I've dissected Prolgic IIx/z using test tones by manipulating the phase and delay and found its actually a fairly simple implementation. Anything the is mirrored in the left and right channels is placed in the center, anything that has a phase difference or changing phase (such as a car driving by from front to back) is routed to the surrounds. Anything that is mirrored in the left or right surrounds is routed to the rear channels, the volume/delay between the left and right surrounds determines which rear channel it's routed to. Anything that still has changing phase angles in the surrounds is then routed to the heights.
One major issue with this is PLII suffers from channel bleed. Sometimes sounds that shouldn't be find themselves in the wrong channel, the center channel sometimes leaks into the right and left, and stuff that should remain in the front sound stage finds its way into the surrounds.
Dolby's surround up mixer lacks all of these problems, the channel steering sounds discrete with absolutely no channel bleed and I've yet to find sounds inappropriately being placed into the top channels. It seems to work perfectly on every demo movie I've tried it out on, which makes me think there is something much more complex than simple matrixing going on.
Out of curiosity, does anyone have any details on how the up mixer is deriving extra channels?
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