Ghetto custom 31 kHz video mode

Could you output color $1D in the center of the screen for the same width as the sync pulse, surrounded by black for the porches, and enable a ghetto low-resolution ~30KHz refresh?

Color $0D plus full darkening from all the color emphasis bits gets you something only half the depth of the NES's normal hsync pulse... so not clear whether that would work.

It's easy enough to make a test ROM, though. I just don't have a screen that parses 30kHz Sync-on-luma to test with. (Also, sprites become obnoxious and you lose color, but meh)

I don't think so, because the line timing would be way out of spec for the NTSC signal, so most TVs would probably reject it, plus, the timing of all the signals is stuck in increments of 8 pixels, with the final nail in the coffin being that $0D isn't the sync level, the most it does is distort the picture on some monitors.

Well, "distort the picture on some monitors" does mean "is being parsed as a sync pulse" ...

The sync signal doesn't need to be active for the entire time in order to perform horizontal retrace, that's why it's sandwiched by the blanking periods. Color $0D as far as I know only slows the gun down, shrinking the scanline at that point; it doesn't trigger a full retrace. So yes, the monitor may be misinterpreting $0D as a weak sync, but it's never enough to actually trigger a new scanline.

Televisions that could have done the right thing when given a 31kHz sync-on-luma input have only existed for the past 10 years or so, and it would be only likely to be discovered by accident with one of the new TVs that automatically switches between composite and component depending on the presence of red/blue cables. I really think a test is necessary before the possibility can be completely ruled out.

There are plenty of other reasons it might not function, but I'd personally be more suspicious of the vertical blanking time than of the too-high fake sync pulse.

I should have been more clear. This is something I would want to test using a modern television that can accept a 31KHz sync-on-luma input, like the component input of an HD CRT or a flat panel display. It would be interesting to see if it is willing to sync to something kind of close to a 31KHz "480p" signal.

In a few weeks I will be picking up an everdrive N8. I might whip up a test ROM for this and give it a try.

Doing some simple math: it looks like the "correct" place for the second fake Hsync pulse should be at x≈81, 24 or 25 pixels long, and should be followed by ≈65 blank pixels.
This leaves two ~80-pixel wide fields making up the visible width, if it works.

Neat! It would be neat to make the world's only 80px-wide "480p" NES game!

Can we split off the "ghetto" custom video modes into a separate thread, please?

Suppose this works and you get an 80x480@60hz picture which works on only a handful of TVs/monitors (and that's if and only if color $0D plus emphasis bits is strong enough to trigger horizontal retrace), which means a doubled vertical resolution at the expense of making pixels 3x wider, and every other scanline is grayscale due to a lack of a colorburst signal and possibly wiggly due to the lack of the pulse signal. Hurrah? Another accomplishment for nesdev?

I'm just saying don't be disappointed when this doesn't work, because it'd be just as disappointing if it did work.

And that's before you have the PPU-variant color-emphasis SNAFU.

To be fair, that only (potentially) affects the red and green bits, so activating all three bits will always result in the darkening, except for RGB PPUs in which it results in complete white.