BitWrangler wrote on 2023-01-28, 03:59:

You've got me wondering what happens if you stick one of those 14.375ish Mhz crystals in there, whether it synchs on a TV or video monitor. Then maybe it has overscan?? IDK

My head is also going toward the simpler Amiga scandoubler plans and the stuff Grant Searle has done for composite display, using atmels, where maybe it could intercept the signal digitally then videofy it.

These are some interesting thoughts! 😁 👍
Since MDA/MGA are 1-Bit in its simplest implementation (pure mono video, omitting intensity for now), an A/D and D/A conversion for TTL<->Video might be possible without so much trouble.
I mean, a framebuffer in its simplest form (holding a 1:1 bit pattern of zeros and ones) would do. Timing/sync are a different story, perhaps, needing some dedicated glue logic.
But I'm optimistic the process of "digitizing" would very well be possible with little code. CGA would be more complex, at least in 4 colour modes, I guess.

Speaking of Arduinos/Atmels, here's something interesting:
External RAM with Atmel: https://spritesmods.com/?art=avrcpm&page=2
Compsoite (VBS, actually) with Arduino/Atmel 328p: https://nootropicdesign.com/video-experimenter/

BitWrangler wrote on 2023-01-28, 03:59:

You've got me wondering what happens if you stick one of those 14.375ish Mhz crystals in there, whether it synchs on a TV or video monitor.

An MDA is not going to synch on a TV or video monitor by just changing the crystal to 14.318 MHz, for multiple reasons:

• The horizontal timing parameters for mode 7 and mode 3 differ.
• If you manage to find the correct crystal frequency (If I calculated correctly, it is 13.783MHz) so that the horizontal timing of an MDA card is dropped from 18.4kHz to 15.6kHz, the vertical frequency will
  have dropped from 42.4Hz, whereas NTSC video timing requires 60 Hz. This is due to the vastly higher scan line count of the MDA mode (350 visible lines) compared to TV timings (around 240 visible lines).

I'm still gonna wonder what happens though. Analog ain't all that notchy, it's like bellmouth vases where if you flip a coin and it lands on the rim it will slip in, even though it's inches off the perfect center. Though yah, it ain't necessarily gonna be repeatable across a range of different CRTs