I've been watching old SNL clips and noticed that whenever there's static text on screen, it vibrates back and forth and you can also see some color artefacts:
Here's an example in this youtube clip:
https://www.youtube.com/watch?v=VUF5KZG_fqY&t=87s
It should automatically go to the right place, but if not, go to 1 min 27 seconds.
Is it something about deinterlacing old tape recordings that is causing it? If it's been deinterlaced with bob that would explain it (although it seems too severe for that), but it's 29.97 fps, so it was probably deinterlaced with an interpolator or blend algorithm.
Or is it something totally different like the tape moving slightly up and down as it's being read (or as it was being recorded). It seems unlikely since only certain parts of the letters are wobbling up and down.
I see this all the time and I'm curious about what's causing it. And is there a way to correct it if you're digitizing tape?
And I guess a broader/newbie question is, why can't analog video stay still.
Thanks!
That clip started life as a 29.97 interlaced recording-- which would have had noticeable vibrating edges on the graphics even under ideal viewing conditions -- because that's the nature of the beast. However, this one then got deinterlaced, upscaled and frame rate changed to 23.98. Most of it looks fairly decent for all that -- but some artifacts remain.
Jackson was on SNL from 1986 - 1992, so there's probably a good D2 or Digibeta master of that piece in the NBC vaults.
Thanks. You're right, it's 24 fps, I thought it was 29.97. Some of of the other classic SNL videos they've uploaded are 29.97, so I just assumed they've been deinterlaced once and the frame rate was a product of the deinterlacing. Why they would then change it to 24 is beyond me, that's just throwing out frames for no reason. But regardless, the vibrating edges don't seem to be related to deinterlacing.Originally Posted by smrpix
What is causing it then, as you say "nature of the beast", why does it happen with analog video?
This clip was from 1986, so I guess it would have been D-1, but probably still analog? (and then transferred to digital at some point). But digital or analog recording wouldn't have affected the artefacting, right? Because it was shot and transmitted in an analog format, the tape only digitized it for storage, but otherwise the entire process was analog.
On the edges of the letters, I'm just seeing rainbowing and dot crawl. Also tons of rainbows on her earrings and some in her hair.
Here's an old post with some images and explanation:
https://forum.videohelp.com/threads/340508-DVD-Playback-Dot-Crawl#post2118655
My YouTube channel with little clips: vhs-decode, comparing TBC, etc.
It happens with interlaced video. Interlaced video scans all the even lines, then all the odd lines -- so they are offset from each other both in space and time. The offset being cycled 30x each second (in NTSC) is what gives the appearance of a wiggle.
It has nothing to do with analog per se (though there is a lot of history as to why almost all analog video is interlaced) -- interlaced digital images exhibit the same artifacts.
But interlacing alone can't produce that effect, it's almost as if the edges of the letters can't decide where to land. With interlacing, especially a completely static image like this, you wouldn't have an issue. This is happening maybe 10 times a second, not 30. Plus, the video has been deinterlaced, so this should have been corrected by deinterlacing. When I watch interlaced video (without de-interlacing), I see combing, but not this.It happens with interlaced video. Interlaced video scans all the even lines, then all the odd lines -- so they are offset from each other both in space and time. The offset being cycled 30x each second (in NTSC) is what gives the appearance of a wiggle.
It has nothing to do with analog per se (though there is a lot of history as to why almost all analog video is interlaced) -- interlaced digital images exhibit the same artifacts.
So I think it's something else maybe?
Ah that makes sense. Are dot crawl and rainbowing related? I assumed the rainbows on her earrings, eyes etc. was just because of the poor chroma resolution of tape, but... they probably used really good tape, so that's probably not it.
Is it normal for dot crawl to vibrate like this? Having watched a bunch of these videos, this effect only happens when they have large letters or other static graphics on screen, otherwise I never see it.
I'm not sure I see the vibration you're describing, but I only viewed the video on my phone.
Yes dot crawl and rainbowing are related. They're both artifacts of composite video, and they're sort of opposites of each other. Dot crawl is chroma incorrectly interpreted as luma, while rainbowing is luma incorrectly interpreted as chroma.
Luma = the black & white portion of the signal
Chroma = the color "layer" added on top
My YouTube channel with little clips: vhs-decode, comparing TBC, etc.
Oh you should see it on a larger screen, it is definitely bobbing up and down. Watching other videos of dot crawl, this seems to be normal, the dot crawl is never static, it always "vibrates". And noticeably slower than the FPS or field rate, so I wonder why it's not static... And this video has been deinterlaced and converted to 24 fps, and it's still there, so I don't think it's related to that, maybe it's just the bleed-over happens over some regular interval for some reason.
I asked in another place and another theory was that the text graphic was made by a computer and digitally superimposed onto analog video in real time, so the bobbing up and down is basically aliasing as a result of the analog recording trying to resolve sharp digital resolution and it can't decide where to put it.
But I think you are correct that it's just dot crawl.
Interlacing combined with an oversharp overlay, chroma subsampling, dot crawl, bad deinterlacing, bad decimation, and a couple generations of compression artifacts can produce stuff like that.
Maybe you never watched late night TV on small local stations on the analog days? It was very common for local ads to have oversharp text that flickered badly and bounced up and down from the interlacing.
Last edited by jagabo; 23rd Jan 2022 at 23:09.
Could I ask you about another clip @Brad or whoever can answer.
https://youtu.be/8iClQNXILfQ?t=232
At 3:52 (and really throughout the video) there are these shadows around edges. I've noticed this on almost all tape recordings from before ~1985 and hardly anything from the late 80s and later (that was properly done). Which leads me to believe it comes straight from the NTSC broadcast, not an issue with the VCR or VHS tape or capture. And it's something they just figured out how to fix at some point.
Is it ringing or ghosting? I don't know enough to make much of my research. What causes it? Input is appreciated.
The clip has halos from over-sharpening. Multiple generations worth. One generation is very common with VHS and other consumer tape formats. Further generations from tape-to-tape recordings, processing amps, etc.
Hm I'm 99% certain that this tape hasn't been copied, it's the original recording from OTA that was digitized. Also, I see the same artefacts in archival footage that is posted directly by TV networks, coming straight from their archival tape.
My theory is that it's ringing artefacts caused by overshoot, and at some point in the mid-80s they switched to Rec. 601 recording which accommodates overshoot. And maybe they cleaned up the broadcast too. I believe VHS sharpening results in overshoot too, so that could be an additional factor, but there's definitely a correlation between time period and these effects, it's very common in 70s and early 80s recordings...
But then, I don't know much about this at all.
I think it's all of the above.
A digital text slide converted to analogue, having issues with chroma and dot crawl (as well as rainbowing) because of it's sharpness and analogue videotape limits. The video preservation was probably done by deinterlacing the video into 60fps then converting it to 24 for some reason
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