Tried a search but didn't really find what I was looking for. Will show my total ignorance of surround modes and how they work, but here goes.

I have a 970/1075 setup with both my DVD and Cable/DVR outputting their audio via optical. I just don't get the surround modes, what the display shows, and why they sometimes change.

For example, for most television, trial and error determined that Dolby PLII-C sounds best. However, for some television the 970 reverts automatically to Dolby Digital, showing Dolby Digital and .0 or 2.0 or 3/2.1. Most of the time this is great as the sound is much improved, however, sometimes the switch will leave nothing on the center channel and a rather "flat" sound, so I manually switch it back to Dolby PLII-C. The manual is lacking in explaining this. I guess my questions are:

1. Is it autoswitching to a source that it recognizes, and if so, is it accurate?

2. What does .0, 2.0, and 3/2.1 indicate (2.0 and 3/2.1 I assume are 2.0 and 5.1 sources)? and what do they mean, the source, or the output?

3. Is it a typo that Dolby PLII-M, listed as Dolby Pro Logic II Movie, says Optimized for Music in the manual?

4. Despite the fact that I have the 970 set for no back surrounds, it still offers PLIIx as an option, is this right?

Any help with this is greatly appreciated, be it in directing me to the answer, or answering in as much detail as anyone is willing to spend the time to explain.