Just found one example that proves my initial worry to be true --- If I have my computer speakers set to 7.1, certain applications will not be in 7.1, and PLIIx will not work in upmixing the 5.1 to 7.1
EIther due to the program sending silent data to the two extra channels, or some other reason... The only fix is to change my PC settings to 5.1 and keep PLIIx, then it works.
I tested with a game that output 5 channels when I told Windows I had 7. From what it seems, it was sending silent data to the other two channels, so my PLIIx would never kick in.
My original intention was to avoid having to switch between telling Windows that I have 7 speakers(when I use native 7.1 content) or that I have 5 speakers(when the above example happens, causing two silent channels).
Unless anyone else has an idea, I think the only solution will be to keep Windows set at 5.1 and apply PLIIx. The downside of this will be that true 7.1 content will end up being 5.1 --> PLIIx upmixed to 7.1. I'm not sure how negative this is, as I don't know the technical facts behind the differences.
Unfortunately it doesn't seem like I have any other options, as one program to another this will differ, but the only way to maintain 7 channel output ALL THE TIME is to force 5.1 and PLIIx.
Can anyone shed some light on the detrimental effects of using 5.1 & PLIIx for true 7.1 content, versus using native 7.1?