I've been playing around with Mid/Side processing, lately. I'm using the Worng Electronics LRMSMSLR to split my stereo signal into one sum (the Mid) and one diff (the Side) signal. Thus Mid and Side can be processed independently and differently, before being converted back (added) to a usual left/right stereo signal. This can create some interesting, special and great effects. I use this method inside my system to gain a wider stereo image by processing the Side signal as follows: cutting its low end, boosting its mid frequencies and raising its volume. Works pretty well. However, I came across one problem, that made me wonder:
Running the Side signal through certain modules inverts the stereo image, i.e. sounds that are in the right channel before processing are in the left channel after processing and vice versa. I investigated that mostly with my filters: North Coast MSK 009, Doepfer A-124 Wasp, Noise Engineering Viol Ruina. Pretty interesting results. Using the MSK 009 inverts the signal, no matter if the HP, BP or LP output is used. Using the Viol Ruina outputs does not invert the signal. Using the BP out on the Doepfer Wasp inverts the signal. Using the LP/HP out on the Doepfer Wasp inverts the signal, when Mix is in LP position (ccw), but does not invert it, when Mix is in HP position (cw).
I understand how filters function in a analytical/mathematical way, but I'm definitely lacking electronical knowledge to comprehend what could be causing this different and strange behaviour.
Can anyone help to clarify?