so for the past month i've been leaving old vhs movies on in the background while i work
i noticed that several tapes had faded audio that got worse over the course of the film, with a lot of humming in the background, to the point of being inaudible after a while. a few web searches suggested either a worn out vhs tape (which I feared), or a poorly tracked vcr (I adjusted the tracking, which didn't improve anything)
i did notice that my older JVC vcr never had the "stereo" light on when playing back movies, and had a suspicion:
so holy crap, til: most VHS tapes past a certain age have two audio tracks: an analog mono track, and a stereo hi-fi track
when the vcr can't track the stereo hi-fi track properly, it switches to the analog mono track on the edge of the tape.
my vcr was always downgrading to the analog track, which on several (ex-rental) tapes had degraded due to mishandling and abuse (being at the edge of the tape path)
i picked up a Sony SLV-778HF today for $20 just to see if switching VCRs would make a difference.
holy COW is this a huge improvement over my old 80s JVC vcr! not only did it pick up the stereo hi-fi track on all of the "bad" tapes perfectly, but it improved the video so much that it looks like a dvd.