Sie sind hier : Startseite →  Hifi Wissen und Technik→  Die Archivierung von Tönen→  Sound-Restoration-Teil 2

2 The overall copying strategy
2.1 The problem to be solved

In this manual I do not propose to discuss the major strategies of running a sound archive; instead, I shall refer you to a book by my mentor Alan Ward (A Manual of Sound Archive Administration, pub. Gower, 1990). But this chapter includes wider issues than just analogue sound reproduction and copying.

Some philosophers have considered the possibility of a replicating machine which might build an exact replica of an original recording, atom by atom. This is science fiction at present, so the only other way is to play such a recording back and re-record it.

But even if we could build such a replicating machine, I suspect that the universe may contain something more fundamental even than sub-atomic particles. Here is a rhetorical question for you to ponder: What is “Information”?

It may even be what holds the Universe together! When certain sub-atomic particles separate under the laws of Quantum Physics, they may be connected by “information” which travels even faster than light, but which does not actually travel until you make the observation. This is still a novel concept amongst the scientific community as I write (Ref. 1); but within a few decades I suspect it will be as familiar to schoolchildren as “Relativity” is now. And, since sound recording is by definition a way of storing “information,” such philosophical issues aren’t completely irrelevant to us.

2.2 General issues

Most of this manual is designed to facilitate the playback process so as to recover the information - the sound - without any intentional or unintentional distortions. It is aimed at the operator whose hands are on the controls, rather than the manager planning the overall strategy. For the latter, politics, cost, space and time are paramount; he is less concerned with “mechanics.” But it would be wrong for me to ignore the operational aspects of overall strategy, if only because in smaller archives the manager and the operator is the same person; so I shall now say a few words on the subject.

First, the law of copyright. This differs from one country to the next, and may also have exemptions for archival applications. For many years the British Library Sound Archive had special permission from The British Phonographic Industry Ltd. to make copies of records for internal purposes, since published records had no “fair dealing” exemptions. Under procedures laid down under the 1988 Copyright Act, archival copying work might always then be possible provided the Secretary for State was persuaded that the archive was “not conducted principally for profit”; but I must stress that, whatever I recommend, it does not absolve you from observing the law of copyright in your country.

The manager will certainly be concerned with cost, perhaps thinking of getting the maximum amount of work done for a particular budget. Frankly, I believe this is inappropriate for an archive dedicated to conserving sounds for centuries, but I recognise this will be a consideration in the commercial world. A manager must therefore understand the principles, so he may see clearly how the work will suffer if the ideal scenario is not followed. It may not be a catastrophe if it isn’t, but there will be trade-offs. The procedure actually used should certainly be documented, and then originals should be kept so that future generations can have another bite at the cherry. So the manager must assess the costs of storing the originals and then financing another bite of the cherry, comparing them with the costs of the ideal scenario.

I am afraid that experience also shows that “unexpected hitches” are frequent. It is usually impossible to copy sounds using production-line techniques. Whatever overall strategy you adopt, your schedule is certain to be disrupted sooner or later by a recording which requires many times the man-hours of apparently-similar items.

2.3 The principle of the “Power-Bandwidth Product”

As I said, the only way of conserving sounds which are at risk is to copy them. “At risk” can mean theft, wilful destruction, accidental erasure, biological attack, miscataloguing, or wear-and-tear, as well as plain chemical breakdown. But if it’s considered there is little chance of these, then there is much to be said for simply keeping the original recording uncopied, for the following reason.

Analogue recordings cannot be copied without some loss of quality, or “information” as engineers call it. Despite the idea of “information” being a fundamental property of matter, to an analogue engineer “information” is an objective measurement of the quality of a recording. It is obtained by multiplying the frequency range, by the number of decibels between the power of the loudest undistorted signal and the power of the background noise. The result is the “power-bandwidth product.” This term is used by analogue engineers to measure the information-carrying capacity of such things as transformers, landlines, and satellites, besides sound recordings.

It is always possible to trade one parameter against the other. To return to sound recording, a hissy disc may have a full frequency range to the limits of human hearing (say 16kHz), but if we apply a high-frequency filter when we play it, the hiss is reduced. In fact, if the filter is set to 8kHz, so that the frequency range is halved, the hiss will also be halved in power. We can therefore trade frequency range against background noise. Of course, there may be other parameters which are not covered by the power-bandwidth formula - such as speed constancy - but because it’s a fundamental limitation, we must always consider it first. It is true that in Chapter 3 we may learn about a potential process for breaching the background-noise barrier without touching the wanted sound; but that process is not yet available, and in any case we must always consider the matter “downstream” of us. In objective terms, there’s no way round it. (For further details, see Box 2.3).

The first strategic point about copying analogue sound recordings is therefore to minimise the loss of power-bandwidth product caused by the copying process. If the hissy disc mentioned above were copied to another hissy disc with the same performance, the hiss would be doubled, and we would irrevocably lose half the power-bandwidth product of the original. An archive should therefore copy analogue recordings to another medium which has a much greater power-bandwidth product, to minimise the inherent losses.

BOX 2.3
THE POWER-BANDWIDTH PRODUCT IN AUDIO RECORDINGS

This box is aimed at engineers. It is relatively easy to assess the information-carrying capacity of analogue devices such as transformers, landlines, and satellites. They tend to have flat responses within the passband, Gaussian noise characteristics, and clear overload points. But sound recordings generally do not have these features, so I must explain how we might quantify the power-bandwidth product of sound recordings.

Analogue media overload “gently” - the distortion gradually gets worse as the signal volume increases. So we must make an arbitrary definition of “overload.” In professional analogue audio circles, two percent total harmonic distortion was generally assumed. As this is realistic for most of the analogue media we shall be considering, I propose to stick to this.

For electronic devices, the bandwidth is conventionally assumed to be the points where the frequency response has fallen to half-power. This is distinctly misleading for sound recordings, which often have very uneven responses; the unevenness frequently exceeds a factor of two. There is another complication as well (see section 2.4). For my purposes, I propose to alter my definition of “bandwidth” to mean the point at which the signal is equal to random (Gaussian) noise - a much wider definition. Yet this is not unrealistic, because random noise is in principle unpredictable, so we can never neutralise it. We can only circumvent it by relying upon psychoacoustics or, for particular combinations of circumstances (as we shall see in Chapter 3). Thus random noise tends to form the baseline beyond which we cannot go without introducing subjectivism, so this definition has the advantage that it also forms the limit to what is objectively possible.

But most recording media do not have Gaussian noise characteristics. After we have eliminated the predictable components of noise, even their random noise varies with frequency in a non-Gaussian way. We must perform a spectral analysis of the medium to quantify how the noise varies with frequency. And because we can (in principle) equalise frequency-response errors (causing an analogous alteration to the noise spectrum), the difference between the recorded frequency-response and the noise-spectrum is what we should measure.

The human ear’s perception of both frequencies and sound-power is a “logarithmic” one. Thus, every time a frequency is doubled, the interval sounds the same (an “octave”), and every time the sound power increases by three decibels the subjective effect of the increase is also very similar to other three-decibel increases. Following the way analogue sound engineers work, my assessment of the power-bandwidth product of an analogue sound recording is therefore to plot the frequency response at the 2% harmonic-distortion level, and the noise spectrum, on a log-log graph; and measure the AREA between the two curves. The bigger the area, the more information the recording holds.

2.4 Restricting the bandwidth

With an older record, we may be tempted to say “There’s nothing above 8 kiloHertz, so we can copy it with the hiss filtered off without losing any of the wanted sound, and make it more pleasant to listen to at the same time.” This is a very common attitude, and I want to take some space to demolish the idea, because it is definitely wrong for archival copying, although it might be justifiable for exploitation work.

The first point is that if it ever becomes possible to restore the frequencies above 8kHz somehow, three considerations will make it more difficult for our successors. First, the copy will add its own hiss above 8kHz, perhaps much fainter than that of the original; but when the original high frequencies are further attenuated by the filter, the wanted signal will be drowned more efficiently. Secondly, by making the high frequencies weaker, we shall make it much more difficult for our successors to assess and pursue what little there is. Thirdly, we actually have means for restoring some of the missing frequencies now - imperfectly and subjectively, it is true; but to eliminate such sounds with filters is an act of destruction exactly analogous to theft, erasure, or wear-and-tear.

The second point is, how do we “know” the “fact” that there is “nothing” above 8 kiloHertz? Actually, there is no known method for cutting all sounds off above (or below) a fixed frequency. Whether the effect is done acoustically, mechanically, or electronically, all such systems have a slope in their frequency responses. A disc-recording cutterhead, for example, may work up to 8kHz, and above that its response will slope away at twelve decibels per octave, so that at 16kHz the cutter will be recording twelve decibels less efficiently. So it is never true to say there’s nothing above 8kHz. In a well-matched system, the performance of the microphone, the amplifier, and the cutterhead will be very similar, so the overall result might be a slope of as much as 36 decibels per octave; but this hardly ever seems to happen. Certainly, experiments have shown that there is audible information above the “official” limit. Often it is highly distorted and difficult to amplify without blowing up the loudspeaker with hiss, but it’s there all right. The question then becomes “how do we go about making the high frequencies more audible”, rather than “where do we cut them off.”

I regret having to labour this point, but a very respected digital sound expert once fell into this trap. He did a computer analysis of the energy of an acoustic recording at different frequencies, and observed that noise dominated above 4kHz, so ruthlessly cut those frequencies off. In his published paper he devoted some puzzled paragraphs to why experienced listeners found the resulting recordings muffled. The reason, of course, is that (using psychoacoustics) human beings can hear many sounds when they are twenty or thirty decibels fainter than noise.

The only possible justification for filtering is if the subsequent recording medium is about to overload. Fortunately digital media are relatively immune from such problems, but it is a perpetual problem with analogue copy media.

In practice, this recovery of sound above an official cut-off frequency is a technique in its infancy. We can do it, but as Peter Eckersley is reported to have said, “the wider you open a window, the more muck blows in.” Practical “muck” comprises both background-noise and distortion-products. The techniques for removing these are in their infancy. Unless such sounds can be restored perfectly, it is probably better that we should not try. But it is equally wrong for us to throw them away. The logical compromise is to transfer the high frequencies “flat” on the “archive copy,” so future researchers will have the raw material to work on. The copy medium must therefore have suitable power-bandwidth characteristics so that it will not alter the noise or distortion of the original medium. From the present state-of-the-art, we suspect that harmonic-distortion removal will depend critically upon phase linearity; therefore the copy medium must not introduce phase distortion either, or if it does it must be documented somehow (e. g. by recording its impulse-response - see section 3.4).

2.5 Deciding priorities

Our strategy is therefore to copy the vulnerable recording to another medium which has ample power-bandwidth product so that we don’t lose very much, and not to filter the recording. Unfortunately, all media have a finite power-bandwidth product, so in fact we shall always lose something. The strategy must therefore balance the inevitable losses against the financial costs of making the copy and the likelihood of the original surviving to another day. This adds another dimension to the equation, because when analogue media degrade, their power-bandwidth product suffers (it’s usually because their background noise goes up). So, one must decide when to do one’s copying programme, depending on the power-bandwidth product of the original, its likely power-bandwidth product after future years in storage, the ability to recover power-bandwidth product at any particular point in time, and the power-bandwidth capacity of the copy medium.

Clearly we must always give top priority to media whose power-bandwidth product seems to be degrading faster than we can reproduce it. At the British Library this means wax cylinders and cellulose nitrate discs. (Acetate tapes are also vulnerable because the base-material is getting more brittle, but this does not directly affect the power-bandwidth product). There is a race against time to save these media, and the matter is not helped by two further circumstances. These media tend to be less-well documented, and it is impossible to play them without risk of damage; so the sounds must be copied before anyone can make an informed judgement on whether it is worth copying them ! Other media are less vulnerable, so we can afford to make a considered judgement about when to start copying them, and the balance will tilt as our knowledge improves. Also it is quite possible (although, in this digital age, less likely) that the technology for obtaining the maximum power-bandwidth product will improve. I am not going to talk about the present state-of-the-art here, because any such discussion will quickly go out of date; but I believe the basic principles will not change.

2.6 Getting the best original power-bandwidth product

The power-bandwidth product of an analogue recording always suffers if it is copied, so we must ensure we are working with “an original”, not a copy. Thus we need to know the provenance of the analogue record. Does an earlier generation exist elsewhere? Does a manufacturer have master-tapes or metal negatives in his vault? Do masters exist of copies donated to your archive? We find ourselves picking up political hot potatoes when we examine this aspect, but the issue must be faced.

A knowledge of sound recording history is vital here, or at least that part of sound recording history which has a bearing upon your particular archive. The ideal strategy would be to collect full lists of the holdings of originals in various collections; but an adequate substitute might take the form of a generalised statement. At the British Library Sound Archive, we have an interest in commercial records made by Britain’s leading manufacturer EMI Records Ltd. It is useful for us to know that: “The British EMI factory has disposed of the metalwork for all recordings of black-label status or below, which were deleted by the outbreak of the Second World War.” This sentence shows us the recordings we must seek and process in order to complete the collection for the nation. I advise you to collect similar statements to describe the genres you are interested in.

The strategy will also be determined by which media have the greatest power-bandwidth product, not just their mere existence. Although the metalwork mentioned in the previous paragraph is amongst the most rugged of all sound recording media, that situation isn’t always the case. From about 1940 onwards, for example, Columbia Records in the United States did their mastering on large nitrate discs in anticipation of long-playing records (“L.P.”s), which they introduced in 1948. These nitrates, if they still survive today, will be nearing the end of their useful life. Similar considerations apply to early tape. Thus a properly-planned conservation strategy will also take account of the lifetime of the “masters.”

The archive must have some idea about the existence or non-existence of such holdings, because I frankly don’t see the point of wasting time recovering the original sound from a second or third-generation copy when a version with a better power-bandwidth product exists somewhere else. The only reason might be to make service copies for use as long as the earlier generation remains inaccessible, or “partially objective” copies for reference purposes in the future (I shall explain this idea in section 2.8).

The overall strategy should be planned in such a way that, if a better version turns up, you can substitute it. There should be minimum disruption despite the obvious difficulties. Re-cataloguing must be possible to ensure the old version doesn’t get used by mistake, but documentation of the old one should not be destroyed.

With nearly all analogue media it is easy to establish the order of the generations by ear - exact provenances aren’t always essential. For example, if an analogue tape is copied onto a similar machine similarly aligned, the hiss will double, the wow-and-flutter will double, and the distortion will double. These effects are quite easy to hear so long as the two tapes are running simultaneously into a changeover switch under the control of the operator. Difficulties only occur when the two tapes are on opposite sides of the world and neither owner will allow them to be moved, or one is suspected of being a copy of the other on a medium with a better power-bandwidth product (but it isn’t certain which is the original and which is the copy), or the original has disappeared and you must choose between two different copies of the same generation.

This latter case, two or more copies of an “original,” is not uncommon. If the original has disappeared, it behoves us to choose the copy with the maximum power-bandwidth product. To put it more simply, if there are two copies available, we must choose the better one. It seems almost self-evident; but it’s a principle which is often ignored.

A further dimension is that it may be possible to combine two copies to get an even better power-bandwidth product than either of them alone, and we shall be looking at this in Chapter 3. There may be political and practical difficulties; but every effort should be put into securing several good copies before the copying session starts.

Copies manufactured in other countries may often be better quality than locally-made ones. Meanwhile, recordings made for foreign broadcasters may only survive in foreign vaults. Thus you may be kept very busy chasing versions in other countries, usually with different catalogue numbers. All this means that someone qualified to do discographical work may be kept just as busy as the actual sound operator. The two should work in close collaboration for another reason as well. Often technical factors depend on the date of the recording, or its publication-date; so the operator (or manager) may need this information before work starts.

2.7 Archive, objective, and service copies

With these considerations in mind, it now seems appropriate to address the issue of the versions we wish to make. (We considered the “three possible copies” in section 1.5) Until now, most copying has been “demand-led” - the demand from listeners and customers dictates what gets copied. While this is all right so far as it goes, the result is usually that only “service copies” are achieved, because copies are tailored to listeners’ needs with subjective and cultural factors incorporated.

In my view, a proper programme of archival copying cannot be demand-led for that reason, and the following as well. The technical standards for service copies can be less critical, so general standards are lowered; I confess I have been guilty of this myself. Service copies are often done “against the clock”, when loving care-and-attention is in short supply. And since the demand always comes from someone familiar with the subject matter, documentation tends to be less rigorously done.

Thus a programme incorporating several separate copies will take longer as well. It may be necessary to do three versions and document them. And it is advisable to have a procedure to prevent the same job being done twice.

On the other hand, there are ways to save time if a proper programme is planned. Demand-led hopping between different media with different characteristics wastes time connecting and aligning equipment, and may mean research and experiment if the plan does not confine itself to known areas. It requires “technical rehearsal time,” which I shall consider shortly. Thus it is best to allocate at least a full working day specifically to archival copying without risk of interruption, and during that time a slab of technically-similar technically-understood work should be tackled.

There are many cases in which the various copy versions may be combined. If a disc record is so good that modern technology can do nothing to improve the sound, then the objective and service copies might as well be identical. Many professionally-made tapes can be copied to fill all three roles.

The overall strategy must always be capable of giving predictable results. If two different operators do the same job with different equipment, there should be no audible difference between their two “archive copies” and their two “objective copies”. This implies that the operators should be supported by technical staff ensuring that all the equipment operates to international standards. A programme of routine measurement of equipment is essential, and if a machine is discovered to have been operated in a misaligned state, all the work done by that machine in the meantime should be checked and, if necessary, re-done. I shall not impose my ideas of the tolerances needed in such measurements, as standards are bound to rise with time; but managers must ensure such checks take place at frequent intervals.

Top-of-the-range copying facilities have high capital costs. These might be diluted by arranging a shift-system, so the equipment is in constant use. Alternatively, one shift might be doing exploitation work while another is doing strict archival work and a third is doing routine maintenance.

2.8 “Partially objective” copies

Sometimes the maximum power-bandwidth product exists on a source without proper documentation, so we are not sure if it qualifies as an “objective copy” or not. This quite often happens when a record manufacturer has had privileged access to metal-parts or vinyl pressings or master-tapes. He may have used them specifically for a modern reissue, but given the reissue subjective treatment using undocumented trade secrets, so we cannot reverse-engineer it to get an objective copy. However, even if we have a poor “original,” we can use it to see whether the reissue qualifies as an “objective copy” or not. I call this the “partially objective” copy. Straightforward comparison with a changeover switch is usually sufficient to determine whether the new version is “objective.” If the manufacturer hasn’t added irreversible effects, we may even be able to re-equalise or alter the speed of his version to match the original, and achieve a better end-result. To assess the re-equalisation objectively, we may need to compare the two versions with a spectrum analyser or use a Thorn-EMI “Aquaid” System (Ref. 2). All this underlines the need for rigorous discographical research before the session.

The Power-Bandwidth Principle shows quite unambiguously the advantages of not copying a recording if we don’t have to. Furthermore, an analogue original will always contain a certain amount of extra information hidden in it which may become available to future technologists. It will often be lost if we copy the original, even if we use the best technology we have. The late talented engineer Michael Gerzon (Ref. 3) claimed that over 99% of the information may be thrown away. Personally I consider this an overestimate; but I could agree to its being in the order of 25%. The difference may partly be because we have different definitions of the word “information.” But, either way, Gerzon’s message agrees with mine - KEEP THE ORIGINALS.

A final point is that it goes without saying that the facilities for cleaning originals, and otherwise restoring them to a ready-to-play state, must be provided (see Appendix 1).

2.9 Documentation strategy

I will not dwell upon the well-known empirical rule, confirmed upon numerous occasions, that it takes at least twice as long to document a recording as it does to play it. Note that I’m only talking about documenting the recorded contents now, not the technical features!

Personally, I am rather attracted by the idea that there should be a version of the documentation on the copy itself. This means that as long as the recording survives, so does the documentation, and the two can never be separated. In olden times this was achieved by a spoken announcement, and there is much to be said for this technique; as long as the copy is playable, it can be identified. For really long term purposes I consider such an announcement should be made by an expert speaker, since questions of pronunciation will arise in future years.

On the other hand, a spoken announcement isn’t “machine-readable.” With a digital copy the documentation might be stored as ASCII text, or as a facsimile of a written document; but as yet I have no practical experience of these techniques. There are (unfortunately) several “standard” proposals for storing such “metadata” in digital form. And the end of Section 3.7 will warn you of potential problems with this idea.

As we proceed through this manual, we shall see that technical documentation could also become very complex, and the strategy for your archive will largely depend on what has gone before. My former employer, the BBC, always had a paper “recording report” accompanying every radio recording. It was useless without one, because it had to have the producer’s signature to say it was ready for transmission before the network engineer would transmit it. But my current employer, the British Library Sound Archive, does not have such a system. It’s probably too late to introduce it, because the idea of a recording-report can only work if alarm-bells ring in its absence.

By adding technical documentation, I don’t wish it to take even longer to document a recording. This is especially important, because a technical report can only be completed by technical staff, and if both the operators and the equipment are unproductive while this goes on, it is very expensive. My suggested alternative is to establish a standard set of procedures, called something simple like “X1”, and simply write down “Copied to procedure X1” followed by the operator’s signature. (I consider the signature is important, since only the operator can certify that the copy is a faithful representation of the original).

By adding technical documentation, I don’t wish it to take even longer to document a recording. This is especially important, because a technical report can only be completed by technical staff, and if both the operators and the equipment are unproductive while this goes on, it is very expensive. My suggested alternative is to establish a standard set of procedures, called something simple like “X1”, and simply write down “Copied to procedure X1” followed by the operator’s signature. (I consider the signature is important, since only the operator can certify that the copy is a faithful representation of the original).

This implies that “Procedure X1” is documented somewhere else, and here we must face the possibility that it may become lost. This actually happened to both my employers. When I was in the BBC the change from CCIR to IEC tape-recording equalisation at 19cm/sec took place (section 7.8), and was implemented immediately on quarter-inch tape; but not immediately on 16mm sepmag film, which happens to run at the same speed. For the next year I got complaints that my films sounded “woolly” on transmission, despite rigorous calibration of the equipment and my mixing the sound with more and more treble. When the truth dawned I was very angry; the problem (and damage to my reputation) could have been avoided by proper documentation. I was determined this should not happen when I joined the Sound Archive. Unhappily, a new director was once appointed who decided there was too much paperwork about, and scrapped most of it. The result is that, to this day, we do not know how-and-when the change to IEC equalisation took place at the Archive, so we often cannot do objective copies.

The two problems of “technical rehearsal” and “time to do the documentation” might both be solved by a system of rehearsals before the transfers actually take place. Thus, a working day might consist of the operator and the discographer working together in a low-tech area to decide on such things as playing-speeds, the best copy or copies to transfer, and whether alternatives are the same or not. The catalogue-entry can be started at the same time, and perhaps spoken announcements can be pre-recorded. There is also time to research anything which proves to need investigation. The actual “high-tech” transfers could then take place at a later date with much greater efficiency.

The advantages and disadvantages of converting analogue sounds to digital are the subject of the next chapter. We shall learn that there are some processes which should always be carried out in the analogue domain - speed-setting, for example - and some best carried out in the digital domain - various types of noise reduction, for example. Thus the overall strategy must take the two technologies into account, so that the appropriate processes happen in the right order. Also digital recordings are often inconvenient for “service copies.” It may be necessary to put service-copies onto analogue media to make it easier to find excerpts, or because analogue machinery is more familiar to users.

This writer happens to believe that the analogue and digital processes should be carried out by the same people as far as possible. Up till now, digital signal processing has been rather expensive, and has tended to be farmed out to bureau services as resources permit. Not only are there communications problems and unpredictable delays which inhibit quality-checking, but a vital feedback loop - of trying something and seeing how it sounds - is broken. The overall strategy should keep the analogue and digital processes as close together as possible, although the special skills of individuals on one side of the fence or the other should not be diluted.

2.10 Absolute phase

The next few chapters of this manual will outline the different techniques for copying sounds, so I shall not deal with them here. But there are three considerations which affect all the techniques, and this is the only logical place to discuss them.

At various times in history, there have been debates whether a phenomenon called “absolute phase” is significant. Natural sounds consist of alternating sound pressures and rarefactions. It is argued that positive pressures should be provided at the listener’s ear when positive pressures occurred at the original location, and not replaced with rarefactions. Many experienced listeners claim that when this is done correctly, the recording sounds much more satisfactory than when the phases are reversed; others claim they cannot hear any difference. I freely admit I am in the latter category; but I can see that the advantages of “absolute phase” could well exist for some people, so I should advise the sound archivist to bear this in mind and ensure all his equipment is fitted with irreversible connectors and tested to ensure absolute phase is preserved.

Since the earliest days of electrical recording, the effect has been so subtle that most equipment has been connected in essentially random ways. Furthermore, bi-directional microphones cannot have “absolute phase,” because the absolute phases of artists on the two opposite sides of the microphone are inherently dissimilar. But this doesn’t apply to acoustic recordings. As sound-pressures travelled down the horn, they resulted in the groove deviating from its path towards the edge of a lateral-cut disc record and going less deep on a hill-and-dale master-recording (due to the lever mechanisms between diaphragms and cutters). Thus, we actually know the absolute phase of most acoustic recordings - those which have never been copied, anyway. I therefore suggest that the copying strategy for all discs and cylinders should follow the convention that movements towards the disc edge for a lateral stylus and upwards movements for a hill-and-dale stylus should result in positive increases in the value of the digital bits. This won’t mean that absolute phase is preserved in all electrical recordings, because electrical recording components were connected in random ways; but it will ensure that this aspect of the originals is preserved on the copies, whether it ever proves to be critical or not.

The English Decca Record Company has recently adopted the standard that positive-going pressures at the microphone should be represented by positive-going digits in a digital recording, and this idea is currently under discussion for an AES Standard. It seems so sensible that I advise everyone else to adopt the same procedure when planning a new installation. There is also a convention for analogue tape (Ref. 4). But virtually no-one has used it, and there is no engineering reason why the absolute phase of a tape-recording should be preserved on a copy when it is only a subjective judgement. Yet because there is a standard, archives should follow it.

2.11 Relative phase

It is even possible to enlarge upon the above ideal, and insist on correct “relative phase” as well. Please allow me to explain this, even though we shan’t encounter the problem very often. Practical recording equipment (both analogue and digital) introduces relative phase shifts between different components of the same sound, which may occur due to acoustic effects, mechanical effects, or electronic effects. Any piece of equipment which “rolls off” the extreme high frequencies, for example, also delays them with respect to the low frequencies - admittedly by not very much, half a cycle at most. Since this happens every time we shout through a wall (for example), our ears have evolved to ignore this type of delay.

Many years ago at my Engineering Training School, our class was given a demonstration which was supposed to prove we couldn’t hear relative phase distortion. The test-generator comprised eight gearwheels on a common axle. The first had 100 teeth, the second 200, the third 300, etc. As the axle rotated, eight pickup coils detected each tooth as it passed. The eight outputs were mixed together, displayed on an oscilloscope, and reproduced on a loudspeaker. The pickup coils could be moved slightly in relation to the gearwheels. As this was done, the relative phases of the components changed, and the waveform displayed on the oscilloscope changed radically. The sound heard from the loudspeaker wasn’t supposed to change; but of course there was one sceptic in our class who insisted it did, and when the class had officially finished, we spent some time in the lab blind-testing him - the result was that he could indeed hear a difference.

But I don’t mention this for the small proportion of listeners who can hear a difference. I mention it because the elimination of overload distortion may depend critically upon the correct reproduction of “relative phase.” So I shall be insisting on reproduction techniques which have this feature, and on using originals (since we usually don’t know the relative-phase characteristics of any equipment making copies).

2.12 Scale distortion

The third consideration has had several names over the years. The controversy flared up most brightly in the early 1960s, when it was called “scale distortion”. It arises from the fact that we almost never hear a sound recording at the same volume as the original sounds. Various psychoacoustic factors come into this, which I won’t expound now, but which may be imagined by knowledgeable readers when I mention the “Fletcher-Munson Curves.” Where does the controversy arise? Because it is not clear what we should do when the sounds are reproduced at the wrong volume. I think everyone agrees that in the ideal world we should reproduce the original volume. The trouble for archivists is that we do not usually have objective knowledge of what this original volume was. A standard sound-calibration would be needed at every location, and this would have to be included on every recording. Such calibrations do occasionally appear on recordings of industrial noises or historic musical instruments, but they are the exception rather than the rule. Yet every time we have even a tiny scrap of such information we should preserve it.

The acoustic-recording system is again a case where this applies. It was not possible to alter the sensitivity of an acoustic recording machine during a “take”, so it would be silly to transfer such a recording without including a calibration signal to link the original waveform with the transferred version. And I would point out that many early commercial electrically-recorded discs were subject to “wear tests” before being approved for publication. At least one studio kept documentary evidence of the settings of their equipment, in case they were forced to retake an item because a test-pressing wore out (Section 6.19). Thus we do have an indication of how loud the performance was, although we have not yet learnt how to interpret this information.

2.13 Conclusion

Unfortunately, in the real world, procedures cannot be perfect, and ad-hoc decisions frequently have to be made. In the remainder of this manual, we shall see there are many areas for which technical information is incomplete. We must avoid making any objective copies unless we have the technology and the knowledge to do them. There must also be a deliberate and carefully-constructed policy of what to do in less-than-ideal circumstances. For example, in today’s world with very restricted facilities, which should have maximum priority: vulnerable media, media in maximum demand, or media which could result in further knowledge? What should be the policy if the archive cannot get hold of a good copy of a record? What should be the policy if appropriate technology is not available? And is this decision affected when fundamental engineering theory tells us such technology is always impossible?

Fortunately, there’s more than enough work for my recommendations to be implemented immediately. We do not have to wait for answers to those questions!

REFERENCES

  • 1: Mark Buchanan, “Beyond reality” (article), London: New Scientist Vol. 157 No. 2125 (14th March 1998), pp. 27-30. A more general article is: Robert Matthews, “I Is The Law” (article), London: New Scientist Vol. 161 No. 2171 (30th January 1999), pp. 24-28.
  • 2: Richard Clemow, “Computerised tape testing” (article), London: One to One (magazine) No. 53 (September 1994), pp. 67-75.
  • 3: Michael Gerzon, “Don’t Destroy The Archives!”. A technical report, hitherto unpublished, dated 14th December 1992.
  • 4: Lipshitz and Vanderkooy, “Polarity Calibration Tape (Issue 2)” (article), Journal of the Audio Engineering Society Vol. 29 Nos. 7/8 (July/August 1981), pp. 528-530.


- Werbung Dezent -
Zurück zur Startseite © 2007/2020 - Deutsches Hifi-Museum Wiesbaden - Copyright by Dipl. Ing. Gert Redlich - DSGVO - Privatsphäre - Zum Telefon der Redaktion - Zum Flohmarkt
Bitte einfach nur lächeln: Diese Seiten sind garantiert RDE / IPW zertifiziert und für Leser von 5 bis 108 Jahren freigegeben - kostenlos natürlich.

Privatsphäre : Auf unseren Seiten werden keine Informationen an google, twitter, facebook oder andere US-Konzerne weitergegeben.