You’ll need two mics: one cardioid, and one bi-directional (“figure 8”). Ideally you want two similar mics, but this isn’t essential; experiment with whatever mics you have that meet the polar pattern requirements.
M-S uses the center or “mid” mic in combination with the bi-directional mic to achieve stereo. As the cardioid mic points right at the sound source, it picks up the direct sound, while the off-axis bi-directional mic picks up the room ambience and reflected sound. M-S stereo “sum and difference” is just the center mic plus the side mic for one channel, and center mic minus the side mic for the second stereo channel, with the center mic being positive polarity and common to both sides. As the left and right sides originate from the same mic, but with the phase inverted, collapsing an M-S recording to mono cancels out the left and right sides from the bi-directional mic, leaving only the positive polarity signal from the center (cardioid) microphone. This significant advantage of M-S recordings insures perfect mono compatibility without any phase issues.
Next, place the figure 8 mic so that the two lobes of the pattern are set 90° relative to the cardioid microphone. M-S is a coincident microphone technique, so you want to get the diaphragms of the two mics as close together as you can. Figure 1 shows a Soundelux E250 (bottom) and ELUX 251 (top) set up as a M-S pair. The cardioid E250 is pointed at the sound source (in this case, the camera), while the pattern selector on the ELUX is set to bi-directional; it’s picking up to the left and right, and its side null point points directly at the sound source/camera.
Now group the two “SIDE” tracks and pan them hard left and right. As you raise the level of the side mic tracks, the stereo width will increase; lowering them decreases it. Being able to adjust the amount of stereo information in the recording after the fact is one of the big ...