Originally Posted by timtape ➡️

I tried that too. It helps but only partially. Again the problem I first heard and then saw on the Spectrograph is multiple cancellations (comb filtering). Interestingly the cancellations appear in each channel but as slightly difference sets of frequencies. If it was originally a mono recording, the different sets of cancellations left and right make no sense. I dont know for sure what caused it. Possibly a transfer or post production issue.

Spectrograph of left and right with vertical axis zoomed to show different null frequencies clearer.