The Interlaced Video Problem - Computerphile

Why do computers have such a hard time showing TV footage? Dr Steve Bagley unlaces the problem.

WARNING There is a short sequence in this film with flashing images.

This video was filmed and edited by Sean Riley.

[Nostalgianerd]

11:20 the irony of having slightly corrupted video whilst talking about how to make video appear as it was originally intended is strong. Well played Youtube.

[knurlgnar]

Interlacing was absolutely genius when it was invented, effectively doubling the temporal resolution while minimally effecting the spatial resolution and reducing phosphor flicker. But those people who allowed it to continue into the HD world released a plague onto the world. I was horrified when I read the HD standard years ago - it was an inexcusable decision.

[wlan]

12:26 "Various different complicated algorithms" is the reason some of us are here! I'd love to see how a sequence of fields is interpolated into a sequence of full-resolution frames; even an 8x8 pixel B&W demo would be instructive.

[pinkdispatcher]

The worst that can happen is downscaling a full interlaced frame as if it were a progressive frame. That does irreversible damage to the image. There are still lots of videos on youtube like that. I sometimes convert legacy interlaced DVD material to progressive video and go to great length of doing the best possible deinterlacing, but that's very CPU-intensive; mencoder has some of the very best algorithms for that called "mcdeint".

[NyanSten]

Now it makes sense why they used odd number of lines. You always skip one line when drawing so in the first field, when you get to the last line (which is odd), the next line is 1 which you skip over and continue with 2. Then you get to the one but last line (which is even), the next one is the last one, you skip over it and continue with line 1. You just drew two fields without any need to synchronize where the first ends and the second begins.

[unvergebeneid]

People, even professionals, not deinterlacing their video drives me mad.

[xotmatrix]

The book is "Digital Video and HDTV Algorithms and Interfaces" by Charles Poynton. It's outstanding.

[MG]

It's 2016. Why is this still a thing? I only remember it from MPEG in the 90's / early 00's

[dipi]

10:25 small correction: the best way to convert film into progressive video is not by taking an interlaced film scan and weaving frames together; the best way is to acquire access to the original film material and scan that one progressively, right from the source.
Progressive hi-rez scans from 20th-Century film classics, maybe even the 70mm stuff: bliss! (-;

[RobotnikPlays]

It's an important point that the frame buffering & interpolation can cause issues when playing video games due to the delay between input/output. Which is why a lot of better TVs these days come with a 'Game mode' or other setting to turn most of the preprocessing off (and another reason why CRTs are still sought after by retro-gamers)

[doougle]

Did you notice the video glitch at 11:20? It's only on the over the shoulder shot. It might just be my computer, but the glitch is repeatable.
Love the channel, BTW. Computers are such an important part of our lives these days but less and less is known about them by society as a whole.

[sacredbanana]

Please make a video about the difference between PAL and NTSC and how colour information is encoded in a video signal :D

[JoeWestcottVFX]

An important point not mentioned is the persistence of vision quality of CRT displays. Since digital monitors display frames discreetly, one at at time, they are not ideal for display of interlaced footage. With CRTs, the previous field would be visible due to the way they work, leaving a glow in the phospher for a short amount of time after being struck by the beam.

[Breznak]

I love the interlaced window in the background

[JimFortune]

Interlacing doesn't reduce the amount of information you need to send. You send an entire screen 25 times each second. But you cut in half the time lag between the top and bottom of the screen, so each frame has only half the distortion to objects moving across the screen.

[MrMonkeybat]

1080i should of never existed, when HD was coming in flat panels were clearly coming to dominate and CRTs where getting progressive scan over component cables anyway. Even if you are being measly about the bandwidth 25 progressive frames digitally compresses better than 50 interlaced frames still looks a lot better on any flat panel or flashed twice on a CRT.

[molletts]

I've noticed that, surprisingly often, TV channels get the field order wrong on outside broadcasts - I've frequently seen reports on BBC News, with their own reporter (so not some video they've borrowed from, say, a US network with opposite parity where it might be almost excusable) where the text overlay is fine but the picture behind it has nasty "double horizontals" symptomatic of swapped fields.

[graingert]

hey, can you content warning flashing images please!

[kasuha]

These are problems I was dealing with 15 years ago and it surprises me how much of end user hardware is still doing that wrong.

One thing you forgot to mention is that a lot of hardware and even software players only replay a video in 25 FPS (or 29.997 for NTSC). That's where most of the problem comes from, that's why there's so much magic in combining fields into progressive frames. Players that can produce 50/60 FPS output have it much easier unless you are striving for more than what the analog original has ever actually offered.

[bradscott]

It's insane they would still use interlaced video in the digital realm.