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04-30-2012, 04:11 PM
Belmont Belmont is offline
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This is actually something I'd wondered for a while. On certain recordings, especially those with large, solid areas of color (such as cartoons or video games), there are sporadic black lines that pop up. They don't move, but flicker onscreen just barely long enough to be noticed. They're removed when the video is denoised or put through a chroma NR filter. Is this physical tape damage (ie: from pausing the tape too long, rough handling, or just a bad tape or recording), or is it from the intensity of the color being too much for the tape to record (it really only happens on strong reds, and occasionally with greens and blues). They don't appear to cause timebase errors like with regular oxide damage (you know, like the sharp gray line and a distorted black and white image, or the rolling block of static).

Included is an example of one of the spots, a filtered video showing how they are mostly removed after noise reduction filter were applied, and an image of what I know to be physical tape damage (it was an EP recording at the end of a cheap tape. Recipe for disaster, much?). Again, I'm going with the SWAT Kats tapes since I can sacrifice them in a pinch, and my more valuable tapes (such as videos of family) don't exhibit that many flaws (I've got exactly 3 family recordings on VHS, all in good condition, since my parents switched to DV tape for camcorders in the '90s, and weren't the types of parents who videotaped their children 24/7 and watched the recordings repeatedly, so we don't have stacks of damaged family VHS's like some others do, just a bunch of TV recordings ). That's another matter for another thread


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File Type: png Screen shot 2012-04-30 at 4.31.09 PM.png (269.1 KB, 58 downloads)
File Type: jpeg tapedamage1.jpeg (75.9 KB, 53 downloads)
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File Type: avi strangespots.avi (4.44 MB, 30 downloads)
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  #2  
04-30-2012, 04:28 PM
NJRoadfan NJRoadfan is offline
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The errors are quite common for VHS in reds and blues (outdoor sky shots in particular). I have several tapes that exhibit it, particularly near the beginning and end of the tape. Reds are a big weakness with VHS, they always seem to bleed and flutter around. Its very noticeable on home movie tapes too. It seems to be a minor versions of this type of tape damage (only affects chroma) that I brought up in a past thread: http://www.digitalfaq.com/forum/vide...ase-error.html
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  #3  
05-02-2012, 03:36 PM
Belmont Belmont is offline
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Well, what makes it different is that the error is always in that same spot. I've always chalked it up to too much color info, damage, or just bad tapes. There's occasionally a timebase error, but mostly, it's just a thin dark spot.
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  #4  
05-05-2012, 04:19 AM
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lordsmurf lordsmurf is offline
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First image:
It's often a mix of physical or signal damage. I've never found a way to correct this, not even with advanced filter work in Avisynth. Generally speaking, those "chroma crinkle" errors (my own term used here) are accompanied by audio fuzz/buzz in the HiFi track. Sometimes, at best, you can hide it with aggressive chroma NR software filters. Ideally, you want to correct in hardware first, which it doesn't appear that you're doing? (There are other chroma errors present that make it look like a non-TBC/non-SVHS deck is being used.)

Second image:
This is physical tape damage. You can't fix this. At best, you can try to hide it with creative restoration or editing techniques.

Also:
The audio on your capture is distorted very badly. The gain is way, way above where it should be.

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  #5  
05-05-2012, 01:14 PM
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Quote:
Originally Posted by lordsmurf View Post
First image:
It's often a mix of physical or signal damage. I've never found a way to correct this, not even with advanced filter work in Avisynth. Generally speaking, those "chroma crinkle" errors (my own term used here) are accompanied by audio fuzz/buzz in the HiFi track. Sometimes, at best, you can hide it with aggressive chroma NR software filters. Ideally, you want to correct in hardware first, which it doesn't appear that you're doing? (There are other chroma errors present that make it look like a non-TBC/non-SVHS deck is being used.)
The errors disappear with just general noise reduction, and there isn't any distortion in the Hifi track, because there is none (the recording is SP mode with linear audio, made on a JVC HR-A41U). On hifi recordings (which make up the majority of my collection), there isn't any buzz either. On errors that do cause audio/video distortion, the static line is more pronounced and moves down the screen.

And yeah, I regret to say I'm using a consumer deck (late '90s Sony Hifi deck) for conversions. I need to upgrade to an AG-1980 stat, I know, but on my high-school freshman budget, no dice For what it is, it's OK. Tracks fairly well, and doesn't over-sharpen the video. My biggest complaint is the fact that when the tracking is adjusted, the display is brought up, and as I don't have the remote, I have no way to turn it off. Really f'ing annoying. I'm looking into better machines, and a hardware TBC, as luma shifting is my biggest enemy (though fixable with software, if done right).

Quote:
Originally Posted by lordsmurf View Post
Second image:
This is physical tape damage. You can't fix this. At best, you can try to hide it with creative restoration or editing techniques.
I'd heard of using different frames with similar images to make a composite image to cover up the static. Very few of my tapes actually have this sort of error (save for at the beginning or end of the tape, which was the case with that image), but if I'm bored enough, I'll try it out. Premiere has an option to export a large sequence of frames as .tif's, IIRC.

Quote:
Originally Posted by lordsmurf View Post
Also: The audio on your capture is distorted very badly. The gain is way, way above where it should be.
I'm not really hearing that much distortion. I monitored the audio level during capture (I keep the input level at or around 75 to avoid clipping), and it never went into the red zone save for once or twice (and even then, only a little bit into the red zone) during explosions. The audio does sound kinda distorted, with the synth sting that can very easily be mistaken for a tracking error, the harsh helicopter sound, and the voices with the walkie-talkie effect don't really help. Linear/mono VHS audio is pretty crap, anyway.

Also, I don't know if this is common knowledge or not, but it seems that the hiss level (~21 dB) is the same on VHS recordings with linear audio, regardless of speed. SP hiss is a higher frequency then EP hiss, though.

Here's a different example of the error I'm talking about, without any filtering. This comes from the same recording as the 2nd image, and be warned when I say it sounds pretty ugly.


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  #6  
05-08-2012, 04:02 AM
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kpmedia kpmedia is offline
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Clipping doesn't always reveal distortion. It's just one way that distortion can be detected. There's plenty of ways to distort audio and still stay within normal (albeit high/loud) levels boundaries. It helps when you're listening on studio-quality reference speakers, too, and not consumer TV sets or computers, which can hide distortion due to the fact that those devices are themselves distorting (and you've become accustomed to hearing it).

Aside from mild signal jitter and chroma noise that could be fixed with an internal TBC, there's not much else that can be done here.

If you're just looking for an explanation, it's simple: VHS is color-under, and chroma flaws are commonplace.

These streaks you're seeing are one of the ways in which color errors present themselves in the VHS format. I don't recall seeing these color-burst streaks on any other format. You do get bursts like this on S-VHS, but it's black/white noise, and never in color.

Some advanced filtering in Avisynth could help hide it, but those same filters have consequences. Fixing that split-second of noise may harm 10 seconds of non-noise video. The professional alternative it to manually repaint the frames, dupe frames, or a mix thereof -- doing whichever maintains the integrity of the image.

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