Re: Weird behavior of this audio track
Reply #1 – 2022-12-12 16:03:57
Down-mixing a stereo recording to mono is frequently problematic. Even if you don't get total cancellation (as in this case), the result may lack some frequency components where that element of the track happens to be in anti-phase between the L and R channels. Not recommended if it can be avoided.
In the studio, each individual voice or instrument is recorded to a separate track with the minimum possible cross-talk. The multi-track recording is then mixed down to L and R channels as desired, with each track given an appropriate position in the stereo field. This might be suitable to then down-mix to mono, because there will be no phase difference per track in the L and R channels. However, if it is a live stereo recording made with just two microphones, there is a very good chance of anti-phase signals according to the frequency and position of the sound source. That is not a problem when played back in stereo, but can't reasonably be mixed to mono. Synthesised sound could be even worse, if phase is used to exaggerate the stereo image. If a recording is ripped from vinyl, the L and R channels come from the stylus cartridge and the stylus itself is moving in directions L+R and L-R. The cartridge (by its construction) extracts L and R from those. According to how it is wired, that might produce L and R or L and -R (perm any combination of + and -). I suspect the rip has recorded L and -R (or whatever), and as the bulk of the signal will be common to L and R, when you sum L and -R of course you get nothing. The solution, as you have found, is to invert one of the channels before summing BUT unless you are dealing with a recording which contains a strong stereo separation, you will be just as well off (and possibly better) just to take either L or R as your mono channel and not mix them.