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Ghetto custom 31 kHz video mode
http://forums.nesdev.com/viewtopic.php?f=21&t=11504
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Author:  mikejmoffitt [ Mon Aug 04, 2014 1:23 pm ]
Post subject:  Ghetto custom 31 kHz video mode

In this post, lidnariq wrote:
Yeah, the NES only emits one voltage—color $1D—for blanking, black, &c.

On the bright side, it's not the hack that was video for the ZX80 (which by default assumed that the TV didn't do any normalizing at all and so just emitted full white during the back porch)

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?

Author:  lidnariq [ Mon Aug 04, 2014 3:32 pm ]
Post subject:  Re: The Quest for the Perfect Palette Generator

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)

Author:  Drag [ Mon Aug 04, 2014 4:21 pm ]
Post subject:  Re: The Quest for the Perfect Palette Generator

mikejmoffitt wrote:
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?

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.

Author:  lidnariq [ Mon Aug 04, 2014 4:37 pm ]
Post subject:  Re: The Quest for the Perfect Palette Generator

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

Author:  Drag [ Mon Aug 04, 2014 5:15 pm ]
Post subject:  Re: The Quest for the Perfect Palette Generator

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.

Author:  lidnariq [ Mon Aug 04, 2014 5:41 pm ]
Post subject:  Re: The Quest for the Perfect Palette Generator

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.

Author:  mikejmoffitt [ Mon Aug 04, 2014 6:35 pm ]
Post subject:  Re: The Quest for the Perfect Palette Generator

lidnariq wrote:
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.

Author:  lidnariq [ Mon Aug 04, 2014 9:20 pm ]
Post subject:  Re: The Quest for the Perfect Palette Generator

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.

Author:  mikejmoffitt [ Mon Aug 04, 2014 9:30 pm ]
Post subject:  Re: The Quest for the Perfect Palette Generator

lidnariq wrote:
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!

Author:  NewRisingSun [ Mon Aug 04, 2014 11:12 pm ]
Post subject:  Re: The Quest for the Perfect Palette Generator

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

Author:  Drag [ Tue Aug 05, 2014 12:57 pm ]
Post subject:  Re: Ghetto custom 31 kHz video mode

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.

Author:  Myask [ Tue Aug 05, 2014 5:00 pm ]
Post subject:  Re: Ghetto custom 31 kHz video mode

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

Author:  Drag [ Tue Aug 05, 2014 6:07 pm ]
Post subject:  Re: Ghetto custom 31 kHz video mode

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.

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