surround question

There are many surround formats.
Today, most mixes are made in the 5.1 format. There are 6 channels : Left Center Right Left and Right surrounds, plus a sub channel (the .1 in 5.1) These are discrete channels.
Once the mix is finished, it can be encoded in the Dolby Digital or DTS format, just like on your DVD. The six channels are embedded in a single stream (called ac3 for Dolby Digital) where audio is compressed, so that it's data fits on the dvd along with the picture or on the film, at the movie theater. On playback, it is decoded as 6 discrete channels by a decoder.
DTS uses a similar process for the dvd, but is very different at the theater. Youshould check both websites, especially http:// which is excellent for answering many of your surround questions.
New technologies like SACD of DVD audio use a different system that has no data compression and they deliver a better sound quality.
When you play back audio from a DVD in stereo, what you hear depends on how you set up your player or what option you chose from the audio menu.
It can be a down mix of the Dobly Digital or DTS 5.1 mix or it can be a seperate PCM stereo track, which could contain a matrixed Dolby Surround mix.
Dolby surround has no Sub channel and it's surround is mono (no matter how many surround speakers you ave) and frequency limited, due to an older but very interesting matrixing technique. Again, read form the Dolby website.

To answer your other question, sometimes, we need to "upgrade" an old stereo mix into 5.1. It is either remixed using the original mutitrack tape or it can be faked using various surround processors. In my opinion remixing does the best job.

Thank you Thomas. I will do some research.

Mike