TMS-RGB vs 'Hans Otten' PCB

A similar method to create RGB is present in some MSX1 computers. I’m the Netherlands the Sony HB-75P and JVC HC-7GB has such circuit. Whilst on this machine you do get RGB the quality of the RGB cannot compare to what we are seeing on machines with V9938 or V9958. Hans’s schematic allows for tuning the individual colors so it might get better results but it is hard to predict without building the board. As you said it will be cheap to build so don’t expect miracles, then again it won’t cost you an arm and a leg you try it out…

One issue that I suspect will happen with this schematic is jail bars. The sync output is derived directly from the Y signal so it is not a clean CSYNC. This also is visible in the 2 computers mentioned.

Is that due to the way the VDC works, or something to do with the technique used for that converter?

On computers without composite sync (such as my FM-7, where the RGB connector has just separate hsync/vsync) I regularly plug the (monochrome) composite video output into the external sync input of my Sony PVM, and I don't recall seeing jailbars when I did that.

Why would that be?

Well, of course, as all displays must. Pretty much every display is going to need separate horizontal and vertical sync signals internally, so the internal circuitry will be separating them. It's not too hard when doing that just to ignore a luma signal mixed in with the sync.

The same often goes for RGB, too. All CRTs and LCD displays (as far as I'm aware) need separate R, G and B luminance signals to drive the tube guns or the pixel matrix, and so a composite or Y/C input will have to be converted to RGB for that purpose. This is why you can mod so many consumer TVs to have an RGB input.

Not actually; the only thing you need is a display that doesn't get upset when you give it 5 V on its inputs, which my PVM does not. (It of course does drive the three luminance signals to maximum brightness, but that's more or less what you want anyway.) That said, yes, I do use a DRGB converter board that brings the levels down to, I think about 1 V P-P. (IIRC, I tried it at the 0.7 V P-P from the monitor's spec sheet but it didn't work so well. But I could be wrong; I've not measured the current settings of the pots on my board.)

But I decided to look around to find out what usually causes jail bars, which took me down a bit of a rabbit hole, including this forum thread and this excellent video. The source of everything there can be summed up as "interference affecting the chroma signal," which makes perfect sense: chroma is by far the highest frequency signal involved (several octaves higher than luma) and also of lower amplitude than the luma, so it's likely to suffer most from any distortion. The grounding fix near the end of that video ("Fourth change") produced a particularly impressive improvement.

But RGB doesn't have a chroma signal, just three separate luma signals, so none of the sources of interference mentioned in the posts and video above appear to be relevant. Are you saying that the jailbars are related to the Y/R-Y/B-Y to R/G/B conversion in some way? What would be the source of the issue, and how would you change the circuit to clean it up?