Mid/Side recording has a long history. originally devised by EMI engineer, Alan Blumlein, an early pioneer of stereophonic and surround sound, the stereo method was patented in 1934 and used on some of the first ever stereophonic recordings. Because it is mono-compatible, the technique been used most extensively in broadcasting. The M/S technique gives users more control over the width of the stereo spread and allows editors to make adjustments at any time after the recording is finished. In television, this can help when matching audio to tight and wide video images.
While XY recording requires a matched pair of microphones to create a consistent image, M/S recording often uses two completely different mics, or uses similar microphones set to different polar pickup patterns. The “Mid” microphone is set up facing the centre of the sound source. Typically, this mic would be a cardioid or hypercardioid polar pattern. The “Side” mic must be a figure-8 pattern. This mic is aimed 90 degrees off-axis from the sound source. Both mic capsules should be placed as close as possible, typically one above the other, with their centres in alignment, to prevent time arrival differences, and therefore phase issues in the recording.
The concept is, that the Mid microphone acts as a centre channel, while the Side microphone’s channel creates ambience, and directional information, 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 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 the Side signal must be split into two separate channels. This can be done either in a Digital Audio Workstation via software, or a hardware mixer by bringing the Side signal up on two channels and reversing the phase of one of them. The two separate side channels are panned, one side hard left, the other hard right. The resulting two channels represent both sides of what ythe figure-8 Side mic is hearing.
Now there are three channels of recorded audio — the Mid centre channel and two Side channels — which must be balanced to recreate a stereo image. 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. The really cool part is, that the width of the stereo field can be altered by adjusting the amount of Side channel in the mix!
An instrument at dead centre (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 centred image. A sound source, 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.
One of the biggest advantages of M/S recording is flexibility. Since the stereo imaging is directly dependent on the amount of signal coming to the side channels, raising or lowering the ratio of Mid to Side channels will create a wider or narrower stereo field. The result is that you can change the sound of the stereo recording after it’s already been recorded, something that would be impossible using other traditional stereo recording techniques.
Another great benefit of M/S miking is that it provides true mono compatibility. Since the two Side channels cancel each other out when you switch the mix to mono, only the centre Mid channel remains, giving you a perfect monaural signal. And since the Side channels also contain much of the room ambience, collapsing the mix to mono eliminates that sound, resulting in a more direct mix with increased clarity.