Each revolution is one frame, so if you compare the data paths next to each other, they don't make up an image but the same single line of several consecutive frames.

Scrolling a still image makes the same line on the screen cover a different part of the image each frame, so you can sort of make out what the original image looked like.

The end credits should show up as a single tall image, since the only limit to the height is the radius of the disc.

The title is a bit misleading. LaserDiscs are digital but CEDs are analog.

Very different technologies.

And it just so happens in this video that the scrolling was at a speed where everything was preserved and also not stretched. Too slow and it would be elongated vertically on the disc and too fast and it would be compressed and you'd miss information.

writeup from the author linked in the video description

https://youtu.be/qZuR-772cks?si=rYM4EjvV7VeTEzx8&t=1570

Long answer: Just jump around. Once you get to the last 1/3rd of the video, there's a lot of close ups of the part of the laserdisc that recorded the credit sequence. The on-screen (laserdisc) text is clearly visible.

The author moves a flashlight around to show how the angle is important; something that won't come across as well in a blog entry.

Maybe it's better now, but that was how it did it recently. To be convinced that it "watches" the video, I would need to see evidence of it referring to facts that are strictly only possible to know from the video, but not guessable from the audio.

I'm so sick of this negative attitude. I get it when it comes to politics or more complex systems or conceptual ideas. But holy hell, we're talking about a "money shot" to get people interested in the subject.

https://m.youtube.com/watch?v=zIsCswtkozI (mimeograph around 3:36:00 mark)

Now imagine if you picked a Y coordinate and extracted the corresponding row of pixels from every frame of a video file, and stacked them vertically. If there was scrolling text, or if the camera moved vertically, you would see a meaningful image, the same way a scanner obtains a complete image by moving a single-row sensor across whatever you're scanning. This is the same effect, just arising from the way the video signal is laid out on the disc.

One other important aspect is that by changing the angle of lighting, he could basically filter out data at a relevant wavelength.

--

At least that's what I got from the video.

If you look at a Constant Angular Velocity disc you can actually see "spokes" radiating out from the centre, with two broad ones 180° apart. The narrow spokes are the horizontal sync pulses occuring every 0.576° - the disc rotates at 25 revs per second and each concentric track is one complete frame. The broader spokes are of course the vertical sync pulses and colour burst occurring every 1/50th of a second.

If you're in the US or Japan, these numbers are 30 revs per second, 0.686° and 1/60th of a second, because of the lower resolution video standard, but it doesn't look like Laserdisc was much of a "thing" in those countries.

Here in the UK, in the 1980s all the schools took part in a thing called "The Domesday Project" [1] - the name is a reference to The Domesday Book, a survey of England and Wales carried out in the 11th century by William the Conqueror.

The Domesday Discs were CAV Laserdiscs that were played in a special player with a SCSI interface, attached to a BBC Micro computer. Because each concentric track was a complete frame it was possible to get perfect still frame video by just keeping the head still, so you could look at photos of places all around the UK and read a bit of information about them genlocked over the top.

[1] https://en.wikipedia.org/wiki/BBC_Domesday_Project

And a Laserdisc with digital sound literally has that CD audio EFM waveform in the lower frequencies.

> And that was because computing power back then was non-existent so they didn't use any kind of compression?

Compression is not a medium-level detail. You can easily store compressed data on a laserdisc.

How are the images encoded?

Without you showing them, they might never see this kind of stuff.

The legible text seen in the microscope images happens because of the combination of LaserDisc recording a raw and uncompressed encoding of the analog video signal, the way that LaserDisc used CAV to store an integer number of frames per track so that the image data for corresponding on-screen locations of subsequent frames would be aligned at the same radial position on the disc, and the credits scrolling vertically at constant speed.

If LaserDisc had used a digital encoding (especially a compressed encoding), the data on disc may still have had discernible patterns but the text would not necessarily have been legible. If it had used CAV but not stored a whole number of frames per track, then temporal and spatial locality on screen would not have corresponded so well to spatial locality on disc. And the vertically-scrolling credits are pretty much the only kind of content that can produce the recognizable and legible images on the disc surface.

I think the fact that the aspect ratio of the text came out approximately right probably is a consequence of the scrolling speed of the credits being chosen to suit the vertical resolution of the video. If the text had appeared squished in the microscope, it would probably have been moving too fast on screen to be clearly legible.

Those can have near-legible images, but most of the time they are not.

CAV discs contained one frame per rotation. While this meant you could only fit half an hour on one side of a disc, it did give you perfect slow-motion and freeze-frames.

I worked in a video store and loved LaserDiscs. The Duran Duran video album was CAV, and the Pioneer LD-700 had such a fast transport mechanism and remote control that I could to DJ-style "scratching" with it.

https://youtu.be/qZuR-772cks?t=1540

The data being written to the disk is the same in CAV or CLV disks, but the player just needs to know how to spin the disk at the right speed so that the laser can read the pits/lands correctly. It is purely a detail about the speed that the disk is spun at so they can cram more data on it with CLV disks.

What CAV LaserDiscs allow for, though, is to make it extremely obvious where scanlines and blanking intervals are in the video signal.

Even small changes in optics can drastically change how light spreads or how uniform illumination appears in a space.

Laser discs are not digital. They encode the analogue video signal’s value as the length of the pit. It is digitized in the time domain - sampled at some frequency, but the “vertical” signal value is stored entirely analogue. In terms of encoding it’s more similar to a VHS tape than a CD. Kinda crazy.

Laser disks are 100% digital (as you said, they store digits in the time domain).

They don't encode their data using binary like a CD does.

"Binary" and "digital" are two separate and unrelated concepts.

The sound was also analog to begin with, then the same encoding as CDs, then after that AC-3 and DTS.

And at that point, most players sold were combo players that could also play CDs.

And there was one more disc format: CD Video. It was a CD-sized digital single that also had a LaserDisc section for the (analog) music video. I have a couple; one is Bon Jovi.

So I don't want to say it's impossible, but I think it would require a lot more creativity.