Operating instructions

D-1
APPENDIX D
ABOUT DOLBY FILM SOUND FORMATS
The application of Dolby A-type noise reduction to film soundtracks, as
explained in Appendix G, “Cinema Sound and the Evolution of Dolby Stereo,”
had a major impact on cinema sound recording and reproduction. The later
application of DolbySR has had still further impact. An understanding of how
these systems work will help you better understand Dolby Stereo film sound
in general, the CP45 in particular, and why the Dolby Level adjustment
procedure is important in maintaining good sound in the theatre.
1. How Dolby Noise Reduction Works
Whenever sound is recorded — on magnetic tape, phonograph records, or film
soundtracks — noise is added to the original signal. Dolby noise reduction
keeps that added noise down to a minimum in a way that has no adverse effect
on the quality of the original signal (e.g., it does not turn down the treble as
filters do).
Dolby noise reduction is a two-step process used both at the time a recording
is made and when it is played back. Just before the signal is recorded, it is
processed by the Dolby noise reduction circuitry. Loud sounds which naturally
hide noise are passed through unchanged. But quiet sounds which would be
marred by added noise are selectively boosted (that is, they are recorded louder
than normal). The resulting recording or film soundtrack is said to be “Dolby
encoded.” Both tracks on Dolby Stereo optical release prints are encoded in this
way.
When the encoded recording or soundtrack is played back, it is decoded by
Dolby noise reduction circuitry switched around to operate as a mirror image
of the circuit used when the recording was made. Loud sounds again pass
through unchanged. But the boosted quiet sounds are reduced in volume to
where they were before they were recorded, a process which simultaneously
reduces any noise added by the recording medium. The CP45 has two channels
of Dolby A-type noise reduction and Dolby SR circuitry to decode the two
soundtracks on Dolby Stereo release prints (the left, center, right, and surround
channels are derived from the two soundtracks after the Dolby noise reduction
decoding).
2. About Dolby SR
The soundtracks on Dolby Stereo SR prints have been encoded with the
DolbySR (spectral recording ) process, rather than Dolby A-type. DolbySR
also boosts quiet sounds when a recording is played and brings them back
down again when it is played back. However, the amount of boost it imparts
is both greater and more selective than Dolby A-type. The result is more than
twice the noise reduction, with the same freedom from side-effects Dolby A-
type is noted for. In addition, Dolby SR incorporates further refinements which