[ndiddy]

The EXT pins only allow setting the backdrop color to one of the palette values in the master PPU's color palette, so the Titler can't use that functionality to superimpose graphics over arbitrary video. It actually uses an RGB PPU which doesn't even have the EXT pins. Instead, it uses a separate video encoder chip for the video superimposing feature. https://www.nesdev.org/wiki/X0858CE

[erik]

Oh, interesting. I assumed the PPU could pass through video the way the TMS9918 can. My mistake!