Farview said:
The hi fi signal is encoded into the video signal. If you are recording hi fi, you are recording on the video portion.
Actually, it isn't encoded into the signal. You're thinking of Beta Hi-Fi. VHS Hi-Fi decks typically have 6 or 7 heads: 4 video, 2 Hi-Fi audio, and an optional flying erase head. (Some cheaper ones have 2 video, 2 Hi-Fi audio.) The video heads don't necessarily have to be laying anything down on the tape (noise notwithstanding).
IIRC (and I could be wrong here), only two video heads are used during recording and normal playback; the other two are used for pausing, slow-mo, and possibly cue/review to minimize signal loss caused by the difference in tape speed and the video signal not quite lining up with the main two heads.
Anyway, normally, the audio gets laid down first in a way that records at the entire depth of the magnetic material. Then the video heads fly by and effectively erase the upper layers of the audio. I'm not sure how they avoid video bleeding into the audio. Supposedly the heads are shaped in a way that minimizes the overlap, but.... Anyway, if the tracking is adjusted even slightly because of the video signal, the audio heads will no longer be in perfect alignment. I -think- that's the cause of the motorboating, but part of it might be leakage. I don't remember. It has been a while.
Most decks should be able to generate a fake edge sync if there is no video signal going in, but not all. For decks that can't, tracking will be a problem, as VHS largely depends on the sync pulse being laid down at one edge of the tape (I forget which) to track the signal, unlike such formats as 8mm and DV that actually do tracking based entirely on the helical signals. That's why a slightly cauliflowered edge will totaly ruin a VHS tape.
Oh, and I think I actually did do an audio-only recording with an HR-S6700U (or maybe with the somewhat crappier S6900U) back in the late 90s. I don't remember why, though....
You can read more about VHS audio (including some comments on the subject of using VHS for audio recording with no video) at
http://www.faqs.org/faqs/AudioFAQ/part7/.