result; but on an unweighted BBC Peak Programme Meter an average EMI shellac pressing of the inter-war years will be improved by about ten decibels. And, as I say, the waveform of the wanted sound is not, in principle, altered.

I should, however, like to make a few points about the practical use of the circuit. The first is that if we play one groove-wall instead of both groove walls, we find ourselves with a “unilateral” medium. Thus we risk even-harmonic distortion, as we saw in section 4.15. Actually, there is a mid-way position on the Packburn such that the two groove walls are paralleled and the whole thing functions as a lateral “push-pull” reproduction process. Theoretical analysis also shows that optimum noise reduction occurs when the groove walls are paralleled whenever they are within 3dB of each other. The problem is to quantify this situation.

The manufacturers recommend you to set the “RATE” control so the indicator-lights illuminate to show left groove-wall, right groove-wall, and lateral, about equally. I agree; but my experience with truncated-elliptical styli is that there is very little even-harmonic distortion reproduced from each groove wall anyway. You shouldn’t worry about this argument; there are other, more-significant, factors.

The next point is that, in the original unmodified Packburn, control-signals broke through into the audio under conditions of high gain, giving a muffled but definite increase in background noise which has been described subjectively using the words “less clarity” and “fluffing”. Therefore the RATE control must be set to the maximum which actually improves the power-bandwidth product and no more. My personal methodology is based on playing HMV Frequency Test Disc DB4037, which we shall be considering in chapter 5. Using a high frequency test-tone, we can easily hear the best noise reduction happens when the three light-emitting diodes are lit for about the same overall time.

Thus the manufacturer’s recommendation is confirmed. Do this on real music, and the optimum power-bandwidth is assured, even though it is less easy to hear the side-effects. Now that “The Mousetrap” manufactured in the UK by Ted Kendall has replaced “The Packburn,” this problem has been eliminated by the use of high-speed insulated-gate field effect transistors (IGFETs).

Another point is that, if the machine is to switch between the two groove walls successfully, the wanted sound on those two groove walls must be identical in volume and phase. (Otherwise the switching action will distort the waveform). The Packburn therefore has a control marked “SWITCHER - CHANNEL BALANCE.” When you are playing a lateral mono disc, you switch the main function switch to VERTICAL, and adjust this control to cancel the wanted signal. Then, when you switch back to LATERAL, the two groove walls will be going through the processor at equal volumes.

All this is made clear in the instruction-book. But what if you cannot get a null? In my view, if the wanted sound is always audible above the scratch, there’s something wrong which needs investigating. Assuming it isn’t a stereo or fake-stereo disc, and you can get a proper cancellation on known mono records (which eliminates your pickup), then either the tracking angle is wrong (most of Blumlein’s discs, section 6.31 below), or you’ve found a record made with a faulty cutterhead (e. g. Edison-Bell’s - section 6.16 below).

The former fault can be neutralised by slewing the pickup cartridge in its headshell. The latter faults have no cures with our present state of knowledge, but cures may be discovered soon, which would be important because sometimes there is useful power-bandwidth product in the vertical plane. In the meantime, all you can do is slew the cartridge as before in an attempt to cancel as much sound as possible, and then try the Packburn in its usual configuration to assess whether its side-effects outweigh the advantage of lower surface-noise.

To decide between the two groove walls, the machine needs access to undistorted peak signals at frequencies between 12kHz and 20kHz. It has been said that even the best stereo analogue tape copy of a disc will mar the efficiency of the unit, because it “clips” the peaks or corrupts their phase-linearity, and it is rather difficult to keep azimuths (section 9.6) dead right. This makes it difficult to treat a record unless you have it immediately beside the Packburn. Actually, I do not agree; I have even got useful noise reduction from a stereo cassette of a disc. But certainly the Packburn isn’t at its best under these conditions.

But digital transfers seem “transparent” enough. So it is practicable to use a two-channel digital transfer for the “archive” (warts-and-all) copy, provided no disc de-emphasis is employed (sections 3.5 or 6.23). Meanwhile, for objective and service copies it is best to place the Packburn following a flat pickup preamplifier with the usual precautions against high-frequency losses. Any frequency de-emphasis must be placed after the Packburn. (This is indeed how the manufacturers recommend the unit should be used).

The “second stage” of the Packburn is “the blanker”, a device for removing clicks which remained after the first stage, either because both groove walls were damaged at the same place, or because it was a stereo disc. The Packburn’s blanker rapidly switches to a low-pass filter, whose characteristics are designed to minimise the subjective side-effects (as paragraph (3) of Box 4.17. It does not restore the original sound wave, so it should only be used for service copies. Likewise, the “third stage” comprises a quite good non-reciprocal hiss reduction system (chapter 10), but this too alters the recorded waveform, so it too should be confined to service copies. To remove the remaining hiss and crackle whilst keeping the waveform, we must use alternative techniques; but the Packburn “first stage” is a very good start.

There are two such alternative techniques. One is to emulate the action of the Packburn first stage, but using two different copies of the same record. I shall be talking about this idea here and in section 4.20. The other is to use computer-based digital processing techniques to synthesise the missing sound.

The first idea is still in the development stage as I write, but the principle of its operation is very simple. Two copies of a disc pressed from the same matrix are played in synchronism. If the discs are mono, each goes through a “Packburn first-stage” (or equivalent). The difficult part is achieving and keeping the synchronism, for which the geometrical errors must be kept very low; but once this is achieved, a third Packburn first-stage (or equivalent) cleans up the result. Using the same example as I had in section 4.16, the result is a further 8dB improvement in signal-to-noise ratio. The noise coming from each disc is not actually steady hiss (although it usually sounds like it), but a very “spiky” hiss which responds to the selection process. If it had been pure white-noise, equal on the two copies but uncorrelated, the improvement would only be 3dB. (Which would still be worth having). Isolated clicks are generally completely eliminated, and no synthesis of the waveform is involved.

For this principle to work, the two discs have to be synchronised with great accuracy - better than 0.2 milliseconds at the very least - and this accuracy must be maintained throughout a complete disc side. Although digital speed-adjustment techniques exist, we saw in section 3.4 these have disadvantages which we should avoid if we can. So use a deck with minimal geometrical errors. For example, use a parallel-tracking arm whose pickup is pivoted in the plane of the disc, or provided with an effective means of keeping it a constant distance above the disc surface, so warps do not have an influence. The sampling-frequency of the analogue-to-digital converter is then “locked” to the turntable speed; there are other reasons in favour of doing this, which I shall mention towards the end of section 5.5. In the British Library Sound Archive’s case, a photoelectric device has been used to look at the stroboscope markings at the edge of the turntable, giving a 100Hz output. The result is frequency-multiplied by 441, giving a 44.1kHz clock-signal for the analogue-to-digital converters. The transfers of the two discs are done using the same stylus and equalisation, and at the same level, through a Packburn first-stage. The results are transferred to a digital audio editor and adjusted until they play in synchronism. The result is fed back through another Packburn at present, although a digital equivalent is being written to avoid unnecessary D-A and A-D conversions.

It has been found advantageous to combine the two discs using methods which operate on different frequency-bands independently. The Packburn only switches in response to noises in the range 12 – 20kHz. But if we have uncorrelated low frequency noises (e.g. rumble introduced during pressing), the switching action will generate sidebands, heard as additional clicks. In a prototype digital equivalent of the Packburn First Stage, we divide the frequency range into discrete octaves and treat each octave separately. The switching action takes place in each octave at the optimum speed for minimising sideband generation, and of course we get the quieter of the two grooves at all frequencies (not just 12-20kHz). We also get the version with the least distortion-effects in each band. The wanted waveform is never touched; all that happens is that background-noises and distortions due to the reproduction process are reduced. But at least two “originals” must be available.

We return now to when we have only one “original.” It is always possible to combine two plays of the same disc with different-sized styli, using the technology I have just described. This imitates the action of a “soft” stylus!

Several systems synthesise the missing section of waveform (previously drowned by the click) and insert it into place. Most digital processes use a technique known as the Fast Fourier Transform, or FFT, to analyse the wanted sound either side of the click. This is a speedy algorithm for a binary computer; in audio work, it is some hundreds of times faster than the next best way of doing the job, so it can run usefully even on a desktop microcomputer. (Ref. 27). When the click is eliminated, random digits are re-shaped according to the FFT analysis, and when the inverse FFT is performed, the missing waveform is synthesised. Both “No-Noise” and “CEDAR” offer realtime implementation, so the operator can switch between the processed and unprocessed versions and check that nothing nasty is happening to the music. Both replace the clicks with synthesised sound, so in principle we are not actually restoring the original waveform; but it’s a matter of degree. Experiments can be set up taking known waveforms, adding an artificial click, and seeing what sort of job the computer does of synthesising the original. The result may be judged aurally, or visually (on a waveform display of some sort).

The CEDAR people have various pieces of hardware and software for click removal. This includes a computer-based platform offering several powerful restoration algorithms (not just clicks), free-standing boxes which cannot be used for any other purpose (the cheaper one has no analogue-to-digital or digital-to-analogue converters), and cards for slotting into a SADiE PC-based hard-disk editor. The last-mentioned is usually the most recent version, since it is easier to make both “beta versions” and proven hardware.

Although “real-time,” the free-standing DC.1 unit may require the signal to be played through the machine three times, since three different algorithms are offered; they should be performed starting with the loudest clicks and ending with the quietest. CEDAR very bravely admit that the DC.1 process does not always synthesise the waveshape correctly for a long scratch, but in 1994 they claimed it was correct for clicks up to fifty samples long. CEDAR have been extending the number of samples; the latest version is in the range 250-300 samples. (This clearly shows archivists must log the version-number of the software). If the results were put to a scientific test on both aural and visual grounds with 100% successful results, there would presumably be no objection to using the algorithm for objective copies as well as service copies.

Unfortunately, since one must start with the biggest clicks, and the DC.1 sometimes blurs these (making it more difficult for future processes to detect them), there are relatively few records for which the DC.1 gives archivally-acceptable results. Malcolm Hobson’s solution is to run his process several times from hard disc in batch mode (this avoids having to accumulate several R-DATs which must work in real time). He starts with an FFT looking for high frequency transients less than six samples long (these are almost bound to be components of crackle), then interpolates these (which is certain to give faithful results for such small clicks). The process then works upwards towards larger clicks. Each time the surrounding music has less crackle, so interpolation is easier. However, much loving care-and-attention is needed for the benign replacement of the largest clicks, which may be done manually. So the trade-off is between leaving blurred clicks and possibly-inaccurate interpolation.

The No-Noise process is obliged to work in conjunction with a Sonic Solutions editing-system, which could be a restriction for some customers; but it is possible to concatenate several processes (for example, de-clicking, de-hissing, and equalisation), and run them all in real-time. This helps the operator to listen out for unwanted side-effects to any one of the processes. No-Noise has an option to mark the start and end of long clicks manually, and then do a trial synthesis of the missing signal, which you can adopt if you are satisfied. I have heard it do a convincing job on many thousands of missing samples, but I do not know how “accurate” this was. Although it has nothing to do with click removal, this process seems to be the best way to synthesise sound for a sector of a broken record which has vanished. This will probably never be an objective technique; but over the years many similar jobs have been done in the analogue domain. No-Noise can help in two ways, firstly by synthesising the missing sound, and secondly by performing edits non-destructively.

CEDAR’s computer-platform system and their free-standing de-crackle unit Type CR.1 offer another process. To oversimplify somewhat, the recording is split digitally into two files, one through a “music model” and the other comprising everything else. It is then possible to listen to the “music model” on its own, and adjust a control so that even the biggest clicks are eliminated. (This file lacks high frequencies and has various digital artefacts along with the music, but it is easy to listen for loud clicks if they are there). When a satisfactory setting has been achieved which eliminates the loudest clicks but goes no further, the two files are recombined. This process has been found empirically to reduce many of the effects of harmonic distortion, as I mentioned in section 4.15.

As we go to press, audio engineers are exploring other mathematical strategies for synthesising missing data. So far, these studies seem to comprise “thought experiments”, with no “before-and-after” comparisons being reported. The only one to have appeared is the newly developed SASS System (invented by Dr. Rudolf Bisping). Prony’s Method is used to analyse the music and express it as a sum of exponentially-decaying frequencies, which enables complete remodelling of the amplitude spectrum, including signals which change in pitch and notes which start or stop during the click. To do all this requires a computer some hundreds of times more powerful than hitherto. The SASS System has a dedicated architecture involving many transputers; but once again I have not had an opportunity to test it for “accuracy.”

Interpolation for replacing the biggest clicks is still not reliable. But it is well-known that interpolation is easier on sounds which change slowly, and that clicks appear subjectively louder on these same sounds. I consider we need a click-replacement process which automatically adapts itself to the subject matter. To end with a crudely-expressed dream, we need an interpolation strategy which automatically knows the difference between clicks during slow organ music, and clicks during a recording of castanets.

4.18 Electronic hiss reduction

No-Noise, CEDAR, and SASS also offer “hiss-reduction” algorithms. I wish to spend some time talking about these, because they offer conspicuously powerful methods of reducing any remaining noise; but frankly I am sure they are wrong for archival storage purposes.

The idea is to carve up the frequency-range into a number of bands, then reduce the energy in each band when it reaches a level corresponding that of the basic hiss. The process can reduce hiss very spectacularly; but it can cut audible low-level signals fainter than the hiss, so listeners sometimes complain there is “no air around” the performance. At its best it can reduce so much noise that it is possible to extract wanted high-frequency sounds which are otherwise inaudible, thereby apparently making a dent in the power-bandwidth principle (section 2.2).

I must also report that an analogue equivalent was being marketed by Nagra at one point (the Elison Model YSMA 18 with eighteen frequency bands); but it did not become available for some reason, which was a pity as it could be operated more intuitively than any digital equivalents.

Unfortunately these processes must make use of psychoacoustics to conceal side-effects. The width of the sample being analysed (in both the time and frequency domains), the amplitude below which attenuation can take place, the degree of attenuation, the times of response and recovery, and the volume at which reproduction is assumed to take place may all need to be taken into account. To make matters worse, recent psychoacoustic experiments suggest that our ears work differently when listening to speech as opposed to music. Most hiss-reduction units have user-adjustable controls for some of these factors. Although offered on a try-it-and-see basis, this subjective approach rules it out for archival applications. The controversies about records which have been processed by both No-Noise and CEDAR are usually attributable to the fact that the operator and the consumer have different psychoacoustic responses and/or listening conditions. Nevertheless, psychoacoustic measurements have made strides in the last decade, and many of the factors can now be quantified with precision, and, equally importantly, with a clear understanding of the variances.

The Fast Fourier Transform requires the number of frequency bands to be an exact power of two, with linear spacing. At present, No-Noise uses 2048 bands and CEDAR uses 1024, giving bands about 11 and 22 Hertz wide respectively when the sampling-frequency is 44.1kHz. This is not the optimum from the psychoacoustic point of view; it is well known that the human ear deals with frequency bands in quite a different way.

To reduce hiss whilst (apparently) leaving the wanted sounds intact, wanted sounds must “mask” unwanted ones. The unit of masking is the “bark”. Listening tests suggest that the human ear has a linear distribution of barks at frequencies below about 800Hz, and logarithmic above that. Thus any computer emulating the masking properties of the human ear needs a rather complex digital filter. Furthermore, the slopes of the filtered sections of the frequency range are asymmetrical, and vary with absolute volume.

The last-mentioned parameter has been circumvented by Werner Deutsch et. al. (Ref. 28), whose team chose values erring on the side of never affecting the wanted sound (“overmasking”). In my view, this algorithm is the best available method of reducing hiss while leaving the (perceived) wanted sound untouched. It is surprisingly powerful. Even when the hiss and the music are equal in volume, the hiss can be reduced by some 30dB; but its quality then becomes very strange. There is also the difficulty that, owing to the vital contribution of psychoacoustics, any frequency or volume changes must be made before the process, not after it.

Bisping divides the spectrum into 24 bands that correspond to critical bands in Corti’s organ of the inner ear, and hiss-removal is inhibited when it is isn’t needed. The trouble is that many experts in psychoacoustics consider 24 bands an oversimplification!

Such processes might be applied to the service copies of recordings meant to be heard by human adults. (But not to other sounds, or to sounds which might go through a second process involving audio masking). Even so, the correct archival practice must surely be to store the recording complete with hiss, and remove the hiss whenever it is played back.

We may yet find that the human ear has evolved to make the best use of the information presented to it, with little room for manoeuvre. We already know that the threshold of hearing is almost exactly at the level where the Brownian-movement of individual air molecules lies. Thus we might find that our pitch-detection and our tolerance to background-noise have evolved together to give a performance which cannot be improved. If so, we could never reduce wideband hiss to reveal completely inaudible sounds. But I very much look forward to further developments, because they might permit a real breakthrough in sound restoration. The limitations of the power-bandwidth principle could be breached for the very first time.

4.19 Eliminating rumble

Apart from the use of linear filters (which affect the wanted sound, of course), there have been very few attempts to neutralise the low-pitched noises caused by mechanical problems in the cutting machine. Not all such noises are capable of being removed “objectively,” but there are a few exceptions. Usually these occur when the machine was driven by gears (as opposed to idler-wheels, belts, or electronic servo-systems). Here the pattern of rumble may form a precise relationship with the rotational speed of the cylinder or disc.

Using the principle of digital sampling being locked to rotational speed, as mentioned in section 0 above, it is possible simply to add together the sounds from each turn of the record. When this is done, you may often find a consistent pattern of low-frequency rumble builds up, which may be low-pass filtered and then subtracted from each of the turns in the digital domain to reduce the noises without affecting the wanted sound. This is particularly valuable when you’re trying to get some bass back into acoustic recordings (Chapter 11).

4.20 De-thumping

This section deals with the side-effects of large clicks when played with many practical pickup arms. The click may shock-excite the arm or disc into resonances of its own, so that even when the click is eliminated, a low-frequency “thud” remains. From one or two cases I have known, I suspect the actual cartridge may also be excited into ringing-noises at much higher frequencies (a few kiloHertz). Sometimes these exhibit extremely high Q-factor resonances, which cause long “pinging” noises. In the ordinary course of events these artefacts are not audible, because they are masked by the click itself. Only when the click is removed does the artefact become audible.

Quite frankly, the best solution is not to generate the thumps in the first place. Very careful microscopic alignment of cracked records may be needed to ensure that the pickup is not deviated from the centre-line of its travel. The cartridge may need to have silicone grease packed in or around it to reduce its tendency to make mechanical or electrical signals of its own. The pickup arm must have well-damped mechanical resonances; alternatively, a “parallel-tracking” arm may be used (section 4.2). This suspends the cartridge above the record in a relatively small and light mechanism, and, all things being equal, has less tendency to resonate. (Some parallel-trackers are capable of dealing with misaligned cracks which would throw a pivoted tone-arm, because they are guided by a relatively inert motor mechanism. This is probably the best way to play a warped and/or broken disc which cannot be handled any other way).

The Tuddenham Processor not only removes clicks; it also has a de-thump option, which applies a transient bass-cut with an adjustable exponential recovery. However, this is a subjective process, which should only be applied to the service-copy.

CEDAR have an algorithm for de-thumping which relies on having a large number of similar thumps from a cracked record. When twenty or thirty are averaged, the components of the wanted sound are greatly reduced, leaving a “template” which can be subtracted from each one. It does not significantly corrupt the original waveform, so it has its place. Sounds intended for such treatment should have the basic clicks left in place, as CEDAR uses them to locate the thumps. The makers of the SADiE digital editor can supply a de-thump card for the machine, but I simply do not have experience of its principles or its ease of operation.

4.21 Future developments

As I write, the industry is excited by the possibilities of adaptive noise-cancellation. This is similar to the hiss-reduction processes I mentioned in section 4.18, except that instead of using a fixed sample of hiss to define the difference between hiss and music, it can be a dynamic process. (Ref. 29). Given a sample of “pure” noise varying with time, the computer can (in theory) do a Fast Fourier Transform of the noise, and use it to subtract the appropriate amount of energy from the signal.

The makers envisage it could be used on the following lines. If it were desired to eliminate the background noise of speech picked up in an aircraft cockpit, for example, the usual noisy recording would be made, plus another track of pure aircraft noise (from another microphone). In theory, it would then be possible to sample the pure aircraft noise and use it to reduce the noise behind the speech, without having to rely upon phase-accurate information. This is actually an old idea (Ref. 30), which hitherto has been used by the US military for improving the intelligibility of radio-telephone communication. Only now is enough computing power becoming available for high-fidelity applications. For once, sound archivists don’t have to plead their special case. It is an application with many uses in radio, films, and television, and I anticipate developments will be rapid.

With mono disc records there are new possibilities, because “pure noise” is largely available. It is possible to extract a noise signal from a lateral disc by taking the vertical output of the pickup, although the rumble of the recording-turntable usually differs between the two planes. It offers the only hope for ameliorating cyclic swishing noises when there is only one copy of the disc, or when all surviving copies are equally affected. Some of the effects of wear might also be neutralised, although I wouldn’t expect to get all the sound back; indeed, we might be left with conspicuous low-frequency intermodulation distortion. Certain types of modulation noise on tape recordings could also be reduced, by splitting a tape track in two and antiphasing them to provide a “clean noise” track. Since it is possible to insert timeshifts into either signal, tape “print-through” might even be reduced. In chapter 11 I shall be considering the feasibility of dynamic expansion; this would almost certainly have to be done in conjunction with adaptive noise-cancellation to conceal the effect of background noise going up and down. But it seems these applications must always be subjective processes, only to be considered when drastic treatment is essential for service copies.

I should stress that adaptive noise-cancellation still has not achieved success in audio. One attempt to reduce long disc clicks failed, because there was insufficient processing power to analyse rapidly-varying noise. Disc clicks are distinguished from wanted sound because they change so rapidly.

At one time CEDAR were developing an algorithm which emulated the dual-processing method described in section 0 above, although it did not actually take the wanted sound from two copies. No “hard-lock” synchronisation was involved, so it could be used on wild-running transfers from two widely different sources. The reason it is not yet available is that it was very computation-intensive, and did not always offer satisfactory results because of difficulties synchronising recordings in the presence of noise.

In the case of disc records, the two copies each underwent click-reduction first, so the whole point of dual processing (to avoid interpolation) was missed. (Refs. 31 and 32). Nevertheless, this process might be used for reducing the basic hiss of already-quiet media, such as two magnetic tapes of the same signal. But it will always be computation-intensive. One can only hope that increased processing power and further research might make this process successful.

4.22 Recommendations and conclusion

This ends my description of how to recover the power-bandwidth product from grooved media, but I have not yet formally stated my views about what the three versions should comprise.

The archive copy should be a representation of the groove reproduced to constant-velocity characteristics (see chapter 6) using a stereo pickup, so that the two groove walls are kept separate. Ideally, there should be several separate transfers done with styli of different sizes, to provide samples at different heights up the groove wall. It is natural to ask how many transfers this should be. Experiments with the earliest version of the program mentioned in section 0 have been done, in which additional counting procedures were inserted to quantify the number of samples taken from each transfer. This was checked by both digital and analogue methods of measuring the resulting signal-to-noise ratio. All three methods suggest that, for coarsegroove shellac discs played with truncated elliptical styli, four such transfers should be done with styli whose larger radii differ by 0.5 thou. Softer media, such as vinyl or nitrate, will have a greater commonality between the transfers, because the styli will penetrate deeper into the groove walls; so four is the most which will normally be needed. Of course, this means a lot of data must be stored; but if you accept that Program J1 (* Editors’ note: program J1 was probably written by Peter Copeland but has not been found.) does the job of combining the transfers optimally, you can use this, and still call it an “archive copy.” To help any future anti-distortion processes, the stylus dimensions, and the outer and inner radii of the disc grooves, should be logged. And I remind you that absolute phase must be preserved (section 2.11).

For the objective copy, the same procedure should be followed, except that known playing-speeds (chapter 5 and recording characteristics (chapter 6) should be incorporated. Clicks may be eliminated, so long as accurate interpolation of the previously-drowned waveform occurs. It is natural to ask what tolerance is acceptable. My answer would be to do some before-and-after tests on the declicker; if subtracting “after” from “before” results in no audible side-effects, then the waveforms were synthesised accurately enough. But I recognise readers might have other ideas.

For the service copy, radius compensation may be applied, speed adjustments for artistic reasons can be incorporated, hiss-reduction may be considered, and sides may be joined up where necessary (section 13.2).

I hope my peek into the future won’t leave you cross and frustrated. Digital techniques are admittedly costly and operationally cumbersome, but there has been enormous progress in the last few years. By the time you read these words, the above paragraphs are certain to be out-of-date; but I include them so you may see the various possibilities. Then you can make some sensible plans, start the work which can be done now, and put aside the jobs which may have to wait a decade or two.

REFERENCES

  • 1: Franz Lechleitner, “A Newly Constructed Cylinder Replay Machine for 2-inch Diameter Cylinders” (paper), Third Joint Technical Symposium “Archiving The Audio-Visual Heritage,” Ottawa, Canada, 5th May 1990.
  • 2: Percy Wilson, “Modern Gramophones and Electrical Reproducers,” (book), (London: Cassell & Co., 1929), pp. 126-128.
  • 3: P. J. Packman, British patent 23644 of 1909.
  • 4: The earliest reference I have found is an anonymous article in Talking Machine News, Vol. VII No. 89 (May 1909), page 77.
  • 5: Carlos E. R. de A. Moura, “Practical Aspects of Hot Stylus,” Journal of the Audio Engineering Society, April 1957 Vol. 5 No. 2, pp. 90-93.
  • 6: A. M. Pollock, Letter to the Editor. London: Wireless World, April 1951, page 145.
  • 7: S. Kelly, “Further Notes on Thorn Needles.” Wireless World, June 1952, pages 243-244.
  • 8: A. M. Pollock, “Thorn Gramophone Needles.” Wireless World, December 1950, page 452.
  • 9: H. E. Roys, “Determining the Tracking Capabilities of a Pickup” (article), New York: Audio Engineering Vol. 34 No. 5 (May 1950), pp. 11-12 and 38-40.
  • 10: S. Kelly, “Intermodulation Distortion in Gramophone Pickups.” Wireless World, July 1951, pages 256-259.
  • 11: F. V. Hunt, “On Stylus Wear and Surface Noise in Phonograph Playback Systems.”
  • Journal of the Audio Engineering Society, Vol. 3 No. 1, January 1955. 12: J. A. Pierce and F. V. Hunt, J.S.M.P.E, Vol. 31, August 1938.
  • 13: J. Walton, “Stylus Mass and Distortion.” Paper presented to the Audio Engineering Society Convention in America in October 1962, but only published in printed form in Britain. Wireless World Vol. 69 No. 4 (April 1963), pp. 171-178.
  • 14: Roger Maude: “Arnold Sugden, stereo pioneer.” London: Hi-Fi News, October 1981 pages 59 and 61. (Includes complete discography)
  • 15: John Crabbe: “Pickup Problems, Part Two - Tracking Error,” Hi-Fi News, January 1963, pp. 541-545.
  • 16: Richard Marcucci, “Design and Use of Recording Styli,” J.A.E.S., April 1965 pp. 297-301.
  • 17: Wireless World, April 1948 page 135.
  • 18: John Crabbe: “Dynagroove Hullabaloo,” Hi-Fi News, November 1963 pages 417 and 419, and December 1963 pages 521 and 523.
  • 19: Harry F. Olsen: “The RCA Victor Dynagroove System” (paper), Journal of the Audio Engineering Society, April 1964, pp. 203-219.
  • 20: Basil Lane, “Improving groove contact,” Hi-Fi News, August 1980 pages 75-77.
  • 21: C. R. Bastiaans, “Factors affecting the Stylus/Groove Relationship in Phonograph Playback Systems,” Journal of the Audio Engineering Society, (1967?), pages 107-117.
  • 22: “Cathode Ray”: “More Distortion . . . What Causes Musical Unpleasantness?” (article), Wireless World Vol. 61 No. 5 (May 1955), pp. 239-243.
  • 23: Girard and Barnes, “Vertically Cut Cylinders and Discs” (book), pub. The British Library Sound Archive.
  • 24: For a general article on the BBC philosophy, see J. W. Godfrey and S. W. Amos, “Sound Recording and Reproduction” (book), London: Iliffe & Sons Ltd (1952), page 50 and pages 80-82. Details of the effect of the circuit for coarsegroove 33s and 78s on the BBC Type D disc-cutter may be found in BBC Technical Instruction R1 (October 1949), page 8; and there is a simplified circuit in Fig. 1.
  • 25: Adrian Tuddenham and Peter Copeland, “Record Processing for Improved Sound” (series of articles), “Part Three: Noise Reduction Methods,” London, Hillandale News (the journal of the City of London Phonograph and Gramophone Society), August 1988, pages 89 to 97.
  • 26: Richard C. Burns, “The Packburn Audio Noise Suppressor” (article), Sheffield, The Historic Record No. 7 pages 27-29. (March 1988).
  • 27: The Fast Fourier Transform was invented in several forms by several workers at several times, and there does not seem to be a definitive and seminal article on the subject. For readers with a maths A-level and some programming experience with microcomputers, I recommend Chapter 12 of the following: William H. Press, Brian P. Flannery, Saul A. Teukolsky, and William T. Vetterling: “Numerical Recipes - The Art of Scientific Computing,” (book), Cambridge University Press (1989). This is available in three editions, the recipes being given in three different computer languages.
  • 28: Werner A. Deutsch, Gerhard Eckel, and Anton Noll: “The Perception of Audio Signals Reduced by Overmasking to the Most Prominent Spectral Amplitudes (Peaks)” (preprint), AES Convention, Vienna, 1992 March 24-27.
  • 29: Francis Rumsey, “Adaptive Digital Filtering” (article), London: Studio Sound, Vol. 33 No. 5, pp. 34-5. (May 1991).
  • 30: (Pioneer adaptive noise cancellation paper) 31: Saeed V. Vaseghi and Peter J. W. Rayner, “A New Application of Adaptive Filters for
  • restoration of Archived Gramophone Recordings” (paper), I.E.E.E Transcriptions on Acoustics Speech and Signal Processing, 1988 pages 2548-2551.
  • 32: UK Patent Application GB 2218307.
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