Subject:How to do the opposite of center cancellation

Posted by: Waltern1

Date:12/17/2003 2:52:32 PM

I know how to remove the center material in a stereo field with phase cancellation or using Channel Converter... but how do I do the opposite, where I want to cancel everything BUT the center? Somehow it seems like it shouldn't be too difficult, but my brain is apparently on holiday. I have a stereo music bed track wth voice-over in dead center. I want to remove as much of the music as possible while retaining the voiceover. (I'm proposing a different music bed and I don't have the original tracks.) Any suggestions?

Subject:RE: How to do the opposite of center cancellation

Reply by: Chienworks

Date:12/17/2003 5:38:51 PM

We went through the math involved in this process pretty thoroughly and we determined that the best you can do is eliminate one channel of the off center material. Unfortunately this also doubles the off center material on the other channel. If you do it both ways and mix it back together you end up with the original material all over again.

Subject:RE: How to do the opposite of center cancellation

Reply by: Rednroll

Date:12/17/2003 5:46:18 PM

Well, here's how it's done in surrond systems, but like removing the center it's not ideal. You basically convert the file to mono.

Here's the math:
C=Center
L=Left
R=Right


Left channel= L+C
Right Channel=R+C

Removing the Center=> Left Channel - Right Channel
(L+C)-(R+C)= L-R (thus center got removed)

Adding more Center=> Left Channel + Right Channel
(L+C)+(R+C)= 2C+L+R (center got twice as loud compared to L and R)

Thus you converted to mono and your Center became twice as loud as it originally was.

Subject:RE: How to do the opposite of center cancellation

Reply by: Waltern1

Date:12/17/2003 7:41:39 PM

Thanks. I'd be fine with mono. I'll try it out, perhaps with a bit of BP EQ I can get enough of the raw voiceover to serve my purposes.

Subject:RE: How to do the opposite of center cancellation

Reply by: wigworld

Date:12/18/2003 10:55:58 AM

Would this work?

Take your file with the center material removed, then invert the phase of that. Add that to the original file (which includes the center material), and everything will be cancelled except the difference between the two files, which should be the center material only.

Subject:RE: How to do the opposite of center cancellation

Reply by: Rednroll

Date:12/18/2003 12:51:22 PM

Well it sounds like it might theorectically work, I even gave it a shot, but this is pretty much what Chienworks was describing and I verified it in Sound Forge. What I did was create a mix with sinewaves in Vegas. Track 1=500 Hz panned hard Left, Track 2= 1Khz panned dead Center, Track 3= 12Khz panned hard right. So now with your theory I should be able to get the 1Khz sinewave and elliminate the 12Khz, and 500 Hz components. It didn't quite work and I found I actually got better results with just converting it to mono.

Here's the math.

Say original mix= L+R+C =>
Left Channel = L+.5C
Right Channel= R+.5C

If we Invert phase the Right Channel we get:
(L+.5C)-(R+.5C)= L-R

Invert phase of L-R= -L+R

Now add that to your original mix
(L+R+C)+(-L+R)= 2R+ C

I doubled the amplitude of my 12Khz sinewave and elliminated the Left. I might be overlooking something in my math, but seems to be what I heard when I was done also. You can pretty easily try a similar test as I did in sound Forge using the simple synthesis tool, and making a similar mix file. Then open the Spectral analysis tool and see where the amplitudes of each frequency is peaking at, then start doing what you suggested either using the channel/convert tool and/or the Invert tool and see where you end up by looking at the new amplitudes on the spectral Analyzer.

Subject:RE: How to do the opposite of center cancellation (The Anti-Karaoke problem)

Reply by: Waltern1

Date:12/19/2003 7:29:13 AM

I too thought something along these lines would be the way to make this work, which prompted the initial question. But the math that Rednroll and Chienworks have described is exactly what comes out in practice -- the track with the center removed, added to the original track, brings back the center in all its unwanted glory.

Somehow, even though the phase cancellation approach sadly appears to be a goose egg, I can't help but feel that perhaps I'm thinking about the whole problem wrong -- and that maybe a different way of thinking about it might get closer to the desired effect -- that of removing the stereo image and preserving the voice over.

(I guess this is may be the evil twin of the ol' chesnut that appears every other week about REMOVING the vocal.)