9. M/S processing
You might think that the left and right channels are always treated the same during mastering. That is not necessarily the case. A technique called M/S processing, which originated in broadcasting and may sound familiar to you from some stereo micing patterns, lets you turn the stereo left-right signal into a so-called mid-side signal (M/S), using software or hardware M/S matrices. Like stereo, it consists of two channels, but one channel (M) contains only the information in the center of the stereo field, while the other channel (S) contains the stereo information. The advantage of this technique is that you can process the mid and side channels separately and then convert them back to a normal left-right stereo signal.
This is where some ingenius possibilities arise. By adjusting the level of the S signal in relation to the M signal, you can adjust the basic stereo width. The entire „stereo stage“ of the final mix can be made more narrow or wider. And once you have the M and S signals separated, you can for example process the outlying guitars with an EQ, without affecting the bass drum, snare drum, vocals or other instruments in the center. You can also compress M and S individually. If you consider what a dynamic EQ or a multiband compressor is capable of when used on an M/S signal, it becomes obvious what level of sonic refinement can be achieved during the mastering process.
