"There's no need to fear, Underdog is here!"

Doug, there is a difference between a rear-center extraction and a side-channel fill-in.

When adding a rear-center speaker, the 'original' rears should be moved to the sides (where some people prefer the surrounds anyway, or have to be if your seating is against the back wall).

The rear signal is encoded as a difference (L - R) signal, as in the aforementioned Dynaco/Hafler 'ambiance' extraction, but it's not necessarily meant to play whatever is 'different' between the two. In fact, a ProLogic decoder will send whatever is IN phase and at nearly equal level (L + R) to the center output. This is what my setup produces in the rears, and differences are steered to the sides.

For sides that are in additional to the stereo rears, they should be reproducing what is common (L + R) to the front-left and rear-left channels, to avoid the 'hole' effect from there being no speaker directly to the side; that's why the surrounds should be to the sides when adding a rear-center.

In other words, 6.1/7.1 surround is NOT the same thing as 7-channel logic (as provided by Lexicon, et al). 6.1/7.1 adds a rear-center (or two) to the existing surrounds, while 7-channel logic adds sides to the existing surrounds. See the difference?

Larry
www.fineelectricco.com