Audio Engineering

Representation of Audio Signals 489

which an apparently high-quality signal can be delivered using fewer bits. The telephone
companies were among the fi rst to employ digital compression and expansion techniques
but the technology has been used for nontelephonic audio purposes. In the A and \i law
converters ( Figure 15.30 ), the quantizing steps, q , do not have the same value. For low
signal levels the quantizing levels are closely spaced and become more widely spaced at
higher input levels. The decoder implements the matching inverse conversion.

Another approach to providing a wide coding range with the use of fewer bits than
would result if a simple linear approach were to be taken is exemplifi ed in the fl ying
comma or fl oating point type of converter. In this approach a fast converter with a limited
coding range is presented with a signal that has been adjusted in level such that most
of the converter’s coding range is used. The full output sample value is determined by
the combination of the value of the gain setting and of the sample value returned by the
converter. The problem here is that the change in gain in the gain stage is accompanied
by a change in background noise level and this too is coded. The result is that the noise
fl oor that accompanies the signal is modulated by the signal level, which produces a
result that does not meet the performance requirement for high-quality audio. A more
subtle approach is exemplifi ed in syllabic companders. The NICAM approach manages a
modest reduction from around 1 Mbs ^1 to 7.04 kbs ^1 and we see in it an early approach
to attempts to adapt the coding process to the psychoacoustics of human hearing.

0 FbFs/2 Fs 2 Fs 3 Fs

Level

0.64

Max

Frequency

Figure 15.28 : Finally the DAC output is fed to a zero order hold circuit which performs a

similar operation to the sample and hold circuit and then to a reconstruction or output

antialiasing fi lter. The plot of the spectral shape of the zero order hold shows that there are

frequency components, at decreasing levels, at harmonic intervals equal to F (^) s.