When we talk about stereo, there is no center. There is only left and right. What lives on one side and not on the other is considered difference.
What lives in the imaginary middle is actually coming out of both sides simultaneously. That’s called the sum. A signal can live entirely on the right or the left or in the middle, but it can also lie somewhere in between.
If we were to add the left and right together, we would have a mono-summed signal. And here’s some simple algebra. Turn that mono-summed signal into a negative number by inverting its phase.
And by adding it back into the mix, you can erase anything that lived in the center. Remember that the next time someone tells you phase doesn’t matter.
