5.1 is 5 discreet mixed channels....Stereo is 2 discreet mixed channels. In other words they're 2 totally different animals. You can mix in 5.1 surround, and those 5 discreet channels get fed to an "Encoder" box. The encoder box, mixes them with phase information into a 2 channel stereo mix. Now you have to play this 2 channel stereo mix (ie CD) through a "Decoder" box, which takes the phase information and closely recreates the original 5.1 surround mix. This is not true 5.1 though. This goes by the names Dolby Pro Logic, or Logic 7. If you want to get into the technical explanation of how this stuff really works, then go visit Lexicon's website at WWW.LEXICON.COM. Logic 7 was developed by Dr. David Griesinger, who works for Lexicon/Harman International. There is a link to Dr. Griesinger's website, where there is many AES reports that he has put together explaning how to take a stereo source and make it surround and such. I work at Harman International also, Harman is the owner of Logic 7 patents and I get to work with this technology every day.