588 Chapter 19
implemented for the cinema fi rst, where it was called Dolby Digital. It was developed
to provide multichannel digital sound with 35-mm prints. In order to retain an analogue
track so that these prints could play in any cinema, it was decided to place the new digital
optical track between the sprocket holes, as illustrated in Figure 19.3. The physical
space limitation (rather than crude bandwidth) was thereby a key factor in defi ning its
maximum practical bit rate. Dolby Labs did a great deal of work to fi nd a channel format
that would best satisfy the requirements of theatrical fi lm presentation. They discovered
that fi ve discrete channels—left (L), right (R), center (C), left surround (LS), and right
surround (RS)—set the right balance between realism and profl igacy! To this they added
a limited (1/10th) bandwidth subwoofer channel; the resulting system being termed 5.1
channels. Dolby Digital provided Dolby Labs a unique springboard for consumer formats
for the new DTV (ATSC) systems.
Like MPEG, AC-3 divides the audio spectrum of each channel into narrow frequency
bands of different sizes optimized with respect to the frequency selectivity of human
hearing. This makes it possible to sharply fi lter coding noise so that it is forced to stay
very close in frequency to the frequency components of the audio signal being coded.
By reducing or eliminating coding noise wherever there are no audio signals to mask
it, the sound quality of the original signal can be preserved subjectively. In this key
respect, a perceptual coding system such as AC-3 is essentially a form of very selective
and powerful Dolby-A type noise reduction! Typical fi nal data-rate applications include
384 kb/s for 5.1-channel Dolby Surround Digital consumer formats and 192 kb/s for two-
channel audio distribution.
Figure 19.3 : Dolby Digital as originally coded on fi lm stock.