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Old 21st July 2017, 07:15   #21  |  Link
jmac698
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I'm not aware of any other source of a checkboard pattern, than dot crawl.

I think you're referring to a letterboxed video. I have one theory, that the comb filtering has nothing to compare the line above it and so can't filter the top line. Does it appear on the bottom edge as well?

I'll also mention a little experiment I did. You can perfectly cancel dot crawl if you have two fields of the same content. The two fields come from points in time where the colour carrier is 180 out of phase. That means if you average them, you get perfect dot crawl removal. This happens naturally in a movie with the pulldown pattern. So every (1/3? I forget these details now) frame can be perfect. Based on that, I had the idea that it could somehow be a guide to remove the dot crawl in the remaining frame, as being some kind of temporal anchor to interpolate against.

Did you read the explanation of dot crawl? I think I'll summarize it here for the benefit of anyone who runs across this thread.

TV was originally monochrome. They wanted to add colour in a backwards-compatible way. They did this by adding a reference short "beep" (called the colour-burst) at the left border. This beep has a frequency of 3.58MHz (NTSC) (PAL came later and used a different method). Then the line is mixed with that frequency of varying phase along the visible line. This shows up as a dotted line pattern in the line. It gets turned into colour, by comparing the "beep" reference phase with the phase along the line. Where they are in phase, it's red, and as they get out of phase, it goes through the colours of the rainbow.

Further, each frame, the entire phase of this process inverts, same with each line. Therefore, if you average two frames together, assuming the image didn't change, you would exactly cancel the checkboard pattern. And if two lines are exactly the same (such as a simple-resized 2x vertically picture) you can average them to remove the dot crawl.

The checkboard pattern is actually a sinewave on one line and a sinewave 180 out of phase on the next line... you could say the edges of the checkboard "squares" smoothly blur along the edges.

They are most visible at the edges of strong colour boundaries because of the inability of techniques to filter out the pattern while interpreting it as colour.

To finish my story in history, they added colour by adding a slight screendoor pattern to the whole picture on older TV's. Newer ones could just filter out that frequency and the picture would look perfect again (for newer monochrome TV's).

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Old 21st July 2017, 17:59   #22  |  Link
wonkey_monkey
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Further, each frame, the entire phase of this process inverts, same with each line.
Isn't that only the case with PAL? Phase Alternating Line... or is that something else.
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Old 27th July 2017, 08:09   #23  |  Link
Katie Boundary
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Originally Posted by jmac698 View Post
I think you're referring to a letterboxed video.
I'm not. I'm referring to the extremely narrow black bars at the right and left sides of the screen in some fullscreen material, which exist due to a phenomenon called overscan (https://en.wikipedia.org/wiki/Overscan).
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Old 27th July 2017, 12:48   #24  |  Link
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It's not overscan, it's nominal analogue blanking. That they are never usually seen is because of overscan, but that's not why they exist. But then I'm blocked, so you'll never learn this.
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Old 27th July 2017, 19:28   #25  |  Link
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Quote:
Originally Posted by jmac698 View Post
I'm not aware of any other source of a checkboard pattern, than dot crawl.

I think you're referring to a letterboxed video. I have one theory, that the comb filtering has nothing to compare the line above it and so can't filter the top line. Does it appear on the bottom edge as well?
I pretty sure I know what she means. It's a vertical pattern one side (or both sides) of the image, at the very edge of the picture, and my memory seems to be insisting it's more often on the right side (at least for PAL).
The "pattern" can vaguely resemble a tiny strip of film at the edge of the picture, several pixels wide, where one "film frame" is picture and the next is black, or something along those lines, and it can appear to strobe or scroll when the video is playing.

That's the best description I can manage, but I've been curious as to what causes it myself. I think I've only ever seen it when the original source was video, and therefore on 4:3 DVDs. I might try to dig out a sample later and post a screenshot.

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Old 28th July 2017, 19:28   #26  |  Link
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South Park was mastered on D3 tapes for a long time which definitely suffered from severe dot crawl. They had no money at first
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Old 31st July 2017, 21:21   #27  |  Link
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South Park was mastered on D3 tapes for a long time which definitely suffered from severe dot crawl. They had no money at first
D3 tapes? Is that like Betamax but worse?
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Old 31st July 2017, 22:56   #28  |  Link
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D3 tapes are actually digital tapes, compared to the analog Betamax or VHS tapes its still quite good quality, but still a "cheap" solution for mastering.
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Old 1st August 2017, 00:19   #29  |  Link
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Yep

Uncompressed digital composite. Totally crazy, but it filled a niche for awhile and was cheaper than D1 / D2.

https://en.wikipedia.org/wiki/D-3_(video)

It was entirely deprecated by Digital Betacam (DigiBeta) which is still in wide use today for SD tape masters.

DigiBeta introduced a perceptually lossless ~2:1 DCT compression which enabled full 4:2:2 10 bit at ~90 Mbps which was totally doable with inexpensive 1/2" tape and transports.

D1 needed 3/4" tape to store totally uncompressed component video (tho only in 8 bits per sample)

D5 later introduced uncompressed 4:2:2 10 bit on 1/2" tape, and was eventually upgraded to support 4:1 perceptually lossless HD

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Old 1st August 2017, 04:39   #30  |  Link
raffriff42
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It was D-2 actually. Quality-wise, D-2 was the same as D-3: as good as PAL/NTSC ever gets, which by today's standards, is not saying much.
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The Making of South Park
The animated shots are transferred to a digital disk recorder and from there to an Avid video editing station where Matt & Trey work with that show's editor to work out the details (the fastest way to get the images into the Avid is as a video signal, so the DDR is used to generate video from the rendered image files). If a shot needs to be changed, it gets sent back to an animator or TD for a 're-take' and then re-rendered. At the same time the video editing is done, the final audio is worked out on the audio Avids and merged into the video Avid's content. Once everything meets the stringent South Park quality standards (i.e. we run out of time), it gets written to a D2 digital video tape and sent off to Comedy Central to be aired.
EDIT Digital Betacam was component, which does not have dot crawl obviously - but required PAL/NTSC video facilities to completely rewire, buy new video peripherals etc, which cost millions. Hence D-2 and D-3: digital recording (no generation loss) while keeping your old video gear.

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