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Dolby Stereo 70 mm Six Track


Dolby Stereo is Dolby Laboratories' trademark for its various stereo sound formats.

Two basic systems used this name: the Dolby SVA (stereo variable-area) system used with optical sound tracks on 35mm film, and Dolby Stereo 70mm, which referred to Dolby noise reduction on 6-channel magnetic soundtracks on 70mm prints.

Of the two, Dolby SVA was by far the more significant, as it brought high-quality stereo sound within the reach of virtually every cinema. Though 7-track magnetic stereo had been used in Cinerama films since 1952, and Fox had introduced 4-track stereo magnetic sound as part of the CinemaScope system in 1953, the technology had proved to be expensive and unreliable. Except in large cities, most movie theaters did not have facilities for playing back magnetic soundtracks, and a majority of films continued to be produced with mono optical soundtracks. Dolby SVA provided a method for putting high-quality stereo soundtracks on optical sound prints.

The optical soundtrack on a Dolby Stereo encoded 35 mm film carries not only left and right tracks for stereophonic sound, but also—through a matrix decoding system (Dolby Motion Picture matrix or Dolby MP) similar to that developed for "quadraphonic" or "quad" sound in the 1970s—a third center channel, and a fourth surround channel for speakers on the sides and rear of the theater for ambient sound and special effects. This yielded a total of four sound channels, as in the 4-track magnetic system, in the track space formerly allocated for one mono optical channel. Dolby also incorporated its A-Type noise reduction into the Dolby Stereo system.

Dolby Labs' original involvement in movie sound was when film studios used Dolby A type noise reduction on studio magnetic film recordings. The first film to use Dolby noise reduction throughout the production process was A Clockwork Orange, though much of the benefit was lost when it was released with a standard "Academy" optical soundtrack. This led to a proposal from Dolby that A type noise reduction be applied to the optical soundtrack on release prints.


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