MANUAL OF ANALOGUE SOUND RESTORATION ... - British Library

If (say) it is desired to copy a digital PCM audio recording to a new PCM format
with a higher sampling frequency, then either the sound must be converted from digital
to analogue and back again, or it must be recalculated digitally. When the new samplingfrequency
and the old have an arithmetical greatest common factor n, then every nth
sample remains the same; but all the others must be a weighted average of the samples
either side, and this implies that the new bitstream must include “decimals.” For truthful
representation, it cannot be a stream of integers; rounding-errors (and therefore
quantisation-distortion) are bound to occur.
The subjective effect is difficult to describe, and with practical present-day systems
it is usually inaudible when done just once (even with integers). But now we have a quite
different danger, because once people realise digital conversion is possible (however
imperfect), they will ask for it again and again, and errors will accumulate through the
generations unless there is strict control by documentation. Therefore it is necessary to
outguess posterity, and choose a sampling-frequency which will not become obsolete.
For the older audio media, the writer’s present view is that 44.1kHz will last,
because of the large number of compact disc players, and new audio media (like the DCC
and MiniDisc) use 44.1kHz as well. I also consider that for the media which need urgent
conservation copying (wax cylinders, acetate-based recording tape, and “acetate” discs,
none of which have very intense high frequencies), this system results in imperceptible
losses, and the gains outweigh these. For picture media I advocate 48kHz, because that is
the rate used by digital video formats.
But there will inevitably be losses when digital sound is moved from one domain to
the other, and this will get worse if sampling-frequencies proliferate. Equipment is
becoming available which works at 96kHz, precisely double the frequency used by
television. Recordings made at 48kHz can then be copied to make the “even-numbered”
samples, while the “odd-numbered” samples become the averages of the samples either
side. The options for better anti-aliasing filters, applications such as wildlife recording,
preservation of transients (such as analogue disc clicks), transparent digital signalprocessing,
etc. remain open. Yet even this option requires that we document what has
happened - future workers cannot be expected to guess it.
Converting to a lower sampling frequency means that the recording must be
subjected to a new anti-aliasing filter. Although this filtering can be done in the digital
domain to reduce the effects of practical analogue filters and two converters, it means
throwing away some of the information of course.
The next problem is pre-emphasis. This means amplifying some of the wanted high
frequencies before they are encoded, and doing the inverse on playback. This renders the
sound less liable to quantisation distortion, because any “natural” hiss is about twelve
decibels stronger. At present there is only one standard pre-emphasis characteristic for
digital audio recording (section 7.3), so it can only be either ON or OFF. A flag is set in
the digital data-stream of standardised interconnections, so the presence of pre-emphasis
may be recognised on playback. And there is a much more powerful pre-emphasis system
(the “C.C.I.T” curve) used in telecommunications. It is optimised for 8-bit audio work,
and 8 bits were once often used by personal computers for economical sound recording.
Hopefully my readers won’t be called upon to work with CCIT pre-emphasis, because
digital sound-card designers apparently haven’t learnt that 8-bit encoders could then give
the dynamic range of professional analogue tape; but you ought to know the possibility
exists! But if the recording gets copied to change its pre-emphasis status, whether
through a digital process or an analogue link, some of the power-bandwidth product will
be lost each time. Worse still, some digital signal devices (particularly hard-disk editors)
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