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Mechanical Scanning Television — Parallax Forums

Mechanical Scanning Television

Invent-O-DocInvent-O-Doc Posts: 768
edited 2013-04-01 20:26 in General Discussion
I can really use some advice here:

I was pretty excited after looking up "Nipkow disc" online and finding www.nbtv.org which is a design (a bit updated from Baird's 1932 model) for a nipkow disk television - which is 32 lines. (they have 60 line systems mono and color). Anyhow the basic operation is to encode video into a WAV sound file or put it on CD. The line level signal goes up to 1 volt. 0.3-1.0 volts is black to white and 0.0 volts is used for a sync for the scan motor between frames. The disk (which has spiral holes) is supposed to rotate at 750rpm and ideally sync with the picture. There is a modulated light source (LEDs or Neon lamp) behind the wheel. Video plays 32 lines at 12.5 frames per second. There are utilities that can convert video to 'narrow bandwidth' format and there is also a color version.

So, here is where I need help. The reference circuity uses a PLL for sync and a bunch of op amps to run everything and drive the LEDS/LAMP. That is a fair analog solution. I was thinking that a prop BOE could be a simpler replacement for the analog stuff. I could PWM the LEDS and I could use the A/D converter on BOE to grab sound voltage (or extract it from WAV files directly). This should be pretty straight forward.

So, here is my problem, how do I get a motor to spin the disk at 750rpm? How do I get it to sync with an encoder? There is a lot of stuff on motor controllers (stepper), but I want to use PWM or something simple to control a DC motor. I was thinking of using PWM to an FET (IRL 510 or something)? Is this reasonable?

Any advice would be appreciated.

Comments

  • ercoerco Posts: 20,256
    edited 2013-03-30 18:00
    Old school way is an AC synchronous motor: http://www.ebay.com/itm/WESTINGHOUSE-AC-MOTOR-P-N-763C742H01-HP-1-3-RPM-750-1140-1-3-A-50-60-HZ-/290854595126?pt=LH_DefaultDomain_0&hash=item43b848ea36 That would be a quick and rock solid solution, but heavy and power-hungry.

    A modern (this millenium!) DC motor and speed control would be smaller and more efficient, given there's no load on the disk. I'm betting a small DC gearmotor with an encoder with speed feedback and a PID loop would hold the RPM and generate the sync signal you need.

    Old TVs are very cool. I've wanted mechnical TVs, but so far all I own is two 1960 tube-based Predicta TVs. Those are awesome.
  • ercoerco Posts: 20,256
    edited 2013-03-30 18:19
    That Ebay link read 750 RPM, but upon closer examination, the motor label reads 950 RPM. Here's a cheaper, smaller motor for less: http://www.surpluscenter.com/item.asp?item=10-1702&catname=electric

    Too bad they're all 230 VAC!
  • Invent-O-DocInvent-O-Doc Posts: 768
    edited 2013-03-30 18:27
    The Predicta is a classic! I had an early motorola color set growing up.

    I'm wondering how to get 750 rpm at maybe 12 volts DC or less.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2013-03-30 18:36
    Get a motor that goes faster than that.
    Use an opto arrangement to sample the speed.
    "In the day" there was no blanking interval, front porch/back porch, or other synchronisation in the receiver.
    They had motors that ran "at or near" and they resigned themselves to getting up and readjusting or just staying at the controls - to adjust the motor speed and rotate the motor on its mount to, in effect, adjust the vertical hold.
  • Invent-O-DocInvent-O-Doc Posts: 768
    edited 2013-03-30 18:39
    That might be worth it. Not sure how to go about it. Is it enough to sample the speed via optical encoder 1x/revolution or does it need to be more frequent?
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2013-03-30 18:45
    1 ought to do it, it's spinning darned fast. You want the reference to be the reference.
    They used to sit there with a normally-open pushbutton and peck at it till it came right.
    It was a real hobby.
  • xanaduxanadu Posts: 3,347
    edited 2013-03-30 19:14
    Doc,

    A 1x encoder will be fine, that's 12.5 samples per second at the target speed.

    Do you have a pic of the motor and wheel you're using?

    This reminds me of the old fish finder displays although I'm not sure it's the same.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2013-03-30 19:21
    The "wheel" will be a custom job, no "COTS" there.
    That's the other part, the placement of those holes, concentricity, all add up appreciable error in short order.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2013-03-30 19:31
    I did this with a Commodore 64 sending the "video" in assembly.
    It was only a character (a letter).
    My final disk had two sets of holes. It was an even smaller picture for that, but it enabled me to use a slower motor speed, having the "raster" in both halves.
  • GadgetmanGadgetman Posts: 2,436
    edited 2013-03-31 05:50
    How big/heavy is the disc?

    Could something like the stepper used in a floppy (5.25") or CD/DVD-drive be used?
  • Invent-O-DocInvent-O-Doc Posts: 768
    edited 2013-03-31 07:21
    5.25" floppy drives ran at 300rpm. The disks can be paper, plastic. I've seen them made of metal and even one made from a vinyl record.

    Also, Phil P used a nipkow disc to make a low speed infrared imager a few years back - it was really cool.
  • ercoerco Posts: 20,256
    edited 2013-04-01 20:26
    PJ Allen wrote: »
    "In the day" there was no blanking interval, front porch/back porch, or other synchronisation in the receiver.

    Sweet info, but I learned everything I need to know about porches from Disney: http://fancysomedisneymagic.tumblr.com/post/24166089822/women-like-a-man-with-a-big-back-porch
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