It's not a glitch with the video, it's datamoshing (SLYT)

June 14, 2022 6:13 AM

"Datamoshing is a technique of damaging video clips to create a glitch effect wherein frames that should change don't. It's most noticeable between cuts and across motion. Datamoshing is the process of corrupting, removing or replacing I-frames, causing P-frames to be applied to the wrong picture."
posted by gestalt saloon (18 comments total)

The first time I saw something like that was when my Grandpa got a shiny new receiver with DigiCipher II for his C Band satellite dish. Sometimes a burst of noise would cause the MPEG decoder to discard a frame or two and hilarity ensued.

Later, I had a friend with a DBS dish literally nailed to a tree in one of the few spots on his property where there was an open spot in the canopy. When the wind blew, branches would get in the way and disrupt the signal for a bit and again, confused decoder.

Later still, I got a digital cable box from my local cable company that didn't wait for an I-Frame to start decoding after a channel change, so every channel change resulted in a couple of seconds of weirdness like this.

It makes me feel a little old that these glitches are now being intentionally introduced to create an aesthetic that just screams "the damn thing is on the fritz again" to me.
posted by wierdo at 7:05 AM on June 14, 2022 [7 favorites]

I'm still waiting so see someone make creative use of the artifacts from the background on zoom calls.
posted by Nancy Lebovitz at 7:06 AM on June 14, 2022

It's nice to put a name to this! @GlitchTVBot (Twitter account, Nitter link) is an account I've enjoyed for years that does this to TV stills with some fun results.
posted by chinesefood at 7:07 AM on June 14, 2022 [2 favorites]

Huh, I appreciate the specific kind of glitch. In particular it's almost like a DJ mix where two videos are playing at once and combining. Unfortunately the effects in the linked video don't really do that. Either one of the two videos is completely unrecognizable. Or else for something like the roller coaster sequences around 15:00 one or the other video entirely dominates and the two don't really talk to each other in an interesting way.

Still looks cool but I was hoping for something more recognizably a mix of the two sources.
posted by Nelson at 7:23 AM on June 14, 2022 [1 favorite]

Looks like the artefacting that happens to my Sky satellite signal when there's a really bad storm.
posted by GallonOfAlan at 7:33 AM on June 14, 2022

The first time I saw this used for artistic effect was in the "Elegant Utensil" video by Chairlift, which still slaps!
posted by jshttnbm at 7:34 AM on June 14, 2022 [4 favorites]

Wasn't this same effect called something else on the blue about 15 years ago? Or is there a distinction I'm missing?
posted by kimota at 7:42 AM on June 14, 2022 [1 favorite]

Wasn't this same effect called something else on the blue about 15 years ago? Or is there a distinction I'm missing?

I wouldn't be surprised if there's been multiple names for it; "datamoshing" has definitely been in play for a long time, as in e.g. this 2009 comment.
posted by cortex at 7:49 AM on June 14, 2022 [2 favorites]

If you can stomach Reddit, there's a /r/glitch_art subforum. Most of it is bleh, some of it is pretty good.
posted by JoeZydeco at 9:27 AM on June 14, 2022

I used to design settop boxes - the systems that seemed to lose iframes on channel change (older Comcast cable plant I think) did it on purpose - you all may have noticed that channel change on digital systems is slower than on old analog systems where it takes ~1/30 of a second - digital systems have to wait for a bunch of things (tuning, PATs/PMTs/ECMs [crypto]) and of course that first new iframe from the stream, and finally audio sync - highly compressed streams (say the NASA channel) send far fewer iframes than lower compressed, more hifi ones (say HBO).

For those who haven't run into this before an iframe is a complete com pressed frame, other mpeg frames are deltas (in time) from that frame, a stream might have an iframe per second, or a one every 5 seconds, it depends on the compression and desired quality - when you change channel you can't display anything until you have one

There are tricks you can play to speed up channel change (caching of a bunch of that stuff) but sadly digital just has to be slower - anyway back when digital set top cable boxes first came out the original Motorola plant did this thing where they used sort of partial iframes (might not be standard mpeg) with something like 100 pixel blocks, every 3 to 4 of them would contain enough blocks to render a whole modern iframe - the result would be that weird thing you saw the new channel coming in block by block (bigger blocks than the glitch video) was because of this and I believe by design so that channel change appeared to happen quickly
posted by mbo at 11:43 AM on June 14, 2022 [7 favorites]

So, I guess all those crappy software MPEG encoders that used to drive me crazy (was in the DVD biz once) are now probably all highly desirable (like my old analog video monitors). I might have some installers kicking about. I also have what was once a very good MPEG hardware encoding station sitting in storage somewhere. Maybe now someone will take it off my hands (and keep it out of a landfill a while longer).
posted by Insert Clever Name Here at 11:48 AM on June 14, 2022

I'd always wondered why there's so much lag like that, mbo! The raw drop in quality of experience with channel flipping is striking enough that I'd figured it had to be *some* tricky technical limit, but it never occurred to me that iframes might be part of that.
posted by cortex at 11:50 AM on June 14, 2022

When we get AI-enhanced video codecs and the stream is basically "Kirk is talking to Spock with emotion #2473" the glitches are going to be wild.
posted by credulous at 12:04 PM on June 14, 2022 [3 favorites]

cortex: it's even more complex than that - typically the video and audio streams are broadcast a bit out of sync with each other - in part because there both forward and reverse interpolation from iframes - you have to get enough audio and video into the rendering pipe before the software PLLs that sync them up can lock - it's why on some systems you can see lip-sync become a bit off and you can fix it by pausing the stream for a little while and then start it up - that delay getting enough stuff into the rendering pipe for the lock adds to the channel flip time

The other gotcha is crypto - you have to wait for a valid decode key (ECM) to arrive and process that too (a crypto decode using the key you were sent when you paid your monthly cable bill) before you can even start to see iframes - ECMs come quite quickly (milliseconds) for this reason and change every few seconds and typically you get ones for the current stream and, just before it changes, ones for the next period (some systems used to wait for the next key change)
posted by mbo at 2:53 PM on June 14, 2022 [1 favorite]

FTR, the particular box I was referring to neglected to drop its buffer on channel changes, so it would apply new P-frames to the previous content. Depending on how much motion and the encoding settings for the particular channel it would sometimes get to a decent approximation of the correct picture. Still very wrong, but at least recognizable.
posted by wierdo at 3:44 PM on June 14, 2022

Regarding techniques of damaging video, using version 5.0 or later of ffmpeg, one can damage or drop frames as needed.

Basic syntax is

ffmpeg -i INPUT -map 0 -c copy -bsf:v "noise=drop='not(mod(n\,10))*not(key)':amount=-1" OUTPUT

This will drop every 10th frame if it is not a keyframe, and will apply some variable amount of damage to the rest.

See docs at http://www.ffmpeg.org/ffmpeg-bitstream-filters.html#noise
posted by Gyan at 9:34 PM on June 14, 2022 [2 favorites]

datamoshing!
* Previously: the datamosh tag
* The Verge wrote about it in 2018
* Waxy pointed out the Connection Interrupted Tik Tok, which uses it to magical effect.
* the Evident Utensil video was posted here, and it has a bunch of examples.

(It's a weird thing to be like "I know I've shared similar things," search to no avail, then find it because I posted the backwards version to Youtube.)
posted by Pronoiac at 1:15 AM on June 15, 2022

On occasion, I see simpler versions of this in my cable tv feed. Just a few seconds of mush trying to connect between where the glitch started and where it finally ends. Probably the best entertainment of the day.
posted by Thorzdad at 4:54 AM on June 15, 2022