Higher than 60Hz on NTSC

[Andrew Davie]

I was thinking about ChronoColour and the effective 20Hz "shimmer" -- that is, changing colours in triplets at 60Hz --> a 20Hz cycle.  If the frame rate increased, then the shimmer would become less obvious. And then I realised I don't think I've seen much discussion about increasing the frame rate on NTSC. Of course we do PAL60, which is pretty much a standard. But what about (say) NTSC 66?  I wonder just how much we can go above 60Hz and still be compatible with most CRT NTSC TVs?

[alex_79]

Not really an answer to the question, as I don't have NTSC TVs, but on most "recent" (from the '90s onward) PAL CRTs I tested, those that can handle PAL60 with correct aspect ratio (and also NTSC over composite/RGB but rarely oover RF), the minimum number of scanlines that doesn't cause a screen roll is 244 (64Hz).

The maximum is 360 lines. And they display with "NTSC aspect ratio" (scanlines farther apart) for scanline counts up to 287, and with "PAL aspect ratio" (scanlines closer together) for counts of 288 and up.

[Thomas Jentzsch]

241 (one version of Artillery Duel) was the minimum back then. And IIRC these cause problems (rolling) on some TVs.

[Yurkie]

I was thinking about ChronoColour and the effective 20Hz "shimmer" -- that is, changing colours in triplets at 60Hz --> a 20Hz cycle.  If the frame rate increased, then the shimmer would become less obvious. And then I realised I don't think I've seen much discussion about increasing the frame rate on NTSC. Of course we do PAL60, which is pretty much a standard. But what about (say) NTSC 66?  I wonder just how much we can go above 60Hz and still be compatible with most CRT NTSC TVs?

What if you use a composite to HDMI like retrotink? Is is possible to go to like 120Hz?

[Andrew Davie]

You can increase the frame rate, at the cost of # scanlines. They are inextricably intertwined.  This is the reason PAL is typically 312 scanline @ 50Hz, and NTSC is 262 @ 60 Hz.  You can work it out; essentially 76 machine (processor) cycles per scanline.  So take the processor speed and divide by 76, this gives you the number of scanlines you get per *second*.   Now divide by desired frame rate, that gives you the number of scanlines per *frame*.  For example, clock speed on NTSC machine is roughly 1193182 Hz.  So, 1193182/76 -> ~15700.  Divide by 60 frames/sec and what do we get?  15700/60 -> 261.67
OK, here we learn NTSC isn't exactly 60Hz . It's 59.94 IIRC.  15700/262 (scanlines) -> 59.92 frames/sec.

So you see, if we want 120 Hz -- assuming the TV/display could handle that -- we would be feeding it with 15700/120 -> ~130 scanline deep frames. Which would be really not that great.

As always with the '2600 there's a really tightly coupled tradeoff with all of the hardware and driving it.  The frame rate and the number of scanlines are, as noted, coupled tightly based on the clock frequency of the processor.  You change one, then you inevitably change the other.

[Yurkie]

thanks for the great explanation.

[SpiceWare]

Had similar thoughts back in 2016 when I did a 128 pixel chronocolor demo while we were experimenting with bus stuffing.  I modified the demo to let you control the scanlines per frame via the joystick and was able to get a 77.82 Hz refresh rate on my C=1084S. It did help, though extra stuff did show up at the top of the screen.