The mid-side (M-S) stereo technique is one of the two formats of “intensity stereo,” that is, stereo in which spatial localization is determined by the differences in the intensity of a sound wave as it arrives in phase at a coincident pair of microphones. Intensity stereo relies completely on the directional characteristics (polar patterns) of the microphone pair to produce this effect, since only intensity differences and not phase differences exist between the channels for any single source arriving at a coincident pair.
(Source: “M-S Stereo: A Powerful Technique for Working in Stereo” by Wesley L. Dooley and Ronald D. Streicher)
The most common situations where M/S (aka M-S aka mid-side) techniques are getting applied are:
- during the recording process when mid-side microphonie is utilized
- on the 2bus during the late mixing or mastering stage
- in audio restauration situations
I’m skipping the mid-side microphonie stuff here and just recommend “A More Realistic View of Mid/Side Stereophony” by Trevor Owen de Clercq which is available online.
While the M/S signal is obtained in a natural fashion during microphone stereophony, in most other cases stereo (L/R) encoded signals are available. In this case and to obtain the advantages of M/S a conversion is necessary.
The conversion is dead easy and is accomplished via a sum-and-difference matrix network, where typically the mid signal is the sum (M = R + L) and the side signal is the difference (S = R – L). The other way around is that simple as well and there are plenty of tools available which handle this stuff for us in the digital audio workstation.
Once a stereo source is encoded in M/S, the door is open to treat mid- and side content individually. For example, this allows for selective correction of some problems encountered on location such as out-of-phase low-frequency noise from the environment. In this case the side channel, which contains the majority of this information, can be passed through a high-pass filter to reduce such unwanted low-frequency content, and this can be done without any alteration of the mid content.
Some other typical mixing/mastering targets ideally achieved in the M/S domain are:
- assuring mono compatibility
- stereo widening and increasing depth perception
- attenuating or emphasizing the signals room information
- increasing intelligibility of voice in a mix
This is accomplished mostly by performing alterations of both channels frequency and dynamic response. The tools to be used just need to have separate controls per channel as long as M/S encoding/decoding is done externally before and after the plugin, other tools already offer internal M/S handling to make things easier: The BaxterEQ is an example for a M/S frequency shaper and Density MKII is an example for a M/S dynamics compressor.