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Hi. 1st post, new to the forum and have really liked what I've heard of the Voxengo plugins. I have a couple of perhaps unusual potential applications for the PHA-979 I'd like to ask about. Apologies for all the words-
I want to decorrelate a single mono surround signal to create two signals.
I've been doing 4 channel surround recording by adding a second stereo pair consisting of a center and rear microphone to a standard left/right stereo spaced or baffled omni pair. Playing the files back on a typical 5 channel playback system, I currently split the mono rear signal to the two (or more) rear surround speakers. The primary thing I want to do is decorrelate the single rear surround signal to create discrete left surround and right surround signals in an attempt to defuse the primarily ambient surround information around the back half of the room. I'm not concerned with imaging back there, instead looking for an open ambient 'feel' of the space in which the recording was made. Sometimes it's a issue of attempting to diffuse the strongly located sound of the occasional loud clapper that was right behind my rig, to spread out and not draw attention to the distraction.
One simple way to decorrelate is to duplicate the mono rear signal and flip the polarity of one of them, but that means one side is 'in phase' with the front 3 channels and the other 'out of phase' - less than ideal. Shifting the rear signal 90 degrees before duplicating it and flipping polarity on one copy would produce two signals out of polarity with each other, but symmetrically 90 degrees out of phase with the 3 main front speakers. Delay or eq could also be used to decorrelate the mono signal but seem like less than ideal solutions that might just make for comb filtering phasy weirdness. If I should consider other good alternate techniques for decorrelating a single mono signal to two signals please let me know. Maybe some 'pseudo stereo' techniques could work?
Part of the reason I'm looking to do it this way is that as I understand it, this constant 90 degree phase shift over the whole spectrum is a technique used by the more advanced matrix surround techniques (such as Dolby PLII, DTS NEO6, CircleSurround, Logic-7) to decorrelate a mono surround signal to two surround speakers. Secondarily, even though my primary goal is a discrete 5 channel output, I'd also like to play around with DIY matrix encoding to stereo for playback on a typical surround decoding receiver. I'm imagining folding the mono center and mono phase shifted surround signal into the right/left signal to create a Lt/Rt matrixed stereo output that might decode reasonably well. I'd guess there are surround tool plugins available for doing matrix encoding but that 'phase' is further down the road and I haven't yet looked into it.
You should probably try splitting the rear channel into two channels, shifted -50 degree and +50 degree (-50 is achieved as 130+180), and then blending a bit of phase-shifted original left to rear left and symmetrically phase-shifted original right to rear right channels.
Note that 50 degree shift is similar to 90 degree shift; but 50 degree is 90 degree mixed with the original (0 degree) signal in a proportion. It's easier to use directly defined phase-shifts than to operate with 90 degree shifts.