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Pro Logic's Center Channel Extraction, Implementation questions
post Jul 23 2011, 05:31
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I'm trying to figure out how Pro Logic implements center channel extraction. Looking at how various different types of sine waves combine (a single one into two other ones), I guess I'll need to implement some type of FIR filter. I'm very new to this. It would be nice if this could be done on a sample-by-sample basis.

Any help?
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post Jul 27 2011, 23:06
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There's really no way to "improve" Pro Logic decoding. Like dc2bluelight said, mixing is done while monitoring through a pro logic decoder, so the mixing engineer knows exactly how it's going to come-out... It's not perfect... It's not a discrete multichannel system like 5.1 digital... It mostly just steers (pans) the sound. With a regular Pro Logic decoder you are hearing exactly what you are supposed to hear.

Problem: When inputting L and R into Lt and Rt, anything perfectly out of phase between L and R gets erroneously sent to the surround channel upon decoding.
It's not a problem, and it's not erronious... That's exactly how Dolby Surround works. If there is no out-of-phase information, there is no rear channel information.

If you want to take the time, you can do you own panning with an audio editor and re-encode to 5.1 digital. (I've done something like this, along with some other "tricks", to make a 5.1 surround track from a mono source.)

1. Phase-shift L by +90 degrees and R by -90 degrees.
2. Find the common signal and extract it.
OK... Now you have L & R 180 degrees out of phase with each other... You can accomplish almost the same thing by inverting one channel... Now what? If you sum the out-of-phase channels you get L-R (or R-L), and if you subtract the out-of-phase channels you get L+R (or -L-R). You still only have 2 "original" channels and you can make various summations (and subtractions) of the left & right signals.

Caveat: Anything originally having a hollow sound will be decoded into the center channel.
What? I don't see how "hollow sound" has any relationship to phase/amplitude of the L & R signals.
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