MANUAL OF ANALOGUE SOUND RESTORATION ... - British Library

3 Digital conversion of analogue sound
3.1 The advantages of digital audio
There is a primary reason why digital recordings appeal to sound archivists. Once digital
encoding has been achieved, they can in principle last for ever without degradation,
because digital recordings can in principle be copied indefinitely without suffering any
further loss of quality. This assumes: (1) the media are always copied in the digital domain
before the errors accumulate too far, (2) the errors (which can be measured) always lie
within the limits of the error-correction system (which can also be measured), and (3)
after error-correction has been achieved, the basic digits representing the sound are not
altered in any way. When a digital recording goes through such a process successfully, it is
said to be “cloned.”
Both in theory and in practice, no power-bandwidth product (Section 2.3) is lost
when cloning takes place - there can be no further loss in quality. However, it also means
the initial analogue-to-digital conversion must be done well, otherwise faults will be
propagated forever. In fact, the Compact Digital Disc (CD) has two “layers” of errorcorrection,
and (according to audio folk-culture) the format was designed to be rugged
enough to allow a hole one-sixteenth of an inch diameter (1.5mm) to be drilled through
the disc without audible side-effects.
For all these reasons, the word “digital” began to achieve mystical qualities to the
general public, many of whom evidently believe that anything “digital” must be superior!
I am afraid much of this chapter will refute that idea. It will also be necessary to
understand what happens when a recording is copied “digitally” without actually being
“cloned”.
The power-bandwidth products of most of today’s linear pulse-code modulation
media exceed most of today’s analogue media, so it seems logical to copy all analogue
recordings onto digital carriers anyway, even if the digital coding is slightly imperfect. But
we must understand the weaknesses of today's systems if we are to avoid them (thus
craftsmanship is still involved!), and we should ideally provide test-signals to document
the conversion for future generations.
If you are a digital engineer, you will say that digital pulse-code modulation is a
form of “lossless compression”, because we don’t have to double the ratio between the
powers of the background-noise and of the overload-point in order to double the powerbandwidth
product. In principle, we could just add one extra digital bit to each sample, as
we shall see in the next section. Now I am getting ahead of myself; but I mention this
because there is sometimes total lack of understanding between digital and analogue
engineers about such fundamental issues. I shall therefore start by describing these
fundamental issues very thoroughly. So I must apologise to readers on one side of the
fence or the other, for apparently stating the obvious (or the incomprehensible).
A digital recording format seems pretty idiot-proof; the data normally consists of
ones and zeros, with no room for ambiguity. But this simply isn’t the case. All digital
carriers store the digits as analogue information. The data may be represented by the size
of a pit, or the strength of a magnetic domain, or a blob of dye. All these are quantified
using analogue measurements, and error-correction is specifically intended to get around
this difficulty (so you don’t have to be measuring the size of a pit or the strength of a tiny
magnet).
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