Stereo variable-area
In the early-1970s, there was renewed interest in improving the quality of optical film soundtracks.
As a consequence by the early 1970s several individuals and organizations were considering improvements to the dated technical standards used for optical sound on 35mm film. In particular both Dolby Laboratories[3] and Kodak[4] were investigating the use of Dolby noise reduction on optical soundtracks.
In the mid-1970s, Westrex Corp. (a wholly owned subsidiary of Litton Industries since 1956, and the successor to Western Electric's cinema sound business unit) re-introduced the ca. 1938 "four ribbon" light valve, and the ca. 1947 RA-1231 sound recorder.[5]
When originally introduced in 1947, the RA-1231 could be configured as a mono 35mm variable-density or variable-area recorder, or a mono 16mm variable-density or variable-area recorder, at the customer's option. However its basic electro-optical arrangement could also be used to create a time-aligned, two-channel variable-area version,[6] and this then became the industry standard device for recording stereo variable-area optical soundtracks.
Variable-density's fate was then sealed as these stereo optical sound prints (as contrasted with stereo magnetic sound prints or mono optical sound prints) became a marketing imperative.
When encoded utilizing Dolby Stereo (itself originally being in part licensed from Sansui), the discrete L and R channels of Westrex's stereo variable-area system were renamed "Left Total" and "Right Total", and when decoded utilizing Dolby's Cinema Processor these produced the L, C, R and S sound image first commonly used by Fox's CinemaScope magnetic stereo system in 1953.
Stereo optical sound prints are compatible with films with any aspect ratio and with normal print film stocks with KS-type film perforations, whereas stereo magnetic sound prints require film stocks with the narrower CS-type film perforations. Film with CS-type perforations can only be run on a projector fitted with special narrow-toothed sprockets or permanent damage will be done to the film. An alternative is LaVezzi's VKF ("Very Kind to Film") sprockets, which perform optimally on KS- as well as CS-perforated prints. Stereo variable-area, therefore, provided for the first time stereo film prints of any aspect ratio (1.37:1/Academy through 2.35:1/CinemaScope, inclusive) which could be run without damage on any normal 35mm cinema projector.
Nearly all original track negatives (OTNs) are now produced as stereo variable-area, and the former Western Electric (Westrex) system has been renamed Photophone and has become the de facto standard for analogue optical soundtracks, world-wide.
The fully implemented case of stereo variable-area (i.e., 4-2-4 encoding/decoding) produces a stereo 3.1 track.
The partially degenerate case of stereo variable-area (i.e., no 4-2-4 encoding/decoding, but discrete left total/left and right total/right) produces a stereo 2.0 track.
The fully degenerate case of stereo variable-area (i.e., no 4-2-4 encoding/decoding, and left total/left equal to right total/right) produces a conventional "dual-bilateral" mono 1.0 track.
