RR 50 MidSide Processing

This section is about MidSide processing. MidSide is an alternative way of looking at stereo. We are used to think of stereo as the combination of a left and a right channel. If both channels carry the same audio signal we hear mono. The audio is positioned in the middle between our two speakers. It's only when the left and the right channel are different that we can hear stereo.

MidSide also uses two channels. The first is named Mid, it contains the audio the left and right have in common. Therefore the Mid is 100% mono. You can create a Mid channel of left and right by simply adding these two: M = L + R.
The second MidSide channel is named Sides, it contains the difference between the left and the right channel. Therefore there is no mono part present. You can create a Sides channel of left and right by subtracting these two: S = L - R.

The advantage of MidSide over normal stereo is it allows for different ways of processing the audio signal. In a stereo drumloop with kick and snare centered and hihats, cymbals and toms panned we can for example add compression to the Mid channel (kick and snare) without compressing the rest too much. Or adding a lot of reverb to the Sides Channel without washing away the kick and snare.

MidSide allows for altering the stereowidth of a signal. By reducing the level of Mid, the stereowidth will become larger. Muting Sides leaves us with 100% mono, yet another way to check what listeners will hear on mono systems.

MidSide can easily be converted back to normal stereo. Mid is panned centered, Side is panned hard left and a copy of Sides with its polarity inverted is panned hard right. Or to put it mathematically: L = M + S and R = M - S.
Of course you could also pan Mid differently if that's what your track calls for.