M/S stereo: decoding two recorded signals into left and right

X/Y uses two directional microphones in a coincident or near-coincident arrangement, and the recorded channels normally become left and right directly. M/S instead keeps a forward Mid signal and a sideways figure-eight Side signal. These are not yet L/R and must pass through a decoding matrix before stereo playback. Both methods can place their capsules close together; the main difference is how the sound field is represented and whether width remains adjustable during decoding.[1][3]

When the positive Side lobe faces left, the basic equations are L = M + kS and R = M − kS. The coefficient k is the Side gain and sets its contribution to each output channel.[1] Different decoders may use 0.5, 1/√2, or another normalization to manage output level, so the equations express the sum-and-difference relationship rather than a universal gain structure. To build it manually, route M to both L and R, duplicate S, send one copy left and an inverted copy right, and adjust the two S gains together.

In the DAW, keep M and S as separate mono tracks and route them to a decoding bus with two inputs and two outputs. D-PZM Decoder’s current Stereo mode performs the M/S-to-L/R matrix at this point.[6] Logic, Pro Tools, Nuendo, and other hosts build buses differently, so a universal instruction such as “put it on the Master” would be misleading. Confirm the input order, Side polarity, and output channels before monitoring.