Dolby Surround Digital: AC-3 in the Home
Dolby Surround Digital, also based on AC-3 coding, is the consumer
equivalent of Dolby Stereo Digital film sound, forming the final link
from multichannel program producer to home listener. Like the film
format, it provides separate channels for left, right, and center
speakers at the front; two surround speakers at the sides; and a
subwoofer at the listener's option.
Unlike analog Dolby Surround with its single band-limited surround
channel (usually played over two speakers), Dolby Surround Digital
features two completely independent surround channels, each offering the
same full range fidelity as the three front channels. As a result, true
stereo surround effects can be achieved for an expanded sense of depth,
localization, and overall realism. And because Dolby Surround Digital
maintains complete separation of the audio channels, it is as suited
to music-only recordings and broadcasts as it is to video formats. Thus
it has the potential to open up new worlds of multichannel sound
reproduction. That isn't all that Dolby Surround Digital can do.
While Dolby Stereo Digital is heard in cinemas with a full complement of
loudspeaker channels, a standardized playback level, and full dynamic
range capabilities, home listening circumstances vary markedly.
Therefore, for Dolby Surround Digital consumer formats, AC-3 has been
designed to satisfy many diverse requirements. At the outset, at least,
while some listeners will have multichannel systems, most will be
listening in mono or conventional stereo. Those with Dolby Surround
systems will want a two channel matrix encoded output from their
decoders. Many listeners may prefer a restricted dynamic range, but
others will wish to experience the full dynamic range of the original
signal. Techniques to satisfy these and other needs have been designed
in from the beginning:
     Data identifying each program's original production format-mono,
stereo, matrixed or discrete surround-can be sent to eliminate
confusion at playback or reception.
     Program material can be coded when it is originally mixed so that
subjectively constant, dialogue-keyed loudness is maintained as the
listener switches between program sources. No alteration of program
dynamics is involved, only playback volume.
     Decoders can be designed to provide optimum mix downs from
multichannel programming, such as a matrix-encoded two-track mix for
analog Dolby Surround decoding, a  conventional stereo mix, or even a
mono mix.
     When programs with wide dynamic range, such as movie soundtracks,
are played at low volume, the system can apply appropriate compression
to preserve low-level content. The degree of compression can be made to
vary according to need.
     The listener can program the Dolby Surround Digital decoder to
route non- directional low bass only to those channels in the system
which have widerange speakers or subwoofers.
     Dolby Surround Digital offers a dramatic step forward in listener
involvement and excitement. It provides program producers, directors,
recording engineers, and performers unprecedented creative
opportunities. And it offers remarkable media adaptability within a
single, far-reaching technological framework.
Copyright © 1995 Lucasfilm, Ltd.
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