Theory
and
Design
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.
Mono Logic works with the left input channel only. If it is used with a stereo
Pro Logic
input, material recorded exclusively in the right channel will be ignored.
The
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
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.
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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