4.3.1. 35mm Optical (monophonic, stereo, or Dolby Stereo (tm) ) (Motion Picture Sound Formats - release prints intended for projection)
Description
This article is from the rec.arts.movies.tech FAQ, by Scott E. Norwood snorwood@nyx.nyx.net with numerous contributions by others.
4.3.1. 35mm Optical (monophonic, stereo, or Dolby Stereo (tm) ) (Motion Picture Sound Formats - release prints intended for projection)
The standard sound-on-film system for 35mm has always been optical
sound. This works like the variable-area system described above under
`16mm optical.' This system is inexpensive and standardized, so that
almost every projection setup in the world is capable of reproducing
it. Of course, the disadvantages are as with any optical sound system:
lousy frequency response, noise, and `pops' when splices pass through
the soundhead.
Eventually, in the 1970's, the standard monophonic track was modified
to permit stereo reproduction. This allowed optical tracks to offer
competition to the four-track magnetic systems in use at the time. The
reproduction of stereo tracks required modification of the projector's
soundhead to accept a stereo solar cell. The optical stereo approach
was not used commercially, however, due to background noise and hiss
issues. In the mid-1970's, Dolby (tm) Laboratories developed methods
of `matrixing' the SVA (stereo variable area) track in order to encode
four tracks worth of information within the twin stereo tracks. This
allowed for the additions of a center (dialogue) track and a rear
`surround' track to the usual left and right stereo tracks. In
addition, Dolby (tm) type `A' noise reduction was used to reduce
background noise.
This `Dolby Stereo (tm) ' system soon became standard, and nearly all
commercially released films since about 1980 have been encoded with
it. Of course, one must use a Dolby (tm) Cinema Processor (or a clone
thereof [e.g. `Ultra Stereo']) in order to decode and reproduce all
four tracks; otherwise, it just reproduces as two-track stereo. `DTS
Stereo (tm) ' uses the same principles as Dolby Stereo (tm) and is
decoded with the same equipment, but the term applies to optical
tracks produced by DTS (tm) , without the use of Dolby (tm) equipment
(Dolby (tm) encoding equipment is usually rented out for higher
rates). Note that `DTS Stereo (tm) ' is distinct from the DTS (tm)
digital sound system described below.
In the late 1980's Dolby Stereo (tm) was improved upon by `Dolby SR
(tm) .' The `SR' stands for `spectral recording,' which incorporated
better channel separation and noise reduction than standard Dolby
Stereo (tm) , but which supposedly retained compatibility with Dolby
(tm) type `A' processors, although this is debatable. A Dolby (tm) `A'
processor can be upgraded to support SR prints, if desired. Type `A'
prints do not reproduce well when played back through a processor set
up for `SR' mode (all modern processors also contain the `A' NR mode
as well).
Incidentally, Dolby (tm) `A' noise reduction is one of several noise
reduction schemes developed by Dolby (tm) Laboratories. It (and SR)
are capable of reducing noise across the entire audible frequency
range. Dolby (tm) also developed type `B' noise reduction, which
reduces the high- frequency noise common to audio cassette tapes, and
type `C' noise reduction which is also used for cassettes, as well as
the Beta SP videotape format.
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