VCRs and speed stability

[echo1434]

I hope I'm posting this in the correct section...

I'll try to keep this simple, but I've always wondered why VCRs virtually always play back tapes at the perfect (original) speed. Even high multigeneration tapes don't seem suffer from this problem.

On the other hand, many audio cassette decks have major problems with playback speed — way to slow, way too fast, and everything in between - sometimes ever-changing during playback. Not to mention all kinds of crazy wow and flutter problems, etc.

I realize these problems are largely absent in the higher end audio cassette decks, but most decks don't really fit into that category. On the other hand, even the lowest end VCRs seem to always play at the right speed, or at least extremely close to it (a normal person would never detect that the pitch of the audio was off).

I'm sure there can be exceptions to this rule, but there's obviously something about VCRs that are much more "precision built," in this regard and I've just never understood how and why this is. I've heard hundreds of off-speed audio tapes in my life, but I don't think I've ever seen an off-speed VHS tape. If I did, either the tape was damaged or the VCR was broken.

So basically, what I'm asking is, how is this, and why? Being an audio hobbyist who digitally archives all kinds of old tapes, it's always boggled my mind!

[edDV]

VCRs run synchronized to a 3.58 MHz (or multiple that) clock. Record circuits sync to the color burst on the incoming video. Consumer VCR decks sync to the internal oscillator during playback. Pro VCR decks sync to an external 3.58 MHz reference (called "black burst").

Still VCR linear audio tracks have wow and flutter similar to cassette decks. The "Hi-Fi" tracks are encoded into the video so are less affected by capstan speed variations.

[davideck]

There are no sync signals in an analog audio cassette recording that identify the correct playback frequency. If the record speed was too fast or too slow, the playback deck has no way of knowing or adjusting.

A video recording contains sync signals that precisely identify what the playback frequency should be. The playback VCR can adjust to align the frequency of these sync signals to the 3.58 MHz reference that edDV mentioned.

[echo1434]

Still VCR linear audio tracks have wow and flutter similar to cassette decks. The "Hi-Fi" tracks are encoded into the video so are less affected by capstan speed variations.

Yes, you're right about that. I was generalizing too much there — I was just mainly wondering why the overall speed was always so darned consistent.

I really had no idea about the synchronization to the color burst and all. Very fascinating, thanks everyone.

But I can't help but wonder, could there have been a similar way to make all audio cassette decks, even the crappy ones, be more consistent in the way VCRs are in terms of playback speed?

If there was, I suppose the added technology (in others words, extra expense) was what would have derailed this possibility(?)

[edDV]

I think davideck answered that question.

There are no sync signals in an analog audio cassette recording that identify the correct playback frequency. If the record speed was too fast or too slow, the playback deck has no way of knowing or adjusting.

Audio recorders intended to support video production need either a good crystal oscillator or better a SMPTE tiime code track.