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Posted by Dave Martindale on 12/17/06 20:02
"Doc" <docsavage20@xhotmail.xom> writes:
>This parade ran to a sound track - I assume digital - as well as the live
>drummers so it was exactly the same every time, but shouldn't there be some
>drift because they were recorded and then duped on consumer analog gear?
>Does running it through the digital camcorder have any bearing on this or is
>this just a fluke?
The overall timing accuracy of record and playback will be determined
by a crystal oscillator, but even cheap crystals are accurate to 100
parts per million or better. 100 PPM is 5 frames over 30 minutes. A 20
PPM crystal would reduce that to 1 frame in 30 minutes.
The thing that is unstable about consumer analog video is that the
spinning drum speeds up and slows down while trying to maintain the
correct speed. Over the long term, it's locked to the crystal, but over
the short term it gets slightly ahead or behind the correct timing.
That, in turn, means that some lines start early and some start late. A
straight vertical line in the scene will "wander" left and right
somewhat within one frame. That's what a timebase corrector corrects
for.
Dave
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