Data Compression 583which stands for near instantaneous companded audio multiplex. In NICAM 782, 14-bit
samples are converted to 10-bit mantissas in blocks of 32 samples with a common 3-bit
exponent. This is an excellent and straightforward technique but it is only possible to
secure relatively small reductions in data throughput of around 30%.
19.3 Psychoacoustic Masking Systems .........................................................................
Wideband compansion systems view the phenomenon of masking very simply and rely
on the fact that program material will mask system noise. But, actually masking is a more
complex phenomenon. Essentially it operates in frequency bands and is related to the way
in which the human ear performs a mechanical Fourier analysis of the incoming acoustic
signal. It turns out that a loud sound only masks a quieter one when the louder sound
is lower in frequency than the quieter, and only then, when both signals are relatively
close in frequency. It is due to this effect that all wideband compansion systems can only
achieve relatively small gains. The more data we want to discard the more subtle must our
data reduction algorithm be in its appreciation of the human masking phenomena. These
compression systems are termed psychoacoustic systems and, as you will see, some
systems are very subtle indeed.
19.4 MPEG Layer 1 Compression (PASC) ...................................................................
It’s not stretching the truth too much to say that the failed Philips ’ digital compact
cassette (DCC) system was the fi rst nonprofessional digital audio tape format. As we
have seen, other digital audio developments had ridden on the back of video technology.
The CD rose from the ashes of Philips Laserdisc, and DAT machines use the spinning-
head tape recording technique originally developed for B and C-Format 1-inch video
machines, later exploited in U-Matic and domestic videotape recorders. To their
credit then, that, in developing the DCC, Philips chose not to follow so many other
manufacturers down the route of modifi ed video technology. Inside a DCC machine,
there’s no head wrap, no spinning head, and few moving precision parts. Until DCC,
it had taken a medium suitable for recording the complex signal of a color television
picture to store the sheer amount of information needed for a high-quality digital audio
signal. Philips ’ remarkable technological breakthrough in squeezing two high-quality,
stereo digital audio channels into a fi nal data rate of 384 kBaud was accomplished by,
quite simply, dispensing with the majority (75%) of the digital audio data! Philips named
their technique of bit-rate reduction or data-rate compression precision adaptive sub-band