9

I know some monitors have a mono/green switch, but I'd like to do this with my 1702. If one doesn't exist, I'll probably try and make one (somehow).

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    Willing to modify the monitor, and able to do so safely? – rackandboneman Apr 20 '17 at 15:23
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    The hack-solution is to just connect the composite video to the green input on the monitors. A lot of monitors have sync-on-green, and the composite signal has the luma (grayscale) in the baseband, plus sync, so you'd end up with a green version plus a little high frequency noise. I'm not writing this as an answer though, because I haven't done any calculations. :) – pipe Apr 20 '17 at 16:28
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    If you turn the "tint" dial all the way to the right, does the image turn completely green? – snips-n-snails Apr 20 '17 at 20:41
  • I had considered modifying my 1702, but I would prefer a little circuit or device to do it. As for tint, it doesn't seem to turn it completely green. – Brian Sturk Apr 21 '17 at 13:32
  • Not for Commodore, but: Such a device exists for the Apple IIc - It takes its video signal, converts it to VGA and allows modifications to it like the one you mention. a2heaven.com/webshop/index.php?rt=product/… – tofro Sep 3 at 12:40
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I don't know if such device exists as a final product, but here's some hints about how to build one:

Composite video signal is made up from the analog sum of two components: luminance and chrominance. We are interested in the luminance component. So you should rip off the chrominance signal by using a low-pass filter with the cut frequency set at 3.5MHz (for NTSC signals) or 4.43MHz (for PAL signals). If no high resolution graphics are going to be displayed (that is, no more than, say 320 pixels per scanline), you may go with a low-pass filter with a cut frequency of 3MHz which will work on both PAL and NTSC systems. Analog Devices offers an online filter generation tool here: http://www.analog.com/designtools/en/filterwizard/ which I have used to generate this filter (I'm not by any means an expert on analog electronics, so this filter may totally fail)

Filter parameters

Filter circuit

The resulting signal is monochrome video. To make it amber or green you need to get the video and sync information on separate signals. To get syncs, you may use an LM1881 video sync separator chip ( http://www.ti.com/lit/ds/symlink/lm1881.pdf) , which will give you an output (pin 1) with sync signals only.

LM1881 circuit

To get the active video signal, use a sync stripper as the one described in Intersil AN9752: http://www.intersil.com/content/dam/Intersil/documents/an97/an9752.pdf

Sync stripper circuit

Now you have a monochrome video signal and a composite sync signal. You can use this video signal as the green channel, keeping R and B channels black (tied to ground), to get a green over black image. For amber, you can route the video signal to the R channel, and using a voltage divider, route half this video signal to the G channel, keeping the B channel black, to get an ambar-like colour.

This way, you will get an RGB signal with the configuration you want. If you need a composite video signal as final output, use an AD724 PAL/NTSC encoder ( http://www.analog.com/media/en/technical-documentation/data-sheets/AD724.pdf ) to get a composite video output from the composite sync signal provided by the LM1881, and the RGB channels, arranged the way you like from the monochrome video signal.

AD724 circuit

  • Looks like I put my comment for this answer in the below answer, my apologies. I appreciate all of the feedback, a lot to digest. – Brian Sturk Apr 21 '17 at 13:48
  • Minor observation: real screens don't necessarily use this sort of frequency filter — things like comb filters were also [eventually] common. Potential easier hack if you're European: plug your computer into a VCR or DVD recorder and use the RGB exported directly from the SCART socket. – Tommy Apr 21 '17 at 18:03
  • If your source is RGB without video on the sync line, what's a good way to get luma to feed into the AD724? Another AD724? – snips-n-snails Apr 21 '17 at 20:04
  • AD724 uses RGB as source video, and the sync line doesn't (and shouldn't) contain video information. If you want to get a "green on black" effect from a RGB source, it's much easier: just use a weighted adder (three or four resistors) to sum all RGB signals and get a monochrome one. That signal may be routed through the G output channel, leaving R and B black. The sync signal would not suffer any changes. If you want a composite video signal as output, then you may use an AD724 to get it. – mcleod_ideafix Apr 21 '17 at 23:10
  • Following up on my own line of thought: if in a NTSC country, the easiest heck of all would be to use S-Video (via a VCR or directly from the machine); leave the luminance alone and output a constant colour-subcarrier frequency sine wave as chrominance. I feel like that produces an amber-ish hue but will check. – Tommy Apr 22 '17 at 20:01
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If you are going to do this "properly", @mcleod_ideafix's pipeline of circuits to extract the luminance and then re-encode it onto just the green channel is the way forwards. I would enhance the design by adding some sliders/pots to allow tuning the exact RGB colour, since "green" monitors and colour monitors did not always use the same shade of green phosphor, and also you might fancy an amber display one day.

However, the way I'd probably do this, at least as a prototype, is to turn the "colour" right down to get a monochrome display and then stick some coloured film over the screen. Hey, it worked for the original Space Invaders! It would probably also have fewer transcoding artefacts than multiple stages of re-encoding a composite video signal.

  • A perfect green filter will block out red and blue and make the screen black in those areas where it should be dark green. – snips-n-snails Sep 3 at 18:05

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