Its actually simple, a multiband m/s processor...idea is like an eq but rather than gain (when listening a stereo track mono) it carrys frequencys to mid or side.
For example: If I move freq.s (by crossover) from mono between 1 khz to 3 khz to the stereo, when listening mono these frequencys will be heard less (like gain decrease with eq).
But besides carrying, these bands also need gain control too, for example: I did when I wrote in previous example but when listening the track stereo, 1 to 3 khz at stereo channel is too high; I might need to decrease these freq.s with a gain control.
Benefits: master can sound better in mono, end result sounds more natural than eq, better stereo separation (when used together with a "widener" stereo processor) and can be very useful for restoration.
I was thinking the gui could look like an eq...anyway if it makes sense to you let me know to provide detail if needed
