Dolby stereo

Dolby stereo is a stereo-sound process for motion pictures created by Dolby Laboratories, Inc., used to improve sound quality; 35mm prints have two optical soundtracks (Dolby can decode and playback on four channels), while 70mm prints have six magnetic tracks for multi-channel playback. By the 1990s, Dolby Stereo was superseded by advanced digitally-recorded sound.