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TTL video capture

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First post, by keenmaster486

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I'm starting this thread to talk about TTL video capture (MDA, CGA, EGA, etc.). There's already a long and extensive thread on VGA, which seems to be a beast no one has yet fully tamed, but TTL is another story.

There's this: https://texelec.com/product/rgbtohdmi-ttl/

The RGBtoHDMI project seems to have leapt ahead of the various cobbled-together VGA solutions that plague the analog side of things, outputting a nice, crisp image that looks in some ways nicer than even a DOSBox capture.

There's one question I haven't been able to get an answer to by reading through various forum threads on this subject, though, and that is: how do we pass a TTL signal back to the monitor after capturing it (edit: this was unclear. I'm talking about real time, not playing back the capture on the original monitor post facto)? There doesn't seem to be an established way to do this. I can't find any examples of people doing this on the internet. Is it technically impossible? Would be nice to play and stream/record at the same time, on the original monitor rather than a VGA or HDMI monitor split off from the capture feed.

Last edited by keenmaster486 on 2023-03-29, 22:52. Edited 1 time in total.

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Reply 1 of 4, by maxtherabbit

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I suspect (but would need to verify) the TTL RGB color lines are high-z terminated just like the syncs. If so, you could just use a simple passive cable splitter

Reply 2 of 4, by Benedikt

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AFAIK, the Cypress chip in those cheap logic analyzers that some have used for TTL video capture can technically do output, too, provided that you feed them appropriate firmware.
However, the jitter introduced by the sample clock, which is usually derived from 48 MHz, would be even more of a problem, i.e. original 16 MHz Hercules would be fine, but everything else problematic.

I'm assuming that you also want to play back the recorded video signal on the old monitor afterwards.
If you simply want to record and display the signal at the same time, a trivial passive splitter should indeed be sufficient.
If it isn't, just add a 74LS244 or something.

Reply 3 of 4, by keenmaster486

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It sounds like a passive splitter may just work. Will have to try this.

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Reply 4 of 4, by keenmaster486

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Update: the RGBtoHDMI works perfectly with a DB9 splitter. The conversion to HDMI is nice and clean. I ordered the wrong gender of splitter, but it still works of course since it’s all just electrically connected to the same place.

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