Mid/side stereo and the M/S matrix

Mid/side (M/S) is a way of recording and handling stereo with two microphones whose output is not left and right but sum and difference. It looks like a detour, but it gives you something rare: a stereo width control that works after the recording.

The microphone pair

• The mid microphone points at the source. It is usually a cardioid, sometimes an omni or wide cardioid.
• The side microphone is a figure-8 with its axis pointing sideways, at 90° to the mid. Its two lobes have opposite polarity: with the decoding below, the positive lobe points to the left: sound from the left gives a positive side signal, sound from the right a negative one.

Both capsules sit as close together as possible, so the pair is coincident and there are no time differences, only level differences. That makes the sum mono compatible, see Mono compatibility.

The matrix

To listen in left/right, the two signals are combined:

Left = Mid + Side
Right = Mid − Side

In a mixing console this is done with a polarity-inverted copy of the side channel, plus two pan pots set hard left and right, or with a dedicated M/S decoder. If you feed an ordinary XY signal in instead, you get the reverse:

Mid = (Left + Right) / 2
Side = (Left − Right) / 2

So an M/S signal can always be converted to XY and back without losing anything.

Adjusting the width

The ratio of side to mid sets how wide the image is:

• Raise the side level: the image gets wider and more diffuse.
• Lower it: the image narrows. With the side at zero you have plain mono.

Because the controls act on the recorded signals, you can change the width at the mix stage, which is why M/S is popular for broadcast, film and anywhere the final width is not known in advance.

What the patterns do to the recording angle

The recording angle of an M/S pair depends on the mid pattern and on how strongly the side is mixed in. A cardioid mid with a figure-8 side behaves like a pair of cardioid or hypercardioid microphones in XY, with the angle depending on the side gain.

Pitfalls

• A figure-8 side microphone picks up the room strongly from the sides and rear, so too much side level exaggerates the reverberation.
• Check the polarity of the matrix: if left and right are swapped, the side channel's polarity is inverted (or the side microphone faces the wrong way). Reversing the side only swaps the channels; the mono sum stays 2 · Mid, so a collapse in mono points to a different problem: one of the decoded outputs (left or right) inverted after the matrix, which turns the mono sum into 2 · Side.
• Keep the mid and side gains well matched while monitoring.

Draft: written by an AI assistant, awaiting technical review.