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From: perkins@bnrmtv.UUCP (Henry Perkins)
Newsgroups: comp.sys.ibm.pc
Subject: Re: millisecond timing
Message-ID: <2279@bnrmtv.UUCP>
Date: Thu, 23-Jul-87 18:45:49 EDT
Article-I.D.: bnrmtv.2279
Posted: Thu Jul 23 18:45:49 1987
Date-Received: Sat, 25-Jul-87 12:39:41 EDT
References: <1@epistemi.UUCP> <2169@bnrmtv.UUCP> <5068@jhunix.UUCP>
Organization: BNR Inc., Mountain View, California
Lines: 35
Summary: Timing accuracy can't be better than 33 milliseconds due to screen refresh interval

In article <1@epistemi.UUCP>, martin@epistemi.UUCP (Martin Shepherd) wrote:
> >> I need to be able to "blank" and "unblank"
> >> the monitor screen so that the onset of a display is more or less
> >> instantaneous.

In article <2169@bnrmtv.UUCP> perkins@bnrmtv.UUCP (Henry Perkins) responded:
> >Your accuracy can't be better than 1/30th second (color monitor)
> >or 1/25th second (monochrome monitor), because that's how long it
> >takes to refresh a screen image.

In article <5068@jhunix.UUCP>, ins_bjjb@jhunix.UUCP (Jared J Brennan) responded:
>    As far as blanking the display goes, well, you'll just have to settle for
> rewriting the strings with blanks (works much better than cls'ing), whether
> by writing to the display memory or puts()'ing or whatever.

The mechanism doesn't matter; you can rewrite just part of screen
memory or blank it all.  The important consideration is that what
you change in screen memory only affects what is actually sent to
the monitor 30 times a second.  You can "display" something and
"erase" it 106 times with NOTHING appearing on the screen if you
do it all in less than one video frame interval.

Yes, you CAN change the timer chip to allow millisecond accuracy.
IT DOESN'T MATTER.  All your timings are + up to 1/30 second (33
milliseconds) since your subjects are responding to screen images,
and you don't know when the images actually get to the screen any
more accurately than that.

"More or less instantaneous" is NOT possible.  "Within 33 milli-
seconds" is what you can get.
-- 
{hplabs,amdahl,3comvax}!bnrmtv!perkins        --Henry Perkins

It is better never to have been born.  But who among us has such luck?
One in a million, perhaps.