Famicom + NESRGB pixel alignment issue

[mikejmoffitt]

I've installed a NES RGB into an original Famicom by hand-wiring it in place, as the existing adaptation solutions don't conveniently allow the eject mechanism to remain functional. I did so after a bodged install of a doujin adapter by "Bakutendo". I'm seeing some odd color glitches.

The Installation
I've done this once in the past - the NESRGB lives on the underside of the Famicom, rotated by ninety degrees. I only had to lift the PPU's CPU-side data pins D0-D7, and the four EXT bus pins; the rest can stay soldered. I also desoldered the transistor that buffers the composite video in the original configuration.

The address lines, control signals, clock, etc. for the PPU have been connected to the NESRGB in parallel, while the lifted data pins are wired such that they match how the NESRGB behaves with a PPU soldered in (filtering the palette data).

I used relatively short wires and grounded the board thoroughly. I would be surprised if this ~3MHz data is too fast for the wiring I've done.

The Problem
From time to time, sprites and background edges will have single-pixel wide color fringes. The leading edge of the fringe is generally a background-palette color while the following edge is generally a sprite-palette color. A technical explanation for this requires a bit of backstory. The NESRGB (ab)uses the original composite video output, paired with a custom palette, to extract the "missing fifth bit" of information from the EXT bus's pixel data, which describes whether or not a pixel refers to a sprite or background palette index.

From my experimentation, it appears that at least on my GPM-revision Famicom, the alignment of the EXT pixel bus and the composite video output is not guaranteed, and it can vary by four degrees of 1/4 of a pixel!

Here's me moving up and down the menu in Dr. Mario, which for whatever reason causes the alignment to neatly jump by 1/4 of a pixel.

https://www.youtube.com/watch?v=Ofhu-S1BRf4

The problem is seen best on games with sprites and scrolling, like Ducktales or Gimmick!.

https://www.youtube.com/watch?v=f2t8wH40zRE
https://www.youtube.com/watch?v=BheaMJnFP4w

These issues do not have anything to do with the analog output, nor the monitor in use - that has been inspected and tried on multiple monitors and capture devices.

Has anyone seen anything like this, and found a resolution?

[viletim]

The NESRGB aligns to the first pixel that comes out of the EXT bus. It happen once at the very start. The only way it can come out of alignment from then on is to miss a clock pulse. Make your clock wire shorter or keep it away from interference.