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| cybertheque |
| Posted: Aug 8 2010, 05:34 PM |
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Advanced Member
Group: Members
Posts: 58
Member No.: 24377
Joined: 16-October 08
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A big problem in digitizing old video tape is repairing dropouts, especially those that look like a horizontal noise bar that moves from the top of the frame to the bottom over several seconds (dependent on tape speed) and which are due to a wrinkle or crease in the tape that affects head contact. I am unaware of any dropout compensation hardware for (s)-vhs decks, but optical media players, such as laserdisc players, often have hardware (sometimes as part of an integral tbc) which stores previous frame data (in the case of laserdisc, analog values) and provides replacement scan line information when signal thresholds from the head drop below some set value indicating a dropout. Heuristics seem relatively straightforward to do something similar on digitized data, even if it is tests of periodicity of horizontal noise or luminance values.
Has anyone tried to build such a filter for virtualdub?
Michael
EDIT: exploring this further... here's a quote from a 'digitalfaq.com' forum post:
"We'd been discussing hardware restoration methods, with most talking centering around the Digital Processing Systems DPS-200 timebase corrector (TBC) that comes with dropout compensation built into the unit.
However, from my tests, that wasn't always enough. It really depends on the tape and the age.
Quote:
If the TBC doesn't take care of your drop-outs entirely, there are some software methods that work to further fix that situation. It's not without its quality drawbacks, however. And of course, I was asked for some details on this.
Quote:
what program do you use on the computer for dropouts? and that's where the emails ended...
It's not so much a program as a method.
One of the easier methods is to load a lossless/uncompressed AVI into VirtualDub. Now you can pre-deinterlace it in Avisynth, or you can use Yadif or Deinterlace Area-based in VirtualDub. You have to convert the video to progressive. This is where the loss comes in -- in motion, resolution/detail, and odd digital artifacting. But sometimes that is the lesser evil than dropouts.
Once the footage is progressive, play with the VirtualDub Median filter.
Median averages a span of pixels, and determines if something is "amiss" and replaces the dropout with pixels from neighboring frames -- pretty much the same thing that the TBC does. However, the TBC operated in an interlaced analog domain. Then again, the software filter is much stronger. You lose strength either way.
It takes time and experiments, and I hope you have faster computers, too. At least a dual-core, preferably a quad-core, to encode with any sort of speed." |
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| phaeron |
| Posted: Aug 10 2010, 04:56 AM |
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Virtualdub Developer
Group: Administrator
Posts: 7773
Member No.: 61
Joined: 30-July 02
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There might be a benefit to making a filter explicitly tailored for dealing with VHS tape, since such dropouts will manifest themselves as slowly going up (down?) the frame across a number of fields due to the helical scan. Such a method would likely require manual intervention to guide the compensation, though.
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