Dear Ken,
I would guess that the frames are dropped when the video noise or loss
on the tape is sufficient to disrupt the sync pulses, or maybe the tape
has stretched causing a timebase change. All modern tvs have a very
"elastic" sync separator that prevents flag waving or hooking (wavy
picture at the top of the screen) and roll due to jitter or wow and
flutter on the tape. Some older tvs used to have a special channel
number dedicated for tape use, where the synchronising was more forgiving.
I would guess that the video capture card is not quite so forgiving and
recommend the use of a VCR with a timebase corrector. These needn't be
expensive (I have a pansonic 950 that cost around £230 UK), most top end
panasonic SVHS VCRs have timebase correctors built in. (perhaps you can
borrow one?).
I don't thinks its a capture problem, its most probably going to be the
source. What you say seems to indicate this....
Regards
Pete
My machine is an Athlon 1.8 (slightly underclocked--should be 1.53 but I had
to run it at less). I'm trying to use an ATI TV Wonder card with bttv, and
am capturing video from my VCR.
My problem is this: Sometimes my capture rate is going way down. In
particular, I have some multiple generation VHS tapes that, when I try to
capture them, end up making me lose lots of frames. The frame loss at any
particular place on the tape stays the same, but can vary over the duration
of the tape--one tape consistently has a 40% loss at one spot and no loss at
all at another. There's no obvious visual distinction between the two
different parts of the tape.
I'm using bttv 0.7.106. It doesn't matter what program I use with it--I've
played in xawtv, captured using avicap, and captured using transcode. Every
one of them gives the same problem.
I am capturing to uncompressed YV12, and I'm not capturing sound at all.
When I use avicap, it claims that all the frame loss is in the capture
thread, and not in the encoder.
I can't switch to bttv 0.9 series because I'm on Mandrake 9.0 and that has
fubar'ed headers which won't let me compile any v4l2 application.
Is this really expected behavior, that some tapes have unacceptable skipping
and others capture perfectly fine? I can understand that the tape might have
more noise, but should this matter if I'm capturing uncompressed? Is it a
driver problem, and would upgrading my system so I can use bttv 0.9 help? Am
I capturing to the wrong format? (If so, then what format should I use?)
--
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