This manual gives details of some of the techniques to be followed when old sound recordings are transferred to more modern carriers. It is aimed primarily at the professional archivist transferring them to conserve them, and it is generally assumed that the purpose is TO PRESERVE THE ORIGINAL SOUND. (That is what I understand to be the function of a “National Sound Archive”, rather than preservation of the artefacts; that would be the function of a “Museum.” Please feel free to disagree with me, though!)

Here is one disagreement I myself accept. In many cases the work of people behind the scenes is just as important as that of the performer, for example in editing defective sections of a performance; so this must often be modified to read “to preserve the original intended sound.” I would enlarge this in two ways. When it comes to the subject matter, it must surely mean “intended by the producer of the recording” (or the film or the broadcast), although this will become rather a subjective judgement. And when it comes to technical matters, it must mean “Intended by the Sound Engineer.” Hence this manual!

I also need to define the word “sound”. Do we mean a psychoacoustic sensation, or objective variations in pressure at the ear(s)? In other words, when a tree fell in a prehistoric forest before animals evolved ears, did it make a “sound” or not? In this manual I use the second definition. It even seems possible that some form of genetic engineering may enable us to develop better ears (and brains to perceive the results, plus ways of storing and reproducing nerve pulses from the ears) in future. But the objective nature of sound pressures is what a sound archivist can (and must) preserve at present.

The arrangement of the book is as follows. After two chapters on overall copying strategy and the conversion of analogue sound to digital, we have five chapters on the techniques for getting the sound accurately from various analogue media. Each has some history and some scientific facts, and shows how we may use this knowledge to help us get back to the original sound today. The section on setting playing speeds, for example, covers both objective and subjective techniques, and contains a summary of our objective knowledge for reference purposes.

Next come three chapters for special techniques to ensure an old recording is heard the way the engineers originally intended. One deals with noise reduction systems, the second where spatial effects occur (e.g. stereo), and the third where the original dynamic range of a compressed recording might be recovered. These are all problems of reproduction, rather than of recording.

There are vast areas where we do not have objective knowledge, and we must rely upon future developments or discoveries. So I have left the discussion of acoustic recording techniques until this point. (I define “acoustic recordings” as “sound recordings made without the assistance of electronic amplification”). Besides our shortage of objective knowledge, the whole subject is much more complicated, and we must use previous vocabulary to express what little we know. Although large amounts of research are under way as I write, I can only indicate what I consider to be an appropriate strategy for a sound archive, which will give better fidelity for listeners until ideal technology becomes available.

In many other cases, we are comparatively close to our goal of restoring the original sound (or the original intended sound). We now have techniques former engineers could never have dreamt of, and even acoustic recordings will probably succumb to such progress. This brings three new difficulties.

In a recent broadcast (“Sunday Feature: Settling the Score” BBC Radio 3, Sunday 4th July 1999), the presenter Samuel West described how he had taken a (modern) performance of a solo piano piece, and deliberately distorted it until it sounded like an acoustic recording. He then played the two versions to a number of musically literate listeners. Not only were all the listeners fooled, but they put quite different artistic interpretations on their two responses, even though the actual performance was the same. This seems to show modern day listeners may have quite different artistic responses to historic records, if progress in sound restoration continues!

However, the programme then went on to outline the second of the three difficulties - the compromises forced upon performers by obsolete recording techniques. So my last chapter is an account of some techniques of former sound recording engineers, so you may judge how they balanced scientific and aesthetic thought processes, and understand some of the differences between the original sound and the intended original sound.

The third of the three difficulties is that sound recording is now becoming subservient to other media, because it is comparatively cheap and easy, and less of a self-contained discipline. Thus audio is becoming centred on applications, rather than technologies. All this means my final chapter will go out of date much faster than the rest of the book.

Considerable research is needed to understand, and therefore to compensate for, the accidental distortions of old sound recording systems. This manual consists largely of the fruits of my research. Although the research has been extensive (and quite a lot of it is published for the first time), I do not intend this manual to fill the role of a formal historical paper. It is aimed at the operator with his hands on the controls, not the qualified engineer at his drawing board, nor the artist in his garret, nor the academic student of history.

So I have tried to describe technical matters in words, rather than using circuit diagrams or mathematical formulae. Some professional engineers will (correctly) criticise me for oversimplifying or using “subjective language.” To these people, I plead guilty; but I hope the actual results of what I say will not be in error.

I have standardised upon some vocabulary which might otherwise cause confusion. In particular, what do we call the people who operated the original sound recording equipment, and what do we call the people restoring the sound today? I have adopted an arbitrary policy of calling the former “engineers” and the latter “operators.” I apologise if this upsets or misrepresents anyone, but there is no universally accepted phraseology. The English language also lacks suitable pronouns for a single person who may be of either sex, so again I apologise if male pronouns suggest I am ignoring the ladies.

This manual includes a certain amount of sound recording history. I shall have to look at some aspects in great detail, because they have not been considered elsewhere; but to reduce unnecessary explanation, I have had to assume a certain level of historical knowledge on the part of the reader. For analogue media, that level is given by Roland Gelatt’s book The Fabulous Phonograph in its 1977 edition. If you want to get involved in reproducing the original sound and your knowledge of sound recording history isn’t up to it, I strongly recommend you to digest that book first.

Psychoacoustics plays a large part, because recording engineers have intuitively used psychoacoustic tricks in their work. They have always been much easier to do than to describe, so the word “psychoacoustic” appears in most of my chapters! But it is very difficult to describe the tricks in scientific language. Furthermore, our present day knowledge is accumulated from long lines of scientific workers following in each other’s footsteps - there are very few seminal papers in psychoacoustics, and new discoveries continue to be made. So I have not given references to such research, but I recommend another book if you’re interested in this aspect: An Introduction to the Psychology of Hearing by Brian C. Moore. However, you do not have to read that book before this one.

I must also explain that human beings are not born with the ability to hear. They have to learn it in the first eighteen months of their lives. For example, as they lie wriggling in their prams, Grandma might shake a rattle to get their attention. At first the child would not only be ignorant of the sound, but would lack the coordination of his other senses. Eventually he would turn his head and see the rattle, coordinating sight and sound to gain an understanding of what rattles are. There are six or seven senses being coordinated here, the sense of sight (which in this case is three senses combining to provide stereoscopic vision - the sense of left eye versus right eye, the sense of parallax, and the sense of the irises “pulling focus”), the sense of hearing (which is stereophonic, combining the difference in times and in amplitudes at the two ears), and the sense of balance and how this changes as the muscles of the neck operate. All this has to be learnt. Individual people learn in slightly different ways, and if an individual is defective in some physiological sense, psychological compensation may occur.

All this combines to make the sense of hearing remarkably complex. It is therefore even more amazing that, in the first 100 years of sound recording history, it was possible to fool the brain into thinking a sound recording was the real thing - and to a higher standard than any of the other senses.

A further difficulty I face is that of the reader’s historical expertise. An expert can take one look at a disc record and immediately pronounce upon its age, rarity, what it will sound like, the surname of the recording engineer’s mother-in-law, etc. An expert will be able to recognise the characteristics of a record just by looking at it. Much of my material will seem redundant to experts. The restoration operators employed by a single record company also do not need such detail, since they will be specialising in recordings whose characteristics are largely constant. But there are innumerable examples of operators getting it wrong when stepping beyond the areas they know. So I consider it important for every operator to read the book at least once, just to see how things may differ elsewhere.

The most difficult part is to know which technology was used for making a particular recording. This is practically impossible to teach. A recipe book approach with dates and numbers is easy to misunderstand, while the true expert relies on the “look and feel” of a particular artefact which is impossible to describe in words. I just hope that experts will not be upset by apparent trivia; but I have made a deliberate attempt to include such details if there is no convenient alternative. I must also confess that the archivist in me wants to get unwritten facts into print while it is still possible.

Yet another problem is caused by the frequent changes in hardware preferred by sound operators. So I shall not give recipe book instructions like “Use a Shure M44 cartridge for playing 78s,” except when there are no alternatives. Instead I shall describe the principles, and leave operators to implement them with the resources they have available. It is my opinion that transferring recordings made during the last century will continue for at least the next century. No-one can predict how the hardware will evolve in that time; but I am reasonably certain the principles will not change.

Past history also shows that you can sometimes have a “low-tech” and a “high-tech” solution to the same problem. Do not assume the “high-tech” solution will always give the best results. Practical skills - in handling and listening to old media - often outweigh the best that modern technology can offer. I can even think of some disgraceful cases where professional sound restoration operators have been thrown out of trade associations or engineering societies, because they favour “low-tech” solutions. Do not allow yourself to be corrupted by such ideas. There is always one optimum solution to a technical problem, and it is up to you to choose that solution. I cannot always teach you the craftsmanship aspects by means of the printed word; but I can, and do, explain the principles of recovering sound with optimum engineering quality. This will allow you to assess your own skills, and balance them against those of others, for yourself.

I cannot consider every sound medium which has ever been invented. I have therefore concentrated upon “mainstream” media, or media which illustrate a particular technical point I wish to make. More advanced or primitive types of sound recordings have been ignored, since when you know the basic principles, you will be able to figure out how to transfer them yourself. So you will not find the first stereo experiments mentioned, or early magnetic recordings, or freakish cylinders, or media never intended to have a long shelf life (such as those for dictation machines).

Finally, I shall only be describing the techniques I consider essential for someone trying to restore the original intended sound. There are many others. People have said to me “Why don’t you mention such and such?” It is usually because I disapprove of “such and such” on principle. I shall be placing great emphasis upon ways giving the most accurate results, and I shall simply ignore the rest - otherwise the length of the book would be doubled.

I had been a professional sound engineer for a quarter of a century before I joined the British Library Sound Archive. I was the first such person to be appointed as a “Conservation Manager,” in overall charge of sound conservation strategy. And I must make it clear that I was regarded as an outsider by the library community. (“A sound expert? Working in a Library!?”)

As soon as I had cleared away the short ends of quarter-inch tape which stopped me putting my feet under my desk, I soon became aware that the “culture” of the sound engineer was not appreciated within the building. So I ask your forgiveness if this manual appears to say “You must do this” or “You should do that” with no rational explanation being given. The culture of a successful analogue sound operator appears to be learnt at his mother’s knee. It isn’t always clear whether he’s generated the rules himself, learnt them from his peers, or had it hammered into him on a formal training course. To illustrate my meaning, the very first task I witnessed my staff doing was a straight copy of a quarter-inch analogue tape. The operator never even looked at the stroboscope of the playback deck to check if it was running at the right speed! However, he did go through a prescribed procedure for checking the performance of the destination machine with tone signals; but he obviously did not understand why this was being done, he only knew who to ask if he got the wrong figures on the meter. All this would be second nature to a professional sound operator from his mother’s knee. So I apologise again if I keep stating what I consider to be obvious.

A broader problem is this. I am the first to recognise that “restoring the original intended sound” may not be the motivation for all transfer operators. The success of Robert Parker in making old recordings accessible to the modern public is proof of that, and he has been followed by numerous other workers bringing various corners of the recorded repertoire out of obscurity. Parker has been the subject of criticism for imposing artificial reverberation and fake stereo effects upon the original transfers. He replies that without these techniques the music would not have had such a wide appeal, and anyway he has lodged the untreated tapes in his vault. I think this is the right attitude. Even if commercial release is anticipated, I consider that recovering the “original sound” should always be the first step, whatever happens to it afterwards. Describing subjective improvements would again double the length of this manual and cause it to go out of date very rapidly (partly because of changes in fashion, and partly because of new technical processes). But I hope my remarks will also be helpful when exploitation is the driving force, rather than preservation.

Since older media often distorted the sound, it is first necessary to decide whether we should attempt to restore the sound in an undistorted form. It is often argued that the existing media should be transferred as they are, warts and all, on the grounds that better restoration technology may be available in the future. Another argument says that such warts are part of the ambiance in which such media were appreciated in the past, and should be preserved as a significant part of the artefact.

Having been a professional recording engineer myself, I challenge these views. I should not wish that the sound recordings I made before I joined the British Library Sound Archive should be reproduced “warts and all”. I should certainly demand that the ravages of time, and the undocumented but deliberate distortions (the “recording characteristics”), should always be compensated, because listeners will then get my original intended sound. So I consider it’s my responsibility to perform similar services for my predecessors. As for attempts to tidy up my work in ways which weren’t possible when I made the recordings, I hold the view that where the warts are accidental (as opposed to deliberate distortions, such as might be applied to a guitar within a pop music balance), I have no objection to their being corrected, so long as the corrections result in more faithful intended sound.

I shall now respond to the assumption of the “warts and all brigade” that future technology will be better than ours. Frankly, I am not fully convinced by this argument, because with a scientific approach we can usually quantify the effects of technology, and decide whether or not future technology can offer any improvement. I only fear technology when it doesn’t exist at all, or when it exists in the form of “trade secrets” which I cannot judge. (I shall be indicating these cases as we come to them). Rather, I fear that sound recording will become more and more “idiot-proof,” and eventually we shall forget the relationships between past artists and engineers. If we misunderstand this relationship, we are likely to misunderstand the way the recording equipment was used, and we will be unable to reproduce the sounds correctly, even with perfect technology. I shall illustrate the point with the same example I mentioned above. Enjoying popular music when I was young, I generally know which distortions were deliberate - the guitar in the pop mix - and I know which were accidental; but I must not assume these points will always be appreciated in the future. Indeed, I strongly suspect that the passage of time will make it more difficult for future operators to appreciate what is now subliminal for us. But few people appreciate these “cultural” factors. They have never been written down; but they’re there, so I shall be making some references to them in the final chapter.

I shall, however, mention one now. Recording sound to accompany pictures is a completely different business from recording sound on its own. I have spent much of my life as a film and video dubbing mixer, and I cannot think of a single case where it would be justifiable to take any of my final mixes and “restore the original sound,” even if it were possible. I would only want people to go as far as indicated above - to undo the ravages of time and equalise the known recording characteristics. All the rest of the “distortions” are deliberate - to distract from compromises made during the picture shooting process, to steer the emotional direction of the film by the addition of music and/or the pace of the mixing, to deliberately drive the dynamics of the sound to fit imperfect pictures, etc. In these circumstances pictures are dominant while sound is subservient - the sound only helps to convey a film’s message. (Films of musical performances seem to be the principal exception).

Most people find their visual sense is stronger than their aural sense, even though sound recording has achieved a higher degree of “fidelity” than moving pictures. Thus films and videos become art-forms with rules of their own, built into them at the time they went through “post-production.” When we do want to restore the “original sound,” rather than the original intended sound, we should clearly divorce the sound from the pictures, and use “rushes” or other raw material in an unmixed state rather than the final mix.

Finally, I should like to mention that some workers have argued that old recordings should be played on old equipment, so we would hear them the way contemporary engineers intended. I have a certain amount of sympathy with this view, although it does not agree with my own opinion. I would prefer my recordings to be played on state-of-the-art equipment, not what I had thirty years ago! But if we wish to pursue this avenue, we meet other difficulties. The principal one is that we have very few accounts of the hardware actually used by contemporary engineers, so we don’t actually know what is “right” for the way they worked.

Even if we did have this knowledge, we would have to maintain the preserved equipment to contemporary standards. There was a great deal of craftsmanship and taste involved in this, which cannot be maintained by recipe book methods. Next we would need an enormous collection of such equipment, possibly one piece for every half decade and every format, to satisfy any legitimate historical demand for sound the way the original workers heard it. And we would inevitably cause a lot of wear and tear to our collection of original recordings, as we do not have satisfactory ways of making modern replicas of original records.

But it so happens that we can have our cake and eat it. If we transfer the sound electrically using precise objective techniques, we can recreate the sound of that record being played on any reproducing machine at a subsequent date. For example, we could drive its amplifier from our replayed copy, its soundbox from a motional feedback transducer, or its aerial from an RF modulator.

I shall first state what I believe to be an extremely important principle. I believe the goal of the present-day restoration expert should be to compensate for the known deficiencies objectively. He should not start by playing the recording and twiddling the knobs subjectively. He should have the courtesy first to reproduce the sound with all known objective parameters compensated. For archival purposes, this could be the end of the matter; but it may happen that some minor deficiencies remain which were not apparent (or curable) to contemporary engineers, and these can next be undone. In any event, I personally think that only when the known objective parameters have been compensated does anyone have the moral right to fiddle subjectively - whether in an archive, or for exploitation.

The aim of objectivity implies that we should measure what we are doing. In fact, considerable engineering work may be needed to ensure all the apparatus is performing to specification. I know this goes against the grain for some people, who take the view that “the ear should be the final arbiter.” My view is that of course the ear should be the final arbiter. But, even as a professional recording engineer deeply concerned with artistic effects, I maintain that measurements should come first. “Understanding comes from measurement” as physical scientists say; if we can measure something’s wrong, then clearly it is wrong.

On numerous occasions, history has shown that listeners have perceived something wrong before the techniques for measuring it were developed; this is bound to continue. Unfortunately, “golden-eared” listeners are frequently people who are technically illiterate, unable to describe the problem in terms an engineer would understand. My personal view (which you are always free to reject if you wish), is that measurements come first; then proper statistically-based double-blind trials with “golden-eared” listeners to establish there is a valid basis for complaining about problems; then only when this has been done can we reasonably research ways to cure the problem. I certainly do not wish to discourage you from careful listening; but accurate sound reproduction must at the very least begin with equipment whose performance measures correctly.

On the other hand, the ear is also important in a rather coarse sense - to get us back on the right track if we are catastrophically wrong. For example, if the tape box label says the tape runs at 15 inches per second and the tape sounds as if it’s at double speed, then it will probably be a fault in the documentation, not a fault in our ears!

For intermediate cases, we should be able to justify subjective decisions in objective terms. For example, if we switch the tape reproducer to 7.5 inches per second and we perceive music at slightly the wrong pitch, then we should proceed as follows. First we check our own sense of pitch with a known frequency source properly calibrated. Then we quantify the error and we seek explanations. (Was it an unreliable tape recorder? or an historic musical instrument?) If we cannot find an explanation, we then seek confirmatory evidence. (Is the background hum similarly pitch-shifted? Does the tape play for the correct duration?) But, at the end of the day, if there is no objective explanation, a sound archive must transfer the tape so that at least one copy is exactly like the original, regardless of the evidence of our senses.

The question then arises, which subjective compensations should be done in the environment of a sound archive? A strictly scientific approach might suggest that no such compensations should ever be considered. But most professional audio operators are recruited from a background which includes both the arts and the sciences. It is my personal belief that this is only to the good, because if these elements are correctly balanced, one doesn’t dominate over the other. But it is impossible for anyone’s artistic expertise to stretch across the whole range of recorded sound. It may be necessary to restrict the artistic involvement of an operator, depending upon the breadth of his knowledge. To be brutal about it, a pop expert may know about the deliberately distorted guitar, whereas an expert in languages may not.

This assertion has not answered the potential criticism that the artistic input should ideally be zero. I shall counter that argument by way of an example. For the past thirty years Britain has had an expert in restoring jazz and certain types of dance music in the person of John R. T. Davies. I do not think he will mind if I say that his scientific knowledge is not particularly great; but as he played in the Temperance Seven, and has collected old records of his own particular genre ever since he was a boy, his knowledge of what such music should sound like has been his greatest asset. Long before the present scientific knowledge came to be formulated, he had deduced it by ear. He therefore systematically acquired the hardware he needed to eliminate the various distortions he perceived, and although the methods he evolved appear a little odd to someone like me with a scientific background, he ends up with very accurate results. John Davies does not claim to be perfect, but he holds the position that his particular musical knowledge prevents him making silly mistakes of the type which might befall a zombie operator.

I therefore accept that a certain level of artistic input is advantageous, if only to insure against some of the howlers of a zombie operator. But my position on the matter is that each individual must recognise his own limited knowledge, and never to go beyond it. We shall be encountering cases in the following pages where specialist artistic knowledge is vital. When such knowledge comes from an artistic expert, I consider it is no less reliable than pure scientific knowledge; but I would feel entitled to query it if I wasn’t certain the knowledge was “right”. Such knowledge may not just be about the original performance. It may also be knowledge of the relationship between the artist and the contemporary recording engineer. It is not always realised that the sounds may have been modified as they were being created, for very good reasons at the time.

The contradictions which can arise between “technical” and cultural” factors have caused passionate debate within the British Library Sound Archive. Indeed, the writer once addressed an external public meeting on the subject, and the audience nearly came to blows. There is often no satisfactory compromise which can be taken between the contradictions. I propose to leave some of the considerations until we get to the final chapter; but in the meantime the best answer seems to be a “quadruple conservation” strategy. This would mean the archive might end up with as many as four versions of a recording for long term storage, although two or more might often be combined into one.

(1) The original, kept for as long as it lasts.
(2) A copy with warts-and-all, sounding as much like the original artefact as possible, which I shall call “The Archive Copy.”
(3) A copy with all known objective parameters compensated, which I shall call “The Objective Copy.”
(4) A copy with all known subjective and cultural parameters compensated, which I shall call “The Service Copy.”

I recognise that such an ambitious policy may not always be possible. As we reach critical decision points during the course of this manual, I shall be giving my personal recommendations; but I am writing from the point of view of a professional sound recordist in a publicly funded national archive. Each reader must make his own decision for himself (or his employer) once he understands the issues.

Fortunately, there are also many occasions where, even from an ivory tower viewpoint, we don’t actually need an original plus three copies. For example, when basic engineering principles tell us we have recovered the sound as well as we can, the objective and service copies might well be identical. Sometimes we do not have the knowledge to do an “objective copy”, and sometimes cultural pressures are so intense that we might never do an “archive copy.” (I shall describe an example of the latter in section 13.2). But I shall assume the quadruple conservation strategy lies behind our various attempts, and I advise my readers to remember these names as the work proceeds.

Given that the purpose of conservation copying is to “restore the original intended sound”, how do we go about this? How can we know we are doing the job with as much objectivity as possible, especially with older media made with temperamental recording machinery, or before the days of international standards? The present-day archivist has the following sources of knowledge to help him, which I list in approximate order of importance.

1. Contemporary “objective recordings.” This generally means contemporary frequency discs or tapes, together with a small handful of other engineering test media for intermodulation distortion and speed. Many large manufacturers made test recordings, if only for testing the reproducers they made, and provided they have unambiguous written documentation, we can use them to calibrate modern reproducers to give the correct result. Unfortunately, not all test records are unambiguously documented, and I shall allude to such cases as they arise. Even worse, many manufacturers did not make test recordings. Yet objective measurements are sometimes available accidentally. For instance, several workers have analysed the “white noise” of the swarf vacuum pipe near an acoustic recording diaphragm to establish the diaphragm’s resonant frequency. And we will be coming across another rather curious example in section 6.47.

2. Contemporary, or near-contemporary, objective measurements of the recording equipment. Written accounts of contemporary measurements are preferable to present-day measurements, because it is more likely the machinery was set up using contemporary methods of alignment, undecayed rubber, new valves, magnetic materials at the appropriate strength, etc. As for the measuring equipment, there are practically no cases where present-day test gear would give significantly different results from contemporary measuring equipment. As the machinery improved, so did measuring equipment; so contemporary measurements will always be of the right order of magnitude.

On the other hand, alleged objective measurements made by the equipment makers themselves should always be subject to deep suspicion (this applies particularly to microphones). This last point reminds us that there may be “specifications” for old recording equipment, besides “objective measurements.” These must be regarded with even more suspicion; but when there were international standards for sound recordings, we must at least examine how well the equipment actually conformed to the standards.

3. Present-day analysis of surviving equipment. This suffers from the disadvantages hinted at above, where the equipment isn’t necessarily in its best state. There are also the “cultural” factors; the way in which the machinery was used was often very important, and may invalidate any scientific results achieved.

4. Analysis of drawings, patents, etc. It is often possible to work out the performance of a piece of recording equipment from documentary evidence; no actual artefact or replica is required.

5. Interviews with the engineers concerned. Reminiscences of the people who operated old recording equipment often reveal objective information (and sometimes trade secrets).

6. Reverse engineering surviving recordings. In general, this is not possible for one recording alone; but if a number of different recordings made by the same organisation exhibit similar characteristics, it is possible to assume they are characteristics of the machine which made them, rather than of the performances. It is therefore possible to reverse engineer the machine and neutralise the characteristics. A similar technique occurs when we have a recording which is demonstrably a copy of another. We can then use the original to deduce the characteristics of the copy, and thence other recordings made on that equipment.

7. Automatic analysis. It is early days yet, but I mention this because mathematical analysis of digital transfers is being invoked to identify resonances in the original. One aim is to eliminate the “tinnyness” of acoustic horn recording. Identifying resonances by such an objective technique is clearly superior to the subjective approach of the next suggestion.

8. Intelligent artistic input. If it’s known that a type of recording had resonances, it may be possible to listen out for them and neutralise them by ear. This implies there is a “general structure” to the recorded characteristics, which can be neutralised by appropriate “tuning.” So in the following pages I have included a few such “general structures.” But I’ve placed this evidence last, because it’s very easy to cross the borderline between objective and subjective compensation. Although the result may be more faithful than no compensation at all, there will be no proof that the particular form of tuning is exactly correct, and this may give difficulties to subsequent generations who inherit our copies. As digital signal processing evolves, it will be feasible to do statistical analysis to determine a “level of confidence” in the results. Then, at some point (which would need a consensus among archivists), the process might even be adopted for the “objective copy”.

If we continue to assume that “recovering the intended original sound” is a vital stage of the process, this implies that we should always keep some documentation of what we have done. It might take the form of a written “recording report” or a spoken announcement. It has three functions. It advises our successors of any subjective elements in our work, enabling them to reverse engineer it if the need arises after the original has decayed away. It also shows our successors the steps we have taken ourselves and our thought processes, so later generations will not be tempted to undo or repeat our work without good reason. I must also say that I personally find the documentation useful for a third reason, although not everyone will agree with me. I find it forces me to think each case through logically.