# Why can't I isolate center-panned audio using inverse phase cancelling?

In Audacity, you can split a stereo song to mono and invert the phase to remove center-panned audio (usually vocals).

Can someone explain why inverting the result and adding the original (in mono) doesn't result in an isolated vocal track?

• Surely it should be "Mono signal" instead of "centre-panned audio" in the title. (?)
– n00dles
Commented Feb 19, 2016 at 15:00

Yeah it would appear to work that way, but the math reveals something else:

You have two channels, Left and Right, and you invert one, that is flip the sign on one channel and mix* them: Left + (- Right), or simply: `L - R`.

What you are doing here is essentially isolating the `side channel`. What was equal in both channels is now removed. What is different stays.

Now pay attention to the "formula" L - R. It will leave the original phase orientation of L intact and flip that of R. If you were to reverse it (which is just as good), R - L, the R orientation would be intact and L flipped.

So, you ask why mixing the inversed side channel equally back into the stereo mix will not produce the isolated center material:

The inverse side channel is equal to `- (L - R)`

Then we calculate new left and right channels:

```Left = L - (L - R), or simply R. Right = R - (L - R), or simply 2*R - L. ```

So that is really not an attractive result ;-) It does not get better if you already mixed* left and right into a mono file:

`(L + R) - (L - R), or simply 2*R`.

And if the side were not inversed:

`(L + R) + (L - R), or simply 2*L`.

(source)

The best you can do is to mix L and R equally to double the volume of the center material. That is what we normally call the `mid channel`.

Moreover you can try some other things on the main LR stereo track:

• Reduce the active frequencies of the side channel (use an RTA or listen and attenuate with and EQ)
• Or boost the frequences of the mid/center material.
• "remove noise" using the side channel as noise profile.

*for the sake of simplicity mixing means adding here, attenuation has been left out.