Dobly Surround Sound:

With Dolby Surround Sound, the AC-3 bitstream encodes for sound to come through all five speakers. There are, however, only four discrete sound tracks used. They are:

1.     Right Front
2.     Left Front
3.     Center
4.     Rear Channel

While stereo sound comes from the two front speakers, reprocessed center track information is fed to the center speaker. The two rear speakers are driven by a single signal and is technically a mono rear sound. This provides depth and reverberation to the room's sound. This format is similar to 4.0 except the rear channel and center are matrix driven not discrete. This means the bitstream is contained primarily in the two front channels and the rear and center information is processed from their. This is the most compatible for systems that only can decode two channel information. It is very similar to Prologic. This is very often found as a secondary track on 5.1 discs. If you are playing the DVD on a two speaker system it is better to use this format.