Mono Logic - Lexicon CP-3 Plus Supplementary Manual

CP-3 PLUS Digital Surround Processor
Mono Logic is a surround conversion mode for monaural film sound tracks.
A quick look through any video rental selection will prove the usefulness of
such a mode; a large percentage 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 mode 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
mode that creates a space the size of a large room or small theater. The room
simulator has outputs for left, right, side and rear surround speakers.
The monaural input sound from the film is unchanged in the center speaker,
so that all the dialog and music that the director expected to come from the
screen still does — with no modification or reverb. Partly because of the
acoustical character of the room synthesizer, the result is often so successful
that switching from Mono Logic to Pro Logic or THX may make a surpris-
ingly small difference.
The most critical adjustment in Mono Logic is the Effect Level. Ideally the
film's music and effects should appear to come from the front but with the
added sense of a large space surrounding you. The side and rear speakers
should not be individually audible.
PLUS
CP-3 Theory
and Design

Mono Logic

During the early days of film stereo,
dialog was sometimes mixed (by
"panning" the monaural dialog
track) to come from the same part of
the screen as the image of the actor.
Subjective reactions to this tech-
nique were varied, and technical
problems with some magnetic sound
tracks helped to discourage the prac-
tice, so modern movies are seldom
mixed with panned dialog. In a home
system with a good Pro Logic de-
coder, however, the effect can work
quite well; recent releases with
panned dialog include "Yellow Sub-
marine" and "Superman I." In most
films though, all dialog comes from
the center channel.
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