Quantcast Checkerboard pattern in VHS, how to repair? - digitalFAQ Forum
  #1  
04-23-2021, 03:03 AM
kingbean kingbean is offline
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I've been noticing a signal noise on some of my VHS captures that I can best describe as a 'checkerboard' pattern. Searching the forums, the most common result seems to suggest s-video crosstalk: POOR Y/C DECODING (S-VIDEO CROSSTALK). However, what I'm seeing on my captures doesn't quite match this.

It appears mostly on warm tones, and doesn't present itself across the whole screen like the other examples I've seen online do.

I'll mention now that the majority of what I'm capturing are concert videos, that have likely been copied from tape to tape on who-knows-what kind of setups.

I've been pulling my hair out trying to find which part of my setup was introducing this effect, but it persisted no matter what I tried. Eventually I came to the conclusion it must have been introduced during an earlier tape dub, and not in my capture chain. I tested this on a third VCR, connected to my TV via RCA, and noticed the same pattern. I was relieved, and moved on.

However, the next tape I tried had the same problem, as did the next two tapes I tried. This is now four different VHS tapes, which have come from three separate sources.

Tape 1: This was the first tape giving me trouble. I suspected it was either from an earlier copy process, or from the actual video camera used.

Screenshot 1:
Visible in orange area on left side of shirt.
Pattern1.png

Screenshot 2:
Visible across arms, shirt, hair and guitar amp.
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Tape 1: Second tape I noticed it on, which came from the same person that sent me Tape 1. This was taped off TV, but may have been copied once. So I suspected it was during the copy process.

Screenshot 3:
Visible on skin and a bit of shirt, and board on right hand side.
Pattern2.png

Tape 3: Satisfied I'm solved the mystery, I moved on to a new tape from a different collection, but noticed the same problem within seconds of playback.

Screenshot 4:
Visible on back of tshirt.
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Tape 4: This tape started out okay but also has almost the whole screen covered.
Screenshot 5:
Visible everywhere!!
Pattern3.png

Tape 5: This is just included to show another capture I've made which does not seem to have any sign of the problem.
Screenshot 5:
Visible... nowhere?
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My first worry was that it was TBC-related, but removing that from my chain made no difference.
Two different VCRs using different sets of s-video cables (though same brand) made no difference.
Playback on a third VCR using RCA made no difference.

So I'm still putting my money on it being present on the actual tapes I'm capturing, and not being introduced doing capture or playback. Not sure if it's related, but these have all been PAL tapes so far. I'm yet to notice it on any of my NTSC captures.

So, my questions are:

- Is this kind of problem so common that it would present itself on four out of five tapes I've picked at random?
- If this is coming from my end, what's the most likely culprit?
- Is there anything I can do post-capture to remedy this?

If it's relevant:

Playback VCRs:
JVC HR-DVS2 (TBC on) (s-video)
JVS HR-S5500AM (s-video)
Some old Sony (RCA)

Capture device:
Tevion DVD Maker+ (ATI clone from LS)

Capture PC:
Windows XP > Virtual Dub 1.9.11 > HuffYUV .avi

Thanks in advance for any advice!!


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  #2  
04-23-2021, 04:16 AM
latreche34 latreche34 is offline
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It's called dot crawl. If these tapes are dubbed and originated from a PAL source it is most likely baked in and there is nothing you can do about it. Cheap standard converters using composite connections did suffer from this interference, Composite a.k.a CVBS is a lower quality video transmission, It is convenient but the interference between chroma and luma can be a disaster. That's the reason why S-VHS VCR's are suggested for capturing tapes, Not that it helps in your case but generally speaking composite is noisy.

Try to lower the chroma down it will help reduce the artifact, though it is baked in luma already.
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  #3  
04-23-2021, 04:32 AM
kingbean kingbean is offline
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Ahhh, thanks for that.

Yeah, I would say that the majority of these 'tape traded' videos were just dubbed from Random VCR 1 into Random VCR 2 using composite video. So it would have been introduced at that stage. These are all PAL tapes that have never seen any NTSC conversion.

Tape #2 is believed to have been taped off-air, so it's the master VHS tape - but I guess using a composite video from Pay TV box > VCR is exactly the same as between two VCRs while dubbing.

It just surprised me I'd never noticed it before, but there it was on four tapes in a row. In the past I've used a Canopus capture box, so it's likely the DV compression masked the dot crawl somewhat. Now that I'm capturing in lossless, it's much more present.

Very relieved to hear this, thanks again. Even though it's a shame the video suffers from dot crawl, at least I know I'm not introducing it during capture.
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  #4  
04-23-2021, 10:13 AM
traal traal is offline
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Are your S-Video cables all double shielded?
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  #5  
04-23-2021, 11:58 AM
hodgey hodgey is online now
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This pattern on strong colors is pretty common on PAL tapes. With the tapes being captures of concerts with strong colored lights there is going to be a lot of strong bright colors on the video than normal I guess.

It is some form of chroma/luma interference, though it's on the tape, not on the capturing side. (Dot patterns you can get when capturing from composite video/rca or with a bad s-video cable are "smaller"/higher frequency/sharper.) I don't know to what extent it is inadequate chroma/luma separation during recording and to what extent it may be the chroma signal on the tape itself interfering with the luma signal on the tape (there is some slight overlap in frequency bands on standard VHS, tho not a lot.)

The built-in noise reduction in VCRs can sometimes help reduce the effect a little (but can also impact actual details) but that's often a bit of a trade-off.
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  #6  
04-23-2021, 05:29 PM
kingbean kingbean is offline
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Thanks. Yeah, that's what I thought from researching online - a bad s-video cable would produce much more defined pattern, and across the whole picture. I use these s-video cables for each connection in my chain.. I've only ever seen a handful of different s-video cables, but these ones feel pretty good:

svid.jpg

I think I'll leave NR off, as I'd rather have the dot crawl as present on the tape rather than trying to mask it, at the cost of also degrading the rest of the picture.

Is there anything I can do in avisynth that would target just the dot crawl, or do I just need to accept it at this stage?

In the lead up to making this post, I'd been scrutinising every piece of video I've captured, as well as other captures/video, so closely I felt I was beginning to see dot crawl everywhere! Very Tetris effect-esque


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  #7  
05-06-2021, 11:15 PM
leeoverstreet leeoverstreet is offline
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I am SOOOO familiar with this phenomenon. And to make a very long story of experimentation much, much shorter... It's just there. On the tapes. Some camcorders and VCRs were worse about it than others, which I assume comes down to how much they inherently boost the luma and chroma to make it look punchier on the average CRT of the time, or possibly some bit of timing being off in the color signal. In my collection of NTSC home videos and tapes via cable TV, this effect doesn't show up as much on tapes from my old 1986 RCA VCR and my grandmother's 1988 RCA camcorder. But my uncle's 1989 Panasonic, and my later 1990 RCA hi-fi VCR have it on almost every tape where the right combination of very saturated and bright colors happen. I only seem to see it on edges or across tiny areas, and you see it more on hot pink / bright orange. Sometimes robin's egg bright blue, but rarely bright green. Reducing the chroma level when editing helps, but it's just kind of THERE. Mocking you.


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  #8  
05-07-2021, 04:18 AM
timtape timtape is online now
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VHS could only handle a certain level of luma and color saturation so when pushed beyond those limits of course it started to produce artifacts.
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