I am getting interested in building a hardware stereo image widener circuit. I have seen a few ideas floating around based on the Middle(M) +Side(S) technique. The basic premise can be summarized into the following equations:
Encoding M and S
Middle = L+R
Side = L-R
Translating that into basic op-amp circuits, the Middle signal could be created with a summing amplifier.
I am not sure if a non inverting summing amplifier will be as stable as inverting amplifier, but its a simple to test to find out. If Rf is a short and Ri is open then the Middle = Left + Right.
For the side signal a difference amplifier can be used.
If all the Resistors are the same value then Side output = Left-Right
Decoding M and S
The basic equations are as follows:
L = M+S
R = M-S
Decoding can be done using the same summing and difference circuits, just with different inputs.