Dolby Stereo/ Dolby Analog was the original analog optical technology developed by Dolby Laboratories for 35 mm film prints in 1976.The optical soundtrack on a Dolby Stereo encoded 35mm film not only carried left and right tracks for stereophonic sound, but also, through a matrix decoding system, a third center channel, and a fourth surround channel (which is heard over speakers on the sides and rear of the theater) for ambient sound and special effects. This yielded a total of four sound channels, in the physical track space previously allocated for just one mono optical channel.Incorporated its A-Type noise reduction into the Dolby Stereo process. The original Dolby Stereo was first used on the 1975 Ken Russell film Lisztomania, in a 3-channel LCR configuration. The success of 1977's Star Wars, which used the four channel system to great effect, did much to encourage movie theaters to convert to the 4-channel LCRS speaker configuration.
Dolby Stereo Key feature of this system was its backward-compatibility: the same print could play anywhere, from an older drive-in theater with mono sound to a cinema which had upgraded its system with a Dolby Stereo processor. Thus, there was no need (nor expense) in carrying a double inventory of prints for distribution. Dolby Stereo was displaced in 35 mm motion picture exhibition by the Dolby SR format (1980s). Dolby SR is still included on all theatrical release prints encoded with Dolby Digital, as the default track, if something goes wrong with decoding the digital track. Also the Dolby SR track is used in theaters not equipped for Dolby Digital playback.
When a Dolby Surround soundtrack is produced, four channels of audio information: left, center, right, and mono surround; are matrix-encoded onto two audio tracks. The stereo information is then carried on stereo sources such as videotapes, laserdiscs and television broadcasts from which the surround information can be decoded by a processor to recreate the original four-channel surround sound. Without the decoder, the information still plays in standard stereo or monaural. The Dolby Surround decoding technology was updated during the 1980s and re-named Dolby Pro Logic. The terms Dolby Surround and LtRt are used to describe soundtracks that are matrix-encoded using this technique.
