584 Chapter 19
coding (PASC). PASC was adopted as the original audio compression scheme for MPEG
video/audio coding (layer 1).
In MPEG layer 1 or PASC audio coding, the whole audio band is divided up into 32
frequency subbands by means of a digital wave fi lter. At fi rst sight, it might seem that
this process will increase the amount of data to be handled tremendously—or by 32 times
anyway! This, in fact, is not the case because the output of the fi lter bank, for any one
frequency band, is at 1/32nd of the original sampling rate. If this sounds counterintuitive,
take a look at the Fourier transform and note that a very similar process is performed
here. Observe that when a periodic waveform is sampled n times and transformed, the
result is composed ofn frequency components. Imagine computing the transform over a
32- sample period: 32 separate calculations will yield 32 values. In other words, the data
rate is the same in the frequency domain as it is in the time domain. Actually, considering
that both describe exactly the same thing with exactly the same degree of accuracy, this
shouldn’t be surprising. Once split into subbands, sample values are expressed in terms of
a mantissa and exponent exactly as explained earlier. Audio is then grouped into discrete
time periods, and the maximum magnitude in each block is used to establish the masking
“ profi le ” at any one moment and thus predict the mantissa accuracy to which the samples
in that subband can be reduced, without their quantization errors becoming perceivable
(see Figure 19.1 ).
Despite the commercial failure of DCC, the techniques employed in PASC are indicative
of techniques now widely used in the digital audio industry. All bit-rate reduction
coders have the same basic architecture, pioneered in PASC: however, details differ. All
Masking
thresholdTone Quantization noiseAmplifierFrequency
Figure 19.1 : Subband quantization and how it relates to the masking profi le.