Operating instructions

E-1
APPENDIX E
HOW FOUR CHANNELS ARE DERIVED FROM DOLBY
STEREO OPTICAL PRINTS
The 35-mm Dolby Stereo optical format calls for the recording in the studio and
the reproduction in the theatre of four separate sound channels, — left, center,
right, and surround — and yet a Dolby Stereo optical print has only two
soundtracks. How is it possible to derive four channels of sound from only two
soundtracks? The answer lies in the application of what are called phase matrix
techniques for encoding four channels of sound onto two soundtracks, much
like the four-channel phonograph records of the early 1970’s.
There are two characteristics of sound which cue the brain to its origin and thus,
its directionality. First is the amplitude or loudness of the sound. If the source
of sound is directly in front of you, it arrives at both ears with equal loudness.
But if the sound is located to the left, for example, the sound that arrives at your
left ear is slightly louder then the sound that arrives at your right ear. This
difference is processed by the brain so you recognize that the sound is coming
from the left. Home stereo sound is based in great part on this principle; two
channels on a recording that is reproduced on two speakers are sufficient for
home listening to convey directional information all across a listening “stage”
in front of you.
The other characteristic of sound which cues the brain to its directionality is
phase. A sound coming from the left not only is louder when it arrives at your
left ear but it arrives slightly earlier. That difference in timing results in what
engineers call phase shift. The brain also processes that difference as a further
directional cue.
The distinction between amplitude and phase as directional cues is not so
important in conventional home stereo as in motion picture stereo. Movie
theatres require more than two channels for good stereo sound. For example,
in a theatre, left and right speakers are so widely spaced that sounds coming
from the center (dialogue in particular) must be more firmly defined than in a
home system. This requirement is met by the addition of a third, or center,
channel and screen speaker. In addition, the use of a fourth channel toward the
rear of the theatre is highly desirable to reproduce ambient sound for more life-
like overall sound and to reproduce special directional effects. Thus, good
theatre stereo requires a total of four channels.
Dolby Laboratories developed a phase-matrix technique for encoding the four
required channels of sound onto two tracks in exactly the same space as the
conventional mono track. The final soundtrack is mixed to four discrete
channels (left, right, center, and surround). Those four channels are then
encoded to the two tracks which ultimately wind up on the release print. Basic
left and right information is recorded unchanged onto the left and right tracks.
Information which is to be reproduced in the theatre as a center channel — that
is, sound which is identical in phase and amplitude — is split between and