Surround music - extracting a center

[Postman]

Maybe Mr. Postman is listening

Here is what I have.
(-L+R) = "side information"
-Side information (note its Ø flip) + (L+R) = center.

Basically the phase flipped left combined with right should isolate anything that is in both channels (phantom center) leaving only "side"

I'm ALWAYS listening! Have you actually tried this? Please try it and see what happens. BTW, this all assumes there is no processing delay between steps. If you are setting these steps up in Pro Tools by routing one signal path into another, there WILL be processing delays. So, assuming no processing delay and always unity gain, dust off those algebra skills because signals add up just like numbers do.

Let's build an equation based on what you said above.
side + sum = center
replace names with variables
(-L+R) + (L+R) = X
since this is simple addition, the order of variables and order of addition on the left side of the equation does not matter. Therefore we can remove parenthesis and restate the order of variables in any way we choose
-L + L + R + R = X
Obviously -L and +L add up to zero, so we can restate this way
0 + R + R = X
Result is that X, which is center, will be identical to the right channel added to itself.

Then the phase flipped "side" added to left and right should cancel out any side information leaving center

Let's build a new equation for this, which starts with the previous equation. I've added parenthesis to group your ideas visually
(side + sum) + (phase flipped side + sum) = X
replace names with variable, like we did above. Once again, parenthesis are optional.
((-L+R) + (L+R)) + ((L-R) + (L+R)) = X
change the order of variables to make our thinking easier
-L + L + L + L + R + R + R - R = X
restate with zeros where appropriate
0 + L + L + R + R + 0 = X

You can see where this is headed I think. Simple addition and subtraction ("flipping the phase") does not give the results you are looking for no matter how you try it.

In the analog world, Dolby Pro Logic uses a clever signal phasing trick during encoding, a kind of intentional processing delay, so that complete cancellation of a channel, as you find above, does not happen. During decode, the center channel is created by simple L+R, but then they use another trick. The louder channel at any moment can sort of "skew" the whole sound field toward it. This is their infamous "steering mechanism" that we all know and love/hate. Without it, the mono sum playing out of the center speaker tends to reduce the perceived width of the soundfield. The mechanism can be fooled easily resulting in softer sounds, like dialog breaths, jumping out of speakers it shouldn't. BTW, in Pro Logic, the center is a sum of L+R, and the surround channel is difference of L-R. That is why reverbs tend to decode as surround information. Any non-coherent sound will remain after L-R and go toward the surround output, unless the steering mechanism decides to do something else with it. You can find documents on Dolby's website, probably in the pro audio area, that explain more closely how it works, and how they altered things for Pro Logic II.

A purely DSP approach might slice the signal into time and frequency domains, examine each slice and compare content and coherency of left and right, and make choices about how to re-assign the slices to output channels. It is not a simple thing to pull off smoothly. It is well beyond my math skills and, as you say, life is too short.

[Newpostguy]

Thanks Mr. Postman. The way you explain this it does seem futile. In theory it should work once the math is correct however as you stated with zero latency. If routing around in PT induces this well... (I never knew / noticed any induced delay while routing a signal around, perhaps I will route a drum loop over and over until It is noticeable by ear as a test)

So it looks like I am going to have to invest in some waves plugs. I really do not like their products, but the waves center extractor looks promising.

Thank you for taking the time with the math.

[infiniteloop]

For real world usefulness, you would be best off looking into the UM226.

[Newpostguy]

Thanks Mr. Loop, I have been window shopping for that plug since I upgraded to 5.1 Will invest sooner or later!!!

[Rick Sanchez]

I use a Penteo stereo to surround converter to extract a center channel.

So far it's the best thing I've found..really the only thing I've found that makes a very clean and downmixable LCR with a real discreet center from a stereo source. It will also pull a 5.1 extraction from a stereo source.

Available as hardware only. Design FX in burbank rents them and it's being evaluated by a couple of other rental houses in town.

If you are in the Los Angeles area and want to check it out, contact me.