Pro Logic - Lexicon CP-3 Plus Supplementary Manual

PLUS
CP-3 Theory
and Design

Pro Logic

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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High frequency attenuation is provided via the variable Treble Rolloff
parameter and the Academy Filter On/Off parameter, which recreates the
proper tonal balance of older, monaural films which were recorded with a
much narrower and brighter frequency response than current films.
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 called Lt and Rt (left total
and right total) 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.
(It's actually a little more complex than that, but the the end result is
essentially the same for this discussion).
The center channel contains the most energy, including the dialog; music is
normally mixed so that it appears to come from the front, with reverberation
or ambience coming from the surrounds. The surround channel ambient
information is a powerful tool for the film sound mixer. It is this information
that helps us believe that the scene in the film is real. 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 almost always
a 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 basic 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.
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