Mid-Side processing separates an ordinary stereo recording into its center and side elements (mid-side). M/S processing gives the engineer a very powerful set of tools for processing stereo signals in mastering and even mixdown when traditional stereo tools fall short as well as creative processing ideas that can even be used in the tracking stage.
As a jumping off point, let’s take a look at the M/S stereo miking technique. Alan Blumlein patented the M/S technique using two microphones. Instead of thinking about the stereo image as having a left half and a right half, the M/S approach essentially considers it as being comprised of central and side elements. The Mid signal is the mono sum of both left and right, and basically describes those elements present in both channels. The Side signal is the difference between the two channels and describes those elements that contribute to the stereo width.
The Mid mic often employs some form of cardioid pickup pattern, while the Side mic is always a bidirectional pattern. Blumlein’s original patent stipulated using an omnidirectional pattern microphone for the Mid mike and you could even use a bidirectional pattern microphone.