My pal and reader Gian Nicola asked about the pros and cons of M-S, or Mid-Side, stereo recording so I thought I’d respond with a passage from the upcoming 3rd edition of The Recording Engineer’s Handbook (due to be released in October).
“M-S stands for Mid-Side and consists again of two microphones; a directional mic (an omni can be substituted as well) pointed towards the sound source and a figure 8 mic pointed towards the sides. Once again the mics are positioned so their capsules are as close to touching as possible (see the graphic on the left).
M-S is great for stereo imaging, especially when most of the sound is coming from the center of the ensemble. Because of this, it’s less effective on large groups, favoring the middle voices that the mics are closest to.
M-S doesn’t have many phase problems in stereo, and has excellent mono compatibility which can make it the best way to record room and ambience under the right circumstances. In many cases it can sound more natural than a spaced pair, which is covered later in the chapter. If the source is extra large, sometimes using M-S alone will require too much distance away from the ensemble to get the whole section or choir into perspective, so multiple mic locations must be used. If a narrower pickup pattern is required to attenuate the hall sound, then a directional mic such as a cardioid, or even a hypercardioid, will work for the “M” mic. Just be aware that you may be sacrificing low end response as a result.
For best placement, walk around the room and listen to where the instrument or sound source sounds best. Note the balance of instrument to room, and the stereo image of the room as well. Once you have found a location, set up the directional mic where the middle of your head was.
M-S Decoding
Listening to either of these mics alone may sound OK, or may even sound horribly bad. That’s because in order to make this system work, the mic’s output signals need an additional decoding step to reproduce a faithful stereo image. The directional creates a “positive” voltage from any signal it captures, and the bi-directional mic creates a positive voltage from anything coming from the left, and a negative voltage from anything coming from the right. As a result, you need to decode the two signals to create the proper stereo effect.
While you can buy an M-S decoder, you can easily emulate one with 3 channels on your console or DAW. On one channel, bring up the cardioid (M) forward-facing mic. Copy the figure 8 mic (S) to two additional channels in your DAW. Pan both channels to one side (like hard left), then flip the phase of the second ‘S’ channel and bring up the level until the two channels cancel 100%.
Now pan the first ‘S’ channel hard left, the second “S” channel hard right, balance the cardioid (M) channel with your pair of “S” channels and you have your M-S decode matrix.
A nice additional feature of this method is that you’re able to vary the amount of room sound (or change the “focus”) by varying the level of the bi-directional “S” mic.”
