A digital audio encoding system from Dolby that provides four channels. Derived from the Dolby Stereo technology used in movie theaters, Dolby Surround was introduced in the early 1980s for video soundtracks, audio cassettes, CDs, TV broadcasts, video games and PC software.
Dolby Pro Logic
The original Dolby Surround decoders in consumer equipment only played back the surround channels. Dolby Pro Logic was introduced shortly after for playback of the center channel as well. Pro Logic II added left and right surround channels (5.1), and Pro Logic IIx added support for 6.1 and 7.1 channels. See Dolby Digital