Re: [Algorithms] FPS Questions

[Thatcher U. <tu...@tu...>]

From: Robert Dibley <RD...@ac...>


> or it can display in non-interlaced mode, so the signal makes the scan
lines
> appear on top of each other, and you don't get the flicker.
>
> thats what most consoles do when not in hi res mode.

Yeah... I got acquainted with the intricacies of non-interlaced NTSC-like
video modes while working on the VR Bike & VR Climber, in which I had to
drive an arcade-style TV monitor from a regular VGA card.  Whereas a regular
NTSC frame has 525 horizontal lines for every two vertical syncs (i.e. 262.5
lines per field), the non-interlaced version drops the extra half-line, so
you have 262 lines per field.  Assuming a fixed horizontal scan rate, you
end up with a slightly higher vertical scan rate.  Directly connecting to a
monitor or a TV is no problem for such a tweaked mode, but it probably would
choke most pro video gear; for those kinds of applications we ran an
ordinary VGA signal through a scan-converter box.  If you diagram out the
timings you can see how it all fits together, and it becomes apparent what a
clever trick interlacing is.

Unfortunately, on larger monitors in non-interlaced modes the individual
scan lines can be excessively obvious.  I did a bunch of experimenting,
trying out modes with more horizontal lines, to combat this effect.  To get
more lines in a frame you have to speed up the horizontal sync or slow down
the vertical sync, which is all within the capabilities of a standard VGA
card.  There are tradeoffs naturally; with a slower vertical sync you have a
slower refresh and more obvious flicker, with a faster horizontal sync you
may have less horizontal resultion depending on how the video card timings
work out.  The big problem for me though was that while all monitors could
sync to a 262-line signal at ~60Hz refresh, they varied in how far
out-of-spec you could drive them before they completely lost sync.  I even
went as far as hacking up a circuit to induce interlace against the will of
the monitor by delaying the vsync signal on every other pulse, but that
didn't really work reliably.  Plus, interlaced modes look a little
"shimmery" to me, not as nice and solid as non-interlaced modes.  I ended up
sticking with the 262-line mode, and not worrying about the obvious scan
lines.

If you look at old arcade games you can see that many of them have similar
issues with scan lines.

Anyway, I don't think I have a point, but I couldn't resist the urge to talk
about the old days.

--
Thatcher Ulrich
http://tulrich.com