Does anybody know how I can go from an analog RGB signal to a TTL RGBI signal?

Is there a good circuit for this out there somewhere?

  • 2
    What is generating the signal, and what do you want to do with it?
    – supercat
    Commented Dec 14, 2018 at 19:16
  • 3
    You want to connect a CGA/RGB monitor to a computer producing a VGA signal? Normally, people would want to go the other way round.
    – tofro
    Commented Dec 14, 2018 at 19:16
  • 2
    If the other way around: retrocomputing.stackexchange.com/questions/5609/… Commented Dec 14, 2018 at 19:41
  • 1
    A use-case for the question as posted: CGA emulation on a computer with VGA output who wants to hook up an old CGA monitor. Or by "CGA" maybe @user11397 really means analog 15 kHz RGB video, commonly used by arcade games and mislabeled "CGA", and the goal is to run MAME on a computer with VGA output and install it in an old arcade cabinet. Commented Dec 14, 2018 at 19:57
  • 1
    Could use some more details in the question. "Analog RGB signal" leaves much to the imagination. The specific devices and your intended use-case/result will help stimulate more useful answers.
    – Brian H
    Commented Dec 14, 2018 at 22:43

1 Answer 1


I assume you want to connect machine emulating CGA image through VGA to the native CGA monitor.

It is of course possible, but mind cost and quality.

To formally downgrade VGA to CGA you need scan frequency go down from 31 kHz to 15 kHz, and thus dot clock will half too, but you will need to read two rows and perform intelligent generation colors of the single CGA row out of two VGA rows (similar operation to be done on dots located within the same row).

Then, color encoding will be lossy, but it will not be a very big problem if you output CGA image onto VGA - you will only need proper conversion table, however this may require tuning or adjustment within various setups in regard to analog signal integrity, strength and noise.

The device, at its complexity level, should be the same complex as RGB to VGA converter (example).

I am sure that off the shelf devices do not exist for simple reason: there's no enough market demand. Probably you can simplify things by using VGA to RGB converter which outputs 15 kHz, then you will only need to convert analog to TTL using conversion table (as converter will perform clock halving and frequency conversion). But note: you must use active converter which halves the clock rate (the box), just direct cable will not work.

P.S. I think this question relates more to SE.EE rather than SE.RC.

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