Has there been any more details or findings on this?
The same thing happens with my RGB modded NES when used on a XRGB3 as well. However, on the XRGB3 I can adjust the “AFC Level” setting and that fixes it. I don’t really know what that setting means, but didn’t know if adjusting it is something that could be replicated on the OSSC.
EDIT: Went and rechecked when using the XRGB3 and it seems it was cutting off some of the overscan. When I made it show all of it at the top, there was still some slight distortion even after adjusting the AFC level, but it was significantly fewer lines affected than when using the OSSC.
Using the vertical mask to hide it does make it less annoying, but then I’m annoyed (to a lesser extent) by the black bars at the top and bottom of the screen :P.
There are some details/hints on previous page such as disabling “Allow TVP HPLL2x” or trying 256×240 optimized mode. However, a proper solution would be to modify NES itself as I wrote earlier. Assuming PPU uses flip-flop clock divider instead of PLL (for 21.477272MHz / 4), there’s a good chance a solution would be as simple as gating four master clock cycles on PPU input during the short scanline to normalize its length (in time domain). To my knowledge there are no timing glitches when NES is used with PlayChoice PPU (which does not have the short scanline) so there should not be any compatibility issues with the mod, but I’ll need to try it for real at some point to verify that.
I’m seeing this exact same thing out of my RGB modded NES. I have my OSSC mated to the TCL 43S405 – a budget 4k TV with a reputation of working extremely well with the OSSC specifically. The distortion appears in x1, x2 and x3, but the picture appears to be slightly cropped in x5 mode and therefore cuts off the area that is affected. I’m usually one that loves to game with every pixel showing, but I’ve been playing in mainly 5x mode due to this as it’s really distracting to see that garbled mess at the top of the screen! Didn’t ever see this on my old Framemeister setup and it was displaying every pixel as well…