Dolby SVA
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 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 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.
Read more about this topic: Dolby Stereo