From Dolby site:
<<Dolby Surround is the consumer version of the original Dolby multichannel analog film sound format (Dolby analog and Dolby SR [Spectral Recording]). 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. These two tracks are then carried on stereo program sources such as videotapes and TV broadcasts into the home, where they can be decoded by Dolby Pro Logic® to recreate the original four-channel surround sound experience.>>
