MANUAL OF ANALOGUE SOUND RESTORATION ... - British Library

3.6 Digital data compression
The undoubted advantages of linear PCM as a way of storing audio waveforms are being
endangered by various types of digital data compression. The idea is to store digital sound
at lower cost, or to transmit it using less of one of our limited natural resources (the
electromagnetic spectrum).
Algorithms for digital compression are of two kinds, “lossless” and “lossy.” The
lossless ones give you back the same digits after decompression, so they do not affect the
sound. There are several processes; one of the first (Compusonics) reduced the data to
only about four-fifths the original amount, but the compression rate was fixed in the
sense that the same number of bits was recorded in a particular time. If we allow the
recording medium to vary its data-rate depending on the subject matter, data-reduction
may be two-thirds for the worst cases to one-quarter for the best. My personal view is
that these aren’t worth bothering with, unless you’re consistently in the situation where
the durations of your recordings are fractionally longer than the capacity of your storage
media.
For audio, some lossless methods actually make matters worse. Applause is
notorious for being difficult to compress; if you must use such compression, test it on a
recording of continuous applause. You may even find the size of the file increases.
But the real trouble comes from lossy systems, which can achieve compression
factors from twofold to at least twentyfold. They all rely upon psychoacoustics to permit
the digital data stream to be reduced. Two such digital sound recording formats were the
Digital Compact Cassette (DCC) and the MiniDisc, each achieving about one-fifth the
original number of bits; but in practice, quoted costs were certainly not one-fifth! While
they make acceptable noises on studio-quality recordings, it is very suspicious that no
“back-catalogue” is offered. The unpredictable nature of background noise always gives
problems, and that is precisely what we find ourselves trying to encode with analogue
sources. Applause can also degenerate into a noisy “mush”. The real reason for their
introduction was not an engineering one. Because newer digital systems were designed so
they could not clone manufactured CDs, the professional recording industry was less likely
to object to their potential for copyright abuse (a consideration we shall meet in section
3.8 below).
Other examples of digital audio compression methods are being used for other
applications. To get digital audio between the perforation-holes of 35mm optical film,
cinema surround-sound was originally coded digitally into a soundtrack with lossy
compression. Initial reports suggested it sometimes strained the technology beyond its
breaking-point. While ordinary stereo didn’t sound too bad, the extra information for the
rear-channel loudspeakers caused strange results to appear. An ethical point arises here,
which is that the sound-mixers adapted their mixing technique to suit the compressionsystem.
Therefore the sound was changed to suit the medium. (In this case, no original
sound existed in the first place, so there wasn’t any need to conserve it.)
A number of compression techniques are used for landline and satellite
communication, and here the tradeoffs are financial - it costs money to buy the powerbandwidth
product of such media. Broadcasters use digital compression a lot – NICAM
stereo and DAB have it - but this is more understandable, because there is a limited
amount of electromagnetic spectrum which we must all share, especially for consistent
reception in cars. At least we can assume that wildlife creatures or analytical machinery
won’t be listening to the radio, visiting cinemas, or driving cars.
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