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?
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?
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.