Stereo Microphone Techniques: A/B, M/S, and X/Y Setups

Common Stereo Microphone Techniques

A/B Stereo (Spaced Pair)

The A/B stereo technique, also known as the time-difference stereo technique, uses two separate microphones (often omnidirectional) to record audio signals. The distance between the microphones creates small differences in phase and timing in the captured audio, which vary according to the direction of the sound sources.

Just as human ears perceive time and phase differences to locate sounds, these recorded time differences act as stereo cues. This allows the listener to capture the space in the recording and experience an immersive stereo image of the sound field, including the position of each individual sound source and the spatial characteristics of the room itself.

Mid-Side (M/S) Stereo

The Mid-Side (M/S) system uses a cardioid microphone as the center (Mid) channel and a bidirectional (figure-eight) microphone at the same point, but angled at 90 degrees, as the side (Side) or ambient channel. The raw M/S signal cannot be monitored directly in a conventional Left-Right (L/R) setup. An M/S matrix decoder uses the phase information between the Mid and Side microphones to produce an L/R signal compatible with conventional stereo systems.

Due to the dedicated center microphone, this technique is highly suitable for stereo recordings that require excellent mono compatibility, making it extremely popular in radio broadcasting. Additionally, M/S processing is commonly used in audio mastering, allowing engineers to individually process the center (mono) and side (stereo) information to solve problems or make precise adjustments.

X/Y Stereo (Coincident Pair)

The X/Y technique is a coincident stereo miking method that uses two cardioid microphones located at the same point to produce a stereo image. Their axes are typically angled at 90°, but other opening angles between the capsules can be used, such as 120°, 135°, or even 180°, which will alter the recording angle and stereo spread.

Theoretically, the two capsules must be in the exact same point—a coincident pair—to avoid phase issues caused by time-of-arrival differences. Since this is physically impossible, the closest approximation is to place one microphone directly above the other with the diaphragms vertically aligned. This arrangement ensures that sound sources in the horizontal plane are captured as if the two microphones were occupying the same point in space.