Mid/Side processing inverts stereo image unintentionally

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?

EDITED Dec 18th 2020, 23:27

Hi Klngvrhltnss,

I have difficulties to pronounce your name ;-)

To be honest, I don't get your point here. I am most probably not understanding what you try to say: How can a mono-filter like the Doepfer A-124 Wasp filter inverts your stereo image. I am having difficulties to understand that...

Kind regards, Garfield Modular.

Hi @GarfieldModular, I'm splitting the stereo signal into its sum (all info, that is the same in L and R) and its diff (all info, that is different in L and R) signal. Those are mono signals of their own, and can be combined, after processing, to form a stereo signal again. That's the wonders of Mid/Side processing, a commonly used mastering techniques. In Eurorack you can do that with modules such as the Worng LRMSMSLR or Shakmat SumDif.

EDITED Dec 19th 2020, 11:29

Hi Klngvrhltnss,

Thanks a lot for the information and the hint, I will give that a try with my SumDif module :-)

I read the link you provide, not all yet, however one question arises... how to take a voice more into the front of the stereo view and how to put for example another (less important sound perhaps) to the back of the stage?

Sorry, that I can't help you with your question. Thank you very much and kind regards, Garfield.

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

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.
-- the-erc

Thanks @the-erc for explaining! This makes perfect sense. Good to know, that it's not a malfunction but a principle based behaviour.

@GarfieldModular Let's talk about tips and tricks for Mid/Side processing in Eurorack in another thread.

EDITED Dec 19th 2020, 11:43