sentences:  It provides an economical and backwards-compatible means for 5.1 soundtracks to carry a sixth, center back surround channel for improved localization of effects.The extra surround channel is matrix encoded onto the discrete "left surround" and "right surround" channels of the 5.1 mix, much like the front center channel on Dolby Pro Logic encoded stereo soundtracks.The result can be played without loss of information on standard 5.1 systems, or played in 6.1 or 7.1 on systems with Surround EX decoding and added speakers.