VHS capturing problem

[Trader]

I have this one very old VHS tape (about 20 years old) and I'm trying to capture it with my ati all-in-wonder 8500. Problem is that while the video looks amazingly good (cosidering the age) on my tv, it's totally jumpy and tearing on my computer through the s-video input on the ati. This has me totally stumped. This is a tape that I copied of an original so it can't be macrovision, dont even think there was macrovision back then. My system is set up so all signals go through my receiver, thus it's the same exact signal going into the s-video port of my TV as the s-video of the ati. So why does it look so good on the tv and so horrible on the computer? Anyone have any ideas?

[snowmoon]

ATI cards ofter mistake poor signal quality ( old tape ) with macrovision.

Can you describe tearing and jumping? If by tearing you mean venetion blinds ( every other line seems to line up ) you might be seeing interlacing ( this is good ).

[Trader]

No the tearing is like the top 1/8 of the pigture jumps to the side and losses synch with the rest of the picture. also I have the defeat macrovisition patch installed and it works great on dvd's and most commercial tapes I've tried. That's why this is driving me crazy. It just doesnt make sense that the same s-video signal looks so good on the tv and so horribe on the ati. Even if it was some form of macrovision, It should still look good if I'm not recording and just using the ati as a monitor. Even before I installed the macrovision patch I was able to WATCH dvd's and most tapes fine through the ati. only when playing back a recording woud everything be haywire. But here its bad all the time. In a way it sort of looks like bad vcr tracking, but then it wouoldn't look good on the tv. I'm stumped

[solarfox]

Well....

I'm afraid that what you're seeing does make sense -- it's just not something immediately obvious if you don't understand the way a TV actually works.

A television set sweeps its electron beam back and forth across the screen with electromagnetic deflection coils, using a "horizontal sweep" signal generated by a sawtooth oscillator. (I.E. it has an output that looks something like this: _/|_/|_/|_ ) While this oscillator is periodically reset by, or compared against, the horizontal-sync pulses on the incoming video signal, it spends most of its time free-running -- rather like the electronic equivalent of a spinning flywheel. And, like a flywheel, this oscillator has "inertia" (because of the expanding and collapsing magnetic fields in the coils, and in the high-voltage transformers that are part of the system) and tends to resist sudden changes in the signal timing -- so the picture tends to remain stable even if the signal timing drifts off for a few lines and then comes back into sync again.

A VCR or video-capture board, of course, doesn't have to worry about sweeping electron beams around, so their horizontal-sync circuits are built entirely differently. No deflection coils, no high-voltage transformers, or other devices whose expanding-and-collapsing electromagnetic fields would provide that "flywheel effect" to help stabilize the image when the sync signals drift off of their proper timing. Instead, they respond directly to each sync pulse as it occurs -- and if you feed it mis-timed pulses, the effect is a mis-timed picture.

Most likely, what happened is that either your incoming video signal was poor to start with, or the VCR it was originally recorded on tended to screw up the sync when it switched heads. (This was/is a common problem with 2-head VCR's; as one head rotates beyond the range of the tape and the second head rotates into range, sometimes there would be a few microseconds where the signals would either overlap each other, or where neither head was close enough to the tape to fully record the video signal.) Your television set helps mask this effect because of the tendancy of its "electromagnetic flywheel" to smooth out sudden shifts in the sync pulses, but your video capture board can't do the same.

If your signal is this far out of whack, you may need a timebase corrector (TBC) to fix it. In a nutshell, TBC's work by recording the entire video signal into digital memory, including all of the sync pulses, then analysing the timing shifts on each line of video, realigning everything back to its proper timing, and outputting a brand-new copy of the properly-realigned signal. Unfortunately, TBC's ain't cheap. You can expect to pay at least $700 for even the cheapest models, and they only go way up from there...

However, there is one other thing you might try if you have (or can borrow) a digital camcorder (mini-DV or Digital-8 ) with analog-video inputs and a firewire output, and a firewire card in your computer. Believe it or not, I've had decent results at "correcting" errant video by dubbing it onto my Sony TRV-103 Digital-8 camcorder, then capturing the DV video through the firewire port. I suspect (though I can't prove, since I don't have access to the schematics, of course!) that this camcorder might have some kind of single-line timebase correction on the analog inputs...

[Trader]

Wow thanks alot for the lesson. Always like to learn something new. I would have driven myself crazy trying to figure this out. The tape was recorded on one of the first generation top loading 2 head vcr's. I'm going to try what you suggested with the camcorder. Obviously the other option is out of my price range just to correct some old tapes. Atleast now I have an answer for someting bugging the heck out of me. Thanks again.