Attempting to reverse mid conversion

Question

I'm new to to DSP in general and have been trying my hand at making some very simple VST's just for practice purposes.

I am trying to convert the stereo channels into Mid (in the left channel) and Side (in the right) without using a placeholder to store output. Here is a very basic version of the code I am currently trying:

    *LeftSample = (*LeftSample + *RightSample) * 0.5;
    *RightSample = (((*LeftSample * 2) - *RightSample) - *RightSample) * 0.5;

The first line assigns the mid output to the left channel, but in the second line I am trying to reverse the mid conversion so that I can use the left channel's original output when performing the L-R operation to create the side channel in the right output.

Am I missing something in thinking that this should be achievable? Is there some reason that

    (*LeftSample * 2) - *RightSample

does not reverse the mid conversion, (L+R) * 0.5? Is it not possible to derive the original left channel using the mid signal and the right signal?

Again, I'm new to this, so I apologize for any errors in terminology I have made, and for anything that has been poorly communicated. I appreciate all feedback and criticism (please, tear any errors I've made to pieces). I've looked around quite a bit, and haven't had any luck; if anyone has any resources or directions to point me in, I would greatly, greatly appreciate it!

Answer 1

You can fully reconstruct L and R from M and S, as you suspect. In your case:

$ M = {{L + R} \over 2} $

$ L = 2M - R $

$ S = {{L - R} \over 2} $

then

$ S = {{2M - R - R} \over 2} $.

So the math checks out. Are you not getting the results you expect?

When I've worked with M&S in the past, I've alyways just done the scaling by 1/2 when returning to L&R. I guess it depends on what you're intending on doing with the M&S signals though. If you want the sum of M and S to have equal energy to the sum of L and R, you should scale by $ 1 \over \sqrt 2 $ when converting both to and from M&S.