582 Chapter 19
frequency spectrum and those that act selectively on ranges of frequencies (subbands) is
true too of digital implementations. However, digital systems have carried the principle of
subband working to a sophistication undreamed of in analogue implementations.
19.2 Intermediate Compression Systems ......................................................................
Consider the 8-bit digital values: 00001101, 00011010, 00110100, 0110100, and
- (Eight bit examples are used because the process is easier to follow but the
following principles apply in just the same way to digital audio samples of 16 bits or,
indeed, any word length.) We might just as correctly write these values thus:
00001101 1101 * 1
00011010 1101 * 10
00110100 1101 * 100
01101000 1101 * 1000
11010000 1101 * 10000If you think of the multipliers 1, 10, 100, and so on as powers of two then it’s pretty easy
to appreciate that the representation given earlier is a logarithmic description (to the log
of base two) with a 4-bit mantissa and a 3-bit exponent. So already we’ve saved 1 bit in 8
(a 20% data reduction). We’ve paid a price of course because we’ve sacrifi ced accuracy
in the larger values by truncating the mantissas to 4 bits. However, this is possible in any
case with audio because of the principle of masking, which underlies the operation of
all noise reduction systems. Put at its simplest, masking is the reason we strain to listen
to a conversation on a busy street and why we cannot hear the clock ticking when the
television set is on: loud sounds mask quiet ones. So the logarithmic representation makes
sense because resolution is maintained at low levels but sacrifi ced at high levels where the
program signal will mask the resulting, relatively small, quantization errors.
19.2.1 NICAM
Further reductions may be made because real audio signals do not change instantaneously
from very large to very small values, so the exponent value may be sent less often than
the mantissas. This is the principle behind the stereo television technique of NICAM,