Lexicon CP-1 PLUS V2.0 Owner's Manual page 58

Theory
and
Design
CP-1 Digital Audio Environment Processor
The Stereo Logic Program can provide an unusual and revealing way to
listen to music. With a multi-speaker surround system the program puts
you in the middle of the music, so that interior elements formerly buried in
the mix become audible. Try setting Front Effect and Rear Effect all the way
up; then mute the center channel by pushing System Mute to turn main and
effects signals off, then Effects Mute to turn everything but the center back
on. This allows you to hear all of the mix (except for strongly centered
material such as vocals) separated and spread throughout the room. This
method also provides an interesting look into the details of film sound
mixes.
The Movie Surround programs: Mono Logic, TV Matrix and Pro Logic, are
Movie Surround
specifically designed for film sound or for systems set up primarily for
enhanced film viewing.
Mono Logic
Mono Logic is a stereo conversion program for monaural film sound tracks.
A quick look through any video rental selection will prove the usefulness of
such a program; the vast majority of titles are mono.
The problem of mono-to-stereo conversion is an old one. One time-honored
solution is to break the incoming signal into frequency bands, sending some
to one channel and the rest to the other. When the filters are complementary
(when the sum of the two output channels equals the original input channel)
this solution can give stereo spread without ruining the tonal balance. When
the filters are non-complementary, they can produce an unpleasant fake
stereo effect.
Some effort has been made to design filter pairs for film sound which leave
voice frequencies unchanged while spreading out the music. More recent
designs have gone in another direction, using digital or analog delay lines
to produce a comb filter effect. So far, these attempts have not been very
successful.
The principal element of film sound is dialog and the principal rule in
reproducing it is to assure that it appears exclusively in the center channel.
Broadcasters, who have an interest in converting mixtures of dialog and
music to synthesized stereo, have built circuits designed to turn off the
stereo synthesizer when voice appears. Unfortunately, the switch from
mono to stereo is often abrupt and the chances of dropping into mono by
mistake during music are high. One basic problem with films, especially
modern ones, is that music or background effects which should be spread
out into the side speakers frequently appear at a low level beneath the
dialog.
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