In order to really test TBC's, I need some kind of signal generator with custom sync so I can create unstable video and see how the TBC reacts.
I've thought of 3 off-the-shelf ways to do this. This is going to be technical so experts only.
One way is, there's programs in Linux to create arbitrary RF/video signals. It's really simple actually, just show a picture on the screen and hack apart your VGA cable. You can set the frequency with your video mode. This gives an instant 100MHz or more signal of any type, except you need some way to pull it negative to make your SYNC levels, and thus will not have much leeway to make picture data, just a few bright shades of white.
People have broadcast working digital TV just with a program and holding the VGA cable to an antenna.
Another way that's very simple is again make the right kind of picture, and just use your Pb output. The Pb signal can vary +-.35V. If you tie Pb and Pr together, you can get +-.7V. There will be a gap in the video you can create, because of the borders where the actual sync would have occurred.
The last way is to use some old computer or game console and program it's video chip yourself. There's a way to create interlaced video on a C128 with the sync anywhere you want it.
Can you think of any other way? What if you find some sync generator chip and give it the wrong clock frequency? TV from uController projects?
I thought of another way to digitize the full raw video - just plug it in together with a DVD. The two signals will mix and you'll see a drifting picture on the screen. You can digitally subtract them and get the 2nd signal. You might need a little levels adjustment so the 2nd signal is ignored by the capture card, just divide it down with a resistor for example. Or you could just feed the 2nd signal into Pb in, and interpret the color as the video.
There's the gaps problem, but if it were a steady signal you'd just have to wait for it to drift all over.