Ok, I'm a little nooby to all this sort of stuff, but I have some knowledge.

Just one that's bugging me, what happens when you play a movie with a 5.1 mix, on a 7.1 system.

Do the extra two speakers sit useless, or do they just play an exact copy of another channel.

Do the extra two speakers sit useless, or do they just play an exact copy of another channel.Depends on the receiver. If you don't choose any 7.1 processing, the rear speakers may go silent or they might simply duplicate the side speakers. Some receivers even let you choose between those two options.

Most 7.1 receivers have Dolby Pro Logic IIx processing, which takes the contents of the 2 surround channels and steers it over all 4 surround speakers. Surround information that would normally phantom image behind you is extracted and sent to the speakers physically located behind you.

Sanjay

On my gear, you can turn the SB channel to Auto, On, or Off. I keep mine on Auto so that only 6.1 encoded material plays through. Otherwise, yes, they (1 in my case...see previous thread) sit (or hang) doing nothing.

If you turn the SB channel(s) on and are using a 5.1 source, the SB channel is a matrixed composite (the sum) of the SL and SR channels. Kinda like a center channel (in Dolby PL speak) for the rear.

I personally found through trials that I prefer it they way it was recorded (i.e. 5.1 to 5.1, 6.1 to 6.1). The added surround back speaker just seemed to add too much (or more) than what was intended.

In DD EX and DTS ES the SB channel is mono (I'm 99% sure) so calling it a "7.1" channel system for these signals is a bit of a misnomer. Yes, there may be 8 speakers, but there are only really 7 channels (including LFE).

I am waiting for others to chime in on my other post to add to my understanding of the purpose of having 2 SB speakers using todays (6.1) non-HD sources.

If you turn the SB channel(s) on and are using a 5.1 source, the SB channel is a matrixed composite (the sum) of the SL and SR channels. Kinda like a center channel (in Dolby PL speak) for the rear.Actually, what happens depends on what type of processing you apply. For example your example does not take place with Dolby PL IIx Movie. Maybe the SB does get the sum (or maybe it's a bit more complex than that - Sanjay probably knows), but what ever it is, it's subtracted from the surround left and surround right channels. This scheme would give you better separation between the surrounds and rear(s).

Ed