8

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).

  • Willing to modify the monitor, and able to do so safely? – rackandboneman Apr 20 '17 at 15:23
  • 1
    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
  • 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
7

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 setted 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 ambar 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 ambar, 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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