Originally Posted by stgdz
So is it just matrix'd in with the left and right channels?
Depends on the processing. Audyssey DSX generates ambience for their heights and wides. Dolby PLIIz extracts ambience for their height speakers.
When listening to 2-channel music, there are certain sounds (lead vocals, instrument solos) that phantom image at the centre of the soundstage. But you have to be sitting in the sweet spot. If you're sitting off-axis, then those sounds will pull towards whichever speaker you're closest to. PLII extracts those sounds and puts them in the centre speaker. This way, those sounds always image at the centre of the soundstage, no matter where anybody's sitting.
Likewise, if you're sitting in the right spot and playing the right content, you'll sometimes hear phantom imaging above you. For example: there are times when watching 'Master and Commander'
that I could have sworn I heard the sound of wood creaking above me. But if I'm sitting at either end of the couch, the phantom image collapses to the nearest surround speaker. PLIIz extracts those decorrelated sounds and sends them to the height speakers. Now, no matter where you're sitting, those sounds seem to come from above.
Not a difficult concept to grasp. You're basically replacing phantom images with hard sources (speakers), for better imaging stability. Nothing more complicated than that.