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Surround sound that's created by using a matrix of two or more channels is called Matrix Surround. The original matrix surround systems were designed to make use of standard home stereo (left and right) channels by adding information which could be interpreted by specially equipped receivers. This includes certain quadraphonic systems in the 1970s, as well as Dolby ProLogic and ProLogic II home theater Surround Sound.
The Center and surround channels in a Matrix Surround system are composed of a composite of the Left and Right stereo signals. In addition, rear (surround) channels can be out of Phase to emphasize the distance between the front and surround speakers.
Modern Matrix Surround
Although Matrix Surround disappeared as a mainstream encoding format with the introduction of DVD and common use of the Discrete Surround Dolby Digital and DTS formats, it's still sometimes used to produce additional surround channels from the discrete Left Surround and Right Surround channels. This method is used by Dolby Digital EX and Dolby Digital Surround EX, as well as by 6.1 Surround and 7.1 Surround receivers, to create additional sound channels.
Dolby Pro Logic II (Matrix Surround)