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 Post subject: Making your own LDs at home?
PostPosted: 21 Aug 2018, 07:24 
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Well, OK, not actual LDs as the size is the problem here :-)

What I am thinking is -- there were CD Video discs, which are essentially Audio CD with an LD track. But there were some of these discs without the Audio part.
So in theory it would be possible to:

  1. Get a CD burner unit from an old PC (plenty of those everywhere these days)
  2. Remove the original electronics
  3. Use an ATMEGA (probably too slow), STM32 or ESP32 and/or a BLDC driver to spin the disc into CAV mode (CAV is the easiest to do just as a proof of concept) -- essentially set a constant speed on the disc, sync to it and per every revolution move the record head 1 step to the outer radius
  4. Use some other CPU or analog processing circuitry to modulate the incoming composite video/audio signal, and a simple opamp/transistor-based amplifier to drive the burner LED
  5. Connect a video source, get a good CD-R (probably a Verbatim or a TDK at least), put it on the spindle,

The questions so far are --

a. How is the signal exactly modulated? This page on the LaserdiscArchive explains it a little but without going into much detail. If there is some patent, or at least a function diagram, that would be a lot more helpful!
b. What is the distance between tracks, that "1 step" that the record head has to move per every revolution of the disc?
c. How is the digital data modulated into the frames, so the player knows the frame number, disc type and so on?
d. Figuring out the pinout of the laser head to control the focus feedback loop, though this is a bit less LD-related.
e. Probably, using some low-level programming under Linux or something, it may be possible to do that without modifying the burner, and just burn a video file right away, using sophisticated enough software? I think I've seen some programs that draw pictures in data patterns on DVD's, though that might be hard to do due to the PCM nature of the CD/DVD vs PWM on the LD. Oh, and I have a "Disc T@Too" DVD burner which can draw text and pictures on the bottom side of DVDs, so maybe it is possible to use one of those?


Update:

1. https://kompendium.infotip.de/laserdisc.html specifies that the distance between tracks is about 1.6um on an LD
2. This page at domesday86 has some really decent info about how LD data is encoded in terms of frequencies used and so on. They are doing a really interesting project of digitizing LDs as an RF signal rather than a video capture, so check them out!
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 Post subject: Re: Making your own LDs at home?
PostPosted: 21 Aug 2018, 13:26 
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 Post subject: Re: Making your own LDs at home?
PostPosted: 21 Aug 2018, 14:29 
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I think it's doable in theory, but it would take a lot of work from someone with specialist hardware programming knowledge. Even if someone did pick it up as a project it probably wouldn't have any value other than as a curiosity as I don't think CD-Vs could hold much more than five minutes of video, and that's in CLV mode.

I imagine the Domesday site you linked to is run by simoni who has been very active in this huge thread about happycube's laserdisc RF signal decoder project. I'm sure you'll find some interesting reading there.
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 Post subject: Re: Making your own LDs at home?
PostPosted: 21 Aug 2018, 15:03 
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I’m pretty sure CAV on a 5” disc is literally impossible.
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 Post subject: Re: Making your own LDs at home?
PostPosted: 22 Aug 2018, 06:39 
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Quote:
How is the signal exactly modulated?

There are hardware RF modulator devices such as the Kenwood DV-3560 (mentioned some time back when one was on Yahoo Japan) which were used in LD mastering, if you want an off the shelf solution for this. By their nature such devices are rare, but not necessarily valuable given their uselessness today, and it would seem Japan might be the best bet for finding one.
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 Post subject: Re: Making your own LDs at home?
PostPosted: 22 Aug 2018, 08:22 
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I don't have the spec for CDV but I'd believe that it'd have to be CLV... they probably spin at a much higher RPM.

It would be rather difficult to make a CDV burner, and even moreso if it really does need a special blank like Pioneer used. It'd be very cool to have one, but considering we live in a world where you can get 64GB microSD cards for <$20US...

Maybe I should've grabbed the DV-3560 afterall, but it's a bit late now apparently. I don't know if any still exist - they haven't been generally useful for over 15 years now :(
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 Post subject: Re: Making your own LDs at home?
PostPosted: 22 Aug 2018, 16:59 
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For a CDV the laser moves right into where the label is on an LD. In CLV you can just spin the disc faster and make the laser track faster in order to read frames at normal refresh rate. For a CAV disc you’d have to make a higher density disc which may even require a higher frequency laser. The closer to the lable you get with CAV frames the smaller the laps get. You can’t shrink the CAV “lap” beyond what the laser can read and the limit is probably pretty close to the beginning of a normal CAV side.

I’m...pretty sure I’m right about all this. The RPM is set at 1800, the TV needs 60hz, and the laser’s frequency is set as well. In that scenario there is going to be a lap too small at some point.
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