[78-L] Videotape as a time reference (was Re: Concert pitch)

[Mike Richter]

Michael Shoshani wrote:

> Assuming that the original tape isn't stretched or otherwise
> dimensionally changed, yes. I believe videotape has some sort of
> sync-pulse signal to maintain correct playback speed relative to the
> recording speed.

Helical-scan videotape presents one field per scan, hence is usually 
synchronized at 50 or 59.94 fields per second. Therefore, audio speeds, 
even with linear tracks, are accurate so long as there is a video 
signal. Problems can occur on high-generation copies and some of those 
cannot be eliminated even with a good TBC, but in general one can count 
on audio speed matching that recorded regardless of tape stretch and 
other factors.

There are many formats for video signals beyond the expected NTSC/PAL; 
one likely to be confusing is Brazilian NTSC which is 50 fields per 
second. There are also several flavors of SECAM, but all I have 
encountered are at 50.

Note, too, that linear video tracks are subject to amplitude variation 
due to the usual tape warping. That does not affect speed except in 
severe cases with IM problems.

Mike
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mrichter at cpl.net
http://www.mrichter.com/