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Comment by onnimonni | Hacker News Reader

I'm not that familiar with CED but the fact that we can see the images with microscopes is because these are analog discs? And that was because computing power back then was non-existent so they didn't use any kind of compression?

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The key point is that there has to be a slow vertical panning happening as actual content. If that happens, then the on-disk representation of a color channel can end up physically below/above what happens before/after in the movie, drawing out the "actual content". This is why end credits were the most likely visible artifacts.

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.

This is 1970s tech, so of course there was no compression. It's basically a composite video signal etched into the medium. Every revolution of the disc is one full frame (or two fields). So when you look at it, the adjacent grooves represent the same lines of video in different frames.

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.

There wasn't any computing involved. The video was recorded as a sampled analogue signal, with pits of varying length setting the "output voltage".

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

Uncompressed digital encoding might still result in recognizable structures, but probably not as nicely as here.

Only the CD had uncompressed data.

CED, not CD. And LDs most definitely also have uncompressed data, resulting in visible text at 22:30 in the video.

This is where we get into semantics, but it sort of has compressed color data, in the analog domain. :)

CD's have repatterened uncompressed data using scrambling codes to keep the waveform stable (i.e. ~50% on/off)

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

This is a distinction without meaning: all digital anything is analogue if you look closely enough

> 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.

No, with digital, you need encoding. How can you even compare binary with embedded images.

> How can you even compare binary with embedded images.

How are the images encoded?

In ways that ensure that they are not visually recognizable on the physical medium afterwards, because the visual layout represents a whole lot of redundancy, and the job of compression is to remove redundancy. If the end result has any recognizable patterns, the compression is not doing its job well.

If you watch the video, you can see that the images are burnt to the medium and can be seen optically. With 1 and 0, you cannot do this. All you'd see would be random dots everywhere.

Your CED or laserdisc player needs to be smart enough to be able to decode whatever you put on it, which- in the era that they were relevant- pretty severely limited what you could do.

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