Mid-Side micing basicly a two mic near coincindent techinque that produces a three part image. you use a figure 8 mic, with a cardiod (or I've also heard of omni) in the centre.

cardiod
^
+ cap
- cap

(pretend the cardiod is facing you)

-then you bring these into you device
-get levels
-mult the figure 8 for a second channel and phase invert it
-pan them left and right

and that should be just about it.

Mid side is a sum and difference technique... You are basically summing the patterns of 2 microphones together.

You can have any pattern in the center as your mid and it faces your source directly. Then at 90 degrees you have a figure-8 capsule. The positive lobe of the fig-8 should be to the left and the negative should be to the right.

When you matrix them, you have your left-right image derived from the sum of the mid plus the side (positive) and the difference of the mid minus the side (negative)... So you have

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

Nice thing about it is that two signals that are 180 degrees out of phase when summed become null... That means you have perfect mono compatability with this technique.

--Ben

Newer understood why it is summed and inverse of phase is differenced, yes i understand this gives mono compatibility, but that is mathematical background there or from acoustic point of view, how this affects imaging?

Don't quite understand vladlv's question, but I want to chime in that, after years of reluctance, I'm a recent M/S convert, and now am a true believer.

The genius of it is the control over the extent of the stereo field. Everything from hard mono to full-on front-of-the-speakers spread is right there.

Imaging with MS is quite good.
Imagine the MS array is your head. Both ears hear what is coming from the front. The left ear hears what comes from the front, and what comes from the left side.

LEFT = M + S

The right ear hears what comes from the front, and what comes from the right side. If the back side of the figure 8 is facing right, then that signal is inverted from what you want, so

RIGHT = M - S

It works - try it.

The easiest way to explain how the stereo image appears is to first understand how a bi-directional mic works.
Since you have negatve polarity on one side, and positive on the other, duplicating the signal and flipping the phase on one of the channels in theory gives you silence.

Now, you pan them left & right and add in the cardiod mid mic until it's phase characteristics actually start reversing the cancellation from the doubled side mics.

this actually places your individual instruments in the appropriate position of the stereo field, as in your Hi-hats will appear to be coming from the left (right hander set up) and your floor tom & ride from the right. then your image goes from as small as you want it to be to as big as you want it to be.