Dolby Stereo was the original analog noise reduction technology that was developed by Dolby Laboratories and used in movie theaters. Also known as Dolby A (or Dolby Analog), it was used for 70 mm film (magnetic) and 35mm film (optical) prints in 1976. Logan’s Run was the first to use Dolby mix and Dolby Stereo was used for the first time on 101 Dalmatians, a Disney Classics film.
The soundtrack on a Dolby Stereo encoded 35 mm film carries left and right tracks for stereophonic sound, a center channel and a surround channel. Dolby Stereo has the Dolby A type noise reduction incorporated into its process. 1975 Ken Russell film Lisztomania used the original Dolby Stereo in a 3-channel LCR configuration. This was followed by the use of four channel LCRS speaker configuration in the 1977’s Star Wars. This system had the advantage of backward-compatibility as it could be played in theaters with mono sound as well as those with upgraded Dolby Stereo processors.
Ultra Stereo Labs emerged as a fierce competitor to Dolby Stereo in 1984, when they introduced Ultra Stereo, a comparable stereo optical sound system. The processor introduced improvements in matrix decoding and greater channel separation. It also had a balancing circuit included that compensated for imbalances between the right and left sound tracks and film weave. These imbalances resulted in voice leakage into the surround channel, which was successfully overcome by the technology.
Dolby Stereo was later displaced by the Dolby SR in the mid-1980s. All theatrical release prints encoded with Dolby Digital include Dolby SR as the default track. This comes handy when something goes wrong with the decoding or in theaters lacking Dolby Digital playback.