Before talking about mid side processing, I would like to give some introduction about stereo processing. During mastering, in stereo processing nothing can be changed other than Left channel and Right channel. Usually when we are listening to a something in a stereo speaker, we get a sound that comes in center because both the left and right speakers are assigned to the same amount of amplitude. Stereo is a mix of two Mono tracks having same amount of amplitude. Thus creating an illusionary image of listening at the center to the audience.
In your normal mix, the vocals, kick and snare will always be panned to center. When we raise their volume level, we get a volume to boost at the center. Similarly we can put EQ, reverb and tweak that channel alone separately.
Limitation of Stereo Processing:
In the mastering session with this stereo processing, the volume of the Vocals, Kick and Snare is going to be in the middle and will be heard clearly. But the guitar and the instruments won’t occupy much space and the sound from them is going to be narrower. To boost and for the left and right panned instruments to occupy much space, we go with the mid side processing.
Mid Side Processing:
The mid side processing is a new technique or plug in which is used to boost or cut the volume, EQ or tweak it with respect to MID and SIDE in order to enhance the quality of your mix. The concept is the channels that are panned center are assigned to MID and the channels that are panned right or left together constitutes to SIDE.
If the vocals and the tracks that are panned in center, then at most of the cases they sounds great and occupies much space whereas the guitar and other instruments occupies less space. To make this even, we can reduce the volume of MID by some 3dB and increase the volume of SIDE by some 5dB. This will occupy the full space and gives you a different good result. With the requirements of the track we can increase or decrease the volume or can add EQ, reverb such that we can occupy the wide space range.