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From: perkins@bnrmtv.UUCP (Henry Perkins)
Newsgroups: comp.sys.ibm.pc
Subject: Re: millisecond timing
Message-ID: <2169@bnrmtv.UUCP>
Date: Mon, 13-Jul-87 17:01:29 EDT
Article-I.D.: bnrmtv.2169
Posted: Mon Jul 13 17:01:29 1987
Date-Received: Thu, 16-Jul-87 03:46:04 EDT
References: <1@epistemi.UUCP>
Organization: BNR Inc., Mountain View, California
Lines: 28
Summary: Can't time things that accurately with standard PC hardware

In article <1@epistemi.UUCP>, martin@epistemi.UUCP (Martin Shepherd) writes:
> I ask because I would like to be able to use a [PC Clone]
> for psychological experiments which involve presenting simple
> stimuli (single words, symbols, etc.) on the monitor and timing
> subjects' keypress responses to the nearest one or two milliseconds.
> I need to be able to "blank" and "unblank"
> the monitor screen so that the onset of a display is more or less
> instantaneous.

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.  You've got further problems with
standard monochrome monitors because they use slow-fade phosphors
-- it can take most of a second for an image to fade away.  Color
monitors will let you change the image completely in the refresh
period, so they're much better suited to your application.

You probably could re-program the timer chip to interrupt more
frequently than the standard 18.2 times per second if you wanted;
however, you'd have to replace the normal timer interrupt service
routine with one of your own creation -- you'd want less overhead,
and your system clock wouldn't keep time if the interrupt rate
were changed.
-- 
{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.