How It Works
It’s not uncommon for musicians to be intimidated by the complexity of MS recording, and I’ve watched more than one person’s eyes glaze over at an explanation of it.
But at its most basic, the MS technique is actually not all that complicated. The concept is that the Mid microphone acts as a center channel, while the Side microphone’s channel creates ambience and directionality by adding or subtracting information from either side.
The Side mic’s figure-8 pattern, aimed at 90 degrees from the source, picks up ambient and reverberant sound coming from the sides of the sound stage.
Since it’s a figure-8 pattern, the two sides are 180 degrees out of phase. In other words, a positive charge to one side of the mic’s diaphragm creates an equal negative charge to the other side. The front of the mic, which represents the plus (+) side, is usually pointed to the left of the sound stage, while the rear, or minus (-) side, is pointed to the right.
The signal from each microphone is then recorded to its own track. However, to hear a proper stereo image when listening to the recording, the tracks need to be matrixed and decoded.
Although you have recorded only two channels of audio (the Mid and Side), the next step is to split the Side signal into two separate channels. This can be done either in your DAW software or hardware mixer by bringing the Side signal up on two channels and reversing the phase of one of them. Pan one side hard left, the other hard right. The resulting two channels represent exactly what both sides of your figure-8 Side mic were hearing.
Now you’ve got three channels of recorded audio– the Mid center channel and two Side channels – which must be balanced to recreate a stereo image. (Here’s where it gets a little confusing, so hang on tight.)
MS decoding works by what’s called a “sum and difference matrix,” adding one of the Side signals—the plus (+) side—to the Mid signal for the sum, and then subtracting the other Side signal—the minus (-) side—from the Mid signal for the difference.
If you’re not completely confused by now, here’s the actual mathematical formula:
Mid + (+Side) = left channel
Mid + (-Side) = right channel
Now, if you listen to just the Mid channel, you get a mono signal. Bring up the two side channels and you’ll hear a stereo spread. Here’s the really cool part—the width of the stereo field can be varied by the amount of Side channel in the mix!
Why It Works
An instrument at dead center (0 degrees) creates a sound that enters the Mid microphone directly on-axis.
But that same sound hits the null spot of the Side figure-8 microphone. The resulting signal is sent equally to the left and right mixer buses and speakers, resulting in a centered image.
An instrument positioned 45 degrees to the left creates a sound that hits the Mid microphone and one side of the Side figure-8 microphone.
Because the front of the Side mic is facing left, the sound causes a positive polarity. That positive polarity combines with the positive polarity from the Mid mic in the left channel, resulting in an increased level on the left side of the sound field.
Meanwhile, on the right channel of the Side mic, that same signal causes an out-of-phase negative polarity. That negative polarity combines with the Mid mic in the right channel, resulting in a reduced level on the right side.
An instrument positioned 45 degrees to the right creates exactly the opposite effect, increasing the signal to the right side while decreasing it to the left.
