>Anyway, this prompts a couple questions:
>1) How do I know I've made _exactly_ the 262 lines per
>frame? It seems TVs are fairly resilient, and will
>allow at least a few lines of error, with ?no difference?
>visible. I mean, how can you know without counting
>through it all, and being ?certain?
The only way to be certain about the number of lines is to count them.
While NTSC TVs may show no difference, when displaying a couple of lines
too much, PAL TVs are a bit more picky. You have to do an even number of
lines, or the TV will loose the colour in that frame, due to the cheap
way Atari implemented the PAL encoder.
>2) Why is overscan differentiated from the main 192 line
>display? Can you not display graphics there? Should
>you blank the screen for that area? Is there anything
>special you should do in that area?
You can take a couple of lines from overscan to display graphics, but not
all TVs might be able to show that many. But usually you wouldn't want
that, because the VBLANK and the Overscan lines are the only time, where
you can use the processor fully for gamelogic. There is no time for that,
when displaying something.
You should blank the screen during overscan, because otherwise TVs, that
can display more than 192 lines (e.g. all PAL TVs) would show the content
of the graphics registers in every line till the end of the screen.
You might already know that, but you can also set up TIM64T timers during
the VBLANK and the Overscan lines, so that you don't have to count the
lines there and therefore are free to do longer gamelogic loops.
Ciao, Eckhard Stolberg
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