Midside

Mid/side stereo, or M/S, is a two-microphone recording and decoding technique that yields a controllable stereo image while preserving strong mono compatibility. It uses a forward-facing cardioid microphone to capture the mid signal, which represents the mono center, and a figure-8 microphone oriented at 90 degrees to the mid mic to capture the side signal, which conveys left–right ambience and width.

The technique was developed by the British inventor Alan Blumlein in the 1930s as part of pioneering stereo research and has since become a standard approach in studios, broadcast, and field recording for its flexibility. M/S can be recorded as two separate signals labeled M and S, or as a pair of channels that can be decoded into standard left and right channels during playback.

Decoding: Left = M + S, Right = M − S. Width is adjusted by changing the relative level of the side signal S before decoding, either during recording or in post-production. A mono signal (S = 0) yields identical left and right channels, preserving mono compatibility. The setup is often achieved with a single MS microphone, a matched pair, or a modular configuration with a cardioid mid and a figure-8 side.

Applications and considerations: M/S is widely used for studio vocal and orchestral recording, film dialog, and location work because it provides flexible control over stereo width without re-recording. Advantages include strong mono compatibility, precise width control, and good isolation of the center source, though accurate microphone alignment and a suitable decoding path are essential; off-axis coloration from the side mic and the handling of the S signal in post can be considerations.