Lexicon CP-1 PLUS V2.0 Owner's Manual page 60

CP-1 Digital Audio Environment Processor
Pro Logic
The CP-1 is one of a few consumer products to offer full Pro Logic Dolby
Surround decoding, and to operate entirely in the digital domain. This has
important advantages, but to understand them we must first take a brief
look at how a film soundtrack is put together.
A Dolby Stereo film sound track has four basic components: Left and right
channels, a center front channel and a surround channel. The first three are
fed to speakers arrayed behind the movie screen, while the surround sound
goes to speakers on the side and rear walls of the theater. The four channels
are recorded on separate magnetic tracks and are combined by the Dolby
Stereo matrix encoder into two stereo channels during the final mixing
process. The original left and right channels go directly onto the left and
right channels of the Dolby Stereo mix. The center channel is fed equally to
both channels, in phase, and the surround track is fed equally to both
channels, but 180 degrees out of phase.
The center channel carries the dialog; music is normally mixed so that it
appears to come from the front, with reverberation or ambience coming
from the surrounds. For special effects, music can be encoded to come from
all around the listener or even from behind. In any case, with music and
ambient effects there is always substantial spread across the front of the
loudspeaker array.
Sound effects can come from any direction around the listener and it is the
job of the decoder to duplicate as closely as possible the film mixer's
placements.
Dolby Surround Decoding
When the movie is shown the two Dolby Stereo tracks must be decoded and
separated into the original four. The Dolby Surround decoder does this in
a rather rudimentary way: it supplies a signal to the center channel which
is just the sum of the two input channels. This signal contains the dialog.
However, the left and right signals still contain dialog too, so the dialog is
spread out among the three front speakers. Similarly, the Dolby Surround
decoder takes all out-of-phase signals and sends them to the surround
speakers, while leaving the original out-of-phase components in the left
and right front speakers.
The basic Dolby Surround decoder has high channel separation between
left and right decoded audio, and between center and surround. The
separation between left or right and center, or between left or right and
surround, however, is only a few dB. The simple Dolby Surround decoder
does pretty well with music (although sometimes the center channel is too
loud) but, because any sound will be reproduced in at least three
loudspeakers, effects are smeared and often unconvincing .
Theory
Design
During the early days of film ste-
reo, dialog was sometimes mixed
(by "panning" the monaural dia-
log track) to come from the same
part of the screen as the image of the
actor. Subjective reactions to this
technique were varied, and techni-
cal problems with some magnetic
sound tracks helped to discourage
the practice, so modern movies are
seldom mixed with panned dialog.
In a home system with a good Pro
Logic decoder, however, the effect
can work quite well; recent releases
with panned dialog include "Indi-
ana Jones and the Last Crusade."
Films originally have four channels:
one for dialog and three for music
and effects. To make a Dolby Stereo
film, these are combined to two.
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