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We're used to thinking of stereo in term of a left and a right channel, but it's also possible to represent it as a sum and a difference channel, or what audio engineers call the M ("mid" or "mono") and S ("side" or "stereo") channels. (For you mathematicians, this is just a change of basis in the vector space of stereo.)
It is sometimes desirable to adjust the levels of M and S independently, or even to apply separate processing to these channels. Stereo Image Munger is a small Swiss army knife for this sort of processing.
There are three typical ways of using it:
Stereo Image Munger 1 -> (filter) -> Stereo Image Munger 2.Set SIM 1 to translate L/R to M/S. Apply filtering in M/S space — EQ on just the S channel, for example. Then set SIM 2 to translate M/S back to L/R.
You can also create novel (if not entirely useful) effects by outputting M/S and leaving it there in the mix, or by outputting odd combinations such as L/S. Inverting channels by setting a negative gain can be fun. Experiment and enjoy.
M = (L + R) / 2 S = (L - R) / 2 L = M + S R = M - S
If the input L/R signal does not clip, then the M/S will not clip either. However, be warned: the reverse is not true — you can have M and S both staying below 0dB, and still have L or R jump above it. You have been warned.
Enjoy and use in good health. Patches and suggestions are welcome, though there's no guarantee I'll get to them in a hurry.