Re: The nextstep in user-friendliness for the iMac

Kragen Sitaker (
Sun, 17 Oct 1999 22:02:11 -0400 (EDT)

Lloyd Wood writes:

If you have, say, a 320x200 screen that's refreshing at 60Hz, you need
to time the light-pen's pulse to within 1/60*320*200 == 1/3_840_000 of
a second to get within-one-pixel positioning information. That's not
child's play, but it's not rocket science either. (It's actually a
little bit worse than this because of the retraces. But I'm not sure
how fast the old CGA screens refreshed, either; I suspect it might have
been slower than this.)

But if you have, say, a 1280x1024 screen that's refreshing at 85Hz, you
need to time the light-pen's pulse to within 1/1280*1024*85 =
1/111_411_200 of a second. That's about nine nanoseconds. This is a
little tougher, or so I've heard, anyway. It seems it would be
especially tough if you're transmitting your timing signals over USB,
which transmits a bit every 83 nanoseconds or so, and thus a byte every
666 ns or so.

I'm a little bit puzzled about why light pens need to be calibrated;
doesn't the video card *know* when it tells the CRT to sweep the beam
past the pixel at (327, 445)? Or is it a matter of compensating for
delays in the light pen itself? Maybe this would be obvious to me if I
knew more about light pens and CRT controllers. Or maybe it's just a
deficiency of FTG Data's designs.

If I get a touch screen, it damn well better be horizontal, btw.

I think this is an exceedingly cool idea, nevertheless.

<>       Kragen Sitaker     <>
Sun Oct 17 1999
23 days until the Internet stock bubble bursts on Monday, 1999-11-08.