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I started paying attention to this stuff back when Dolby Pro Logic was new, which was a pretty clever way to get surround effects using only left and right audio channels. Left and right channels went directly to the front left and right speakers, but it also compared wave forms coming from the left and right channels. Any wave forms that matched got sent to the center channel (like most on-screen dialog) and any that mismatched got sent to the rear surround speakers (noise, ambience, etc). It wasn't perfect by any measure, but it was a pretty clever hack.