Post

Some filters invert the signal. This is the cause of the stereo image flipping in mid/side chain.

Here's the maths. There should be a lot of dividing by two to make this work exactly, but for simplicity I'm going ignore that. To start with :

M = L+R
S = L - R

Normally I retrieve the stereo signal by doing the same trick again :

M + S = (L+R) + (L -R) = L
M - S = (L+R) - (L -R) = R

But if I pass S through an inverter to get

S' = -S = R - L

When I recombine I now :

M + S' = (L + R) + (R - L) = R

which is not the side I was expecting!

Incidentally, a simple way to make a high-pass from a low-pass is to invert the low-passed signal and add it to the unfiltered signal. I guess this is exactly what the Doepfer Wasp is doing.

EDITED Dec 19th 2020, 03:07