Lexicon CP-1 Owner's Manual page 48

Theory
and
Design
The Surround
Programs
Page 44
sides to the front, while leaving the side outputs on.)
The Surround programs: Mono Logic, Stereo Logic and Pro Logic, are
specifically designed for film sound or for systems set up primarily for
Mono Logic
enhanced film viewing.
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.
The Mono Logic program electronically identifies certain properties of film
speech and removes it from the stereo synthesis. This allows music and
effects in the dialog to be spread out while leaving the dialog centered. The
remaining music and effects are directed to the input of a room simulation
program that creates a space the size of a large room or small theater. The
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