Here's an interesting curiosity. As you know, when compressing a stereo signal, a two-channel compressor must
have its sidechains linked, otherwise heavy compression in one channel will cause an image shift in the stereo
sound stage. Both channels must, at all times, be compressed equally. Of course, this assumes that you are
handling stereo as left and right channels - let's call this LR stereo. Not as popular but certainly very useful is
mid-side or MS stereo, where the M channel is the mono sum of the whole sound stage and the S channel
represents the difference between left and right. MS is a useful microphone technique and is sometimes used at
other points in the signal chain for modifying the width of the stereo image. (It's a funny thing that proponents of
MS often forget that you can do that to LR stereo signals with the pan controls.) But what about compressing a
signal in MS format? Is it possible? Does it have anything new to offer?
Yes, it is possible to compress MS signals without converting them to LR. Just pass the M signal through one
channel of the compressor and the S signal through the other. Once again, you will need to link the sidechains or
funny things will happen, but it will all work perfectly. Some might say that it works better than compressing LR
stereo since, even when sidechains are linked, it is not guaranteed that analog compressors will handle both
channels absolutely equally and some image shift may persist. But, if you compress in MS domain then any
disparity between the channels will result not in an image shift, but in a variation in the width of the stereo image,
which is arguably less obtrusive. But why not take this a stage further and do something really wacky like
compressing the S signal only. What happens now? If you compress the S signal only, then anything panned
center is unaffected and compression only affects signals panned left or right, or signals that are out of phase.
Loud signals in these modes will cause a momentary reduction in level of the S channel resulting in a narrowing of
image width. I can't say that I recognize any useful function for this myself, but in the hands of more creative