Analog: Dolby's Original Surround Sound Format
The original Dolby multichannel film sound format revolutionized the moviegoing experience, and is still included today on nearly all 35 mm film prints.
This analog optical technology encodes the soundtrack into four channels: Left, Center, and Right, which play back on speakers located behind the screen, and Surround, which is heard over speakers at the sides and rear of the theatre for ambient sound and special effects.
Dolby analog soundtracks now incorporate Dolby SR technology, which significantly improves the dynamic range. The analog soundtrack is still used in cinemas not equipped for digital playback. In digitally equipped theatres, it serves as a backup in case of problems with the digital track.