From tests I’ve done with the mid res dot clock, I’ve noticed some things. First off, is the color burst signal. This resides at the start of the line. There’s a toggle where every scanline has an out of phase color subcarrier signal. Of course, this matches the out of phase color burst signal so that it’s still aligned. This means the errors from the Y to C frequency ratio are in phase only every other line. The phase difference is exactly 1 pixel too, in 7.159mhz mode. This is why my checker board dither pattern didn’t get alternating color errors on every line, going down the screen. It remained constant, even though all dithering in my test image was offset by 1 pixel every other scanline.
Now, this was with H-filter off. When H-filter is on, the toggle mechanism still works the same. But with one small difference. When H-filter is on, there are now 263 scanlines. This translates into the next frame having the color subcarrier offset being reversed for the scanline layout. Then on the next frame, it’s back to the original again. And so on, and so fourth.
Basically, it takes 30hz to correct errors in the composite signal. Scrolling at anything above 30hz breaks the pattern/filter. My test image, the cat from MooZ’s forum post, is full of checker board dither all over the place. So when you turn *on* the h-filter in mid res mode, there’s massive flicker (I’m not doing anything to the image). Because there are huge areas where interference between Y and C are being corrected by the filter. No good. The image looks best with H-filter turned off. But… you need to take caution to adjust/make sure you’re in the right phase first. Again, this will probably require user validation on a test pattern, since you’ll never know where you are in the phase toggle/setup. Else, you’re image will be tinted either to red side or the blue side. The red side looks best IMO.
Another observation, comb filters on my capture card seem to remove the artifact from simple luma changes across a wide (scanline) pattern – max white to dark color (I noticed this with my capture card+hardware comb filter on+ Ccovell’s screen test demo). It turns it to solid gray when the comb filter is on. But doesn’t effect with my test patterns of color dithered image, though I don’t have any direct white/black dithering patterns. And, because of the phase errors – you simply can’t just mix colors assume they’ll average like that. There’s a ‘tinting’ involved (because of which phase of the VCE you’re on). It’d take a bit of work to calculate what colors you can come up with. But the more important feature out of all of this, is for transparency. In mid res mode, you’ll get a nice solid transparency mask with any sort of checker board overlay. More solid than what Genesis games attempt to do in similar fashion.