Lexicon CP-1 User Manual download pdf (Page 49)

Page 45

CP-1 Digital Audio Environment Processor

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 a monaural input with Mono Logic to a

stereo input with Pro Logic may make a surprisingly 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.

Theory

and

Design

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.

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 “Yel-

low Submarine” and “Superman

I”. In most films, though, all dialog

comes from the center channel.

Mono Logic works with the left input channel only. If it is used with a stereo

input, material recorded exclusively in the right channel will be ignored.

The CP-1 is one of a few consumer products to offer full Pro Logic Dolby

Surround decoding, and it is the only one that operates 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

Pro Logic

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Lexicon CP-1 User Manual Page 49

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