Hi everyone. I know I'm a couple years late, but I wanted to share a few results from a very
similar project that we just undertook over at the AtariAge forums over the last six months. Unfortunately I didn't find this topic until after the project was complete or it would have saved a lot
of effort! You guys did a great job over here. Most of our lessons learned were exactly the same as honestbob's findings, though hopefully I can add one or two new points.
The project started
as a kind of survey of all the known approaches to get the 9928A/9929A to output RGB. We knew about the old "Interface to Color Monitors
" document from TI. We'd also seen the Hackaday circuit, which ended up being the most helpful/influential. I ended up building five
(!) different adapters on breadboards and the final result used at least one idea from each one.
The first discovery was that the TI datasheet for these VDPs has a major error: the 9928A does
have a color burst on (only) the B-Y line. My guess is that it's an unintentional bug in the silicon. (Perhaps after removing the red color burst, they thought their job was done and forgot to also remove it from the blue side?) So the NTSC version of the chip has no choice but to handle color bursts the same as the PAL version. And with only one color burst, the "burst averaging" solution proposed a couple pages ago here wouldn't be possible. So, the sample-and-hold idea from the Hackaday circuit became the best bet.
Like honestbob, we also saw lots of noise from the VDP. The filter on the TMS7374 cleaned most of it up. There are still some faint jailbars in solid black areas though, which can usually be cleaned up by tweaking settings on the receiver (OSSC, Framemeister, etc.).
One nice detail we were able to borrow from a project called ColecoRGB was the idea of packing the whole adapter/mod PCB into the same DIP-40 footprint as the VDP so that it can be soldered right onto the underside of the motherboard. This makes it compatible with anything
that uses a 9928A/9929A. Granted, it requires some pretty small surface mount components to get everything to fit.
The whole "TMS-RGB" project is completely open. Schematics and Eagle files are on its GitHub page
and I recorded a full set of build/install videos on the official site
Hopefully that's useful information to someone. It'll be nice to have it in the same place as the rest of this discussion for people that might spot this in the future. In any event, nice work again honestbob!