I know what is the end result regarding the sound coming from the surround speakers, but I'd like to know a little more about what kind of processing is taking place in these expansion schemes.
Dolby Surround decoding was pretty simple:Center = Left + RightRear = Left - Right, low-pass filter it at around 7 kHz, and delay it by 20 to 50 milliseconds or so. The filtering may have been mostly because the analogish "bucket-brigade" delay lines ran at rather low sample rates.
And then there's the 3D sound processes. This explains them a lot better than I could without plagiarizing Wikipedia: http://en.wikipedia.org/wiki/3D_audio_effect