MS Stereo Basics

Sound Devices 442 and 302 mixers along with the 7-Series Digital Recorders have the ability to accept, monitor, and decode MS stereo signals. MS (mid-side) stereo is a popular stereo microphone technique because of its good spatial imaging, mono compatibility, surround-sound compatibility, and ability to adjust the stereo field in post production. MS stereo is a two microphone capsule technique using a primary microphone as the mid-signal and a bi-directional microphone for the side-signal. A stereo signal is generated combining these two microphones, with different polar responses, in an electrical matrix. Whether decoded during recording or in post production, the amount of stereo spread can be adjusted, a great benefit of M/S stereo over other stereo microphone techniques.

M = Mid (Middle, Main)

The choice of mid signal microphone is arguably more critical than the selection of side microphone. The mid-signal microphone is often a flat-frequency-response cardioid {ρ = a(1 + cos θ)}, but can be nearly any polar pattern, including hyper-cardioid, super-cardioid, line-gradient (shotgun) or omni-directional. Because of the inherent mono compatibility of MS stereo, when an MS stereo signal is folded to mono, only the Mid microphone signal remains.

S = Side

In very close proximity (coincident) to the mid microphones element, a bi-directional microphone element is placed at a right angle to the Mid microphone. The side microphone signal is added to the mid microphone to construct the stereo image.

M + S = Left Channel M - S = Right Channel

To construct a stereo (left/right) signal the Mid and Side signals are combined in an electrical matrix which sums the following signals:
  1. Mid signal sent to both left and right equally,
  2. Side signal sent to the left channel,
  3. Side signal with polarity reversed sent to right channel.
The on-board MS matrix in the MP-2 and 442 can decode MS stereo for monitoring in headphones or to the outputs. The microphone gain controls adjust the ratio between mid signal and side signal - varying the stereo field. There are several single point MS stereo microphones on the market from several vendors including Audio-Technica, Neumann, Pearl, Shure, Sony. Whether using an MS microphone or a cardioid and a bi-directional mic, MS is a useful stereo recording technique.