The original Dolby Surround system which used four audio channels carried on a stereo optical track on 35mm film, using Dolby perceptual encoding. On 70mm film, six audio channels are recorded on discrete magnetic tracks laid onto the film. In the broadest and most common usage, the trademark that appears on movie prints, advertisements, and posters which means that a given film has been released in prints that employ Dolby A-Type noise reduction encoding. Beginning in 1987, Dolby-SR has been available on 35mm stereo optical prints. Dolby Stereo on 70mm usually means four discrete primary channels (LCRS) with the left-center and right-center tracks dedicated to low-frequency information (below 250 Hz). The four tracks are normally use A-Type encoding, although selected 70mm films, since 1987, have utilized Dolby-SR encoding. See film soundtrack.
