MS: Middle and Side

M is the mono sum of two stereo channels, and S is the difference between them.

M = (L + R) ÷ 2
S = (L + (-R)) ÷ 2

L = (M + S) ÷ 2
R = (M + (-S)) ÷ 2

Generating fake MS from a mono signal

Split the output of the mono source into two paths and route one to the main output, panned centrally so it is equal on both left and right; this is the M element.
The second path needs to be high-pass filtered around 100Hz or so and delayed by about 7ms or more (not less). This is the fake S element. Take the output of the delay and split it in two again. Route one output to the left mix bus where it will combine with the M signal, and the other to the right mix bus but with inverted polarity. Thus we have M+S on the left mix bus and M+(-S) on the right mix bus. Ideally, these two S signals should be routed through a stereo channel so that they can both be faded up and down together.
Ensure the levels of +S and -S are exactly equal so that if monitored in mono they cancel each other out completely.
With the fake S channels faded out you have the original mono source as a central phantom image. As you fade the S channels up the image will appear to broaden out and become far wider. As the level of the S signal approaches that of the M signal the source will fill the entire space between the speakers.
Pushing the level of the S signal beyond this point will result in a vague and diffuse image that is impossible to localise. Listeners in mono will only ever hear the original mono source, since the S signal is designed to cancel out completely in mono.

Separating the centre and sides of a stereo signal

To convert you need to create sum and difference signals using the various mix busses and phase-inversions.
Starting with left and right…

(L+R)-3dB = M
(L+ (-R)) -3dB = S

The -3dB level shift is important to avoid running out of headroom. The -R is a phase inverted version of R.
To convert MS back to LR do the same again…

(M+S)-3dB = L
(M+(-S))-3dB = R

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