Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!cs.utexas.edu!uunet!ginosko!infinet!rhorn
From: rhorn@infinet.UUCP (Rob Horn)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: interrupt-driven vs. polled I/O performance
Message-ID: <1501@infinet.UUCP>
Date: 2 May 89 20:09:12 GMT
References: <25231@amdcad.AMD.COM> <421@logicon.arpa>
Reply-To: rhorn@infinet.UUCP (Rob Horn)
Organization: Infinet, Inc.  North Andover, MA
Lines: 41

In article <25231@amdcad.AMD.COM> rpw3@amdcad.UUCP (Rob Warnock) writes:
>But the echo delay doesn't need to be kept to a *minimum*, but only
>well below the objectionable level to the human user.
 [...]
>I claim that 100ms for echo is not noticable to a user of a video
>display

I went through this extensively for some CAD/CAM and Command & Control
applications.  The dominant factor was the refresh rate of the CRT and
phosphor decay rate (or their equivalents for non-CRT technologies).
Flicker perception, rate timing, and absolute timing perceptions are
each different and impose different update rate issues.  There is a further
problem with eye-hand coordination if the update lags by more than a few
hundred milliseconds.

Most CRT's refresh near either 15 msec or 30 msec.  Most phosphors are picked
to decay in one to two refresh cycles.  Any delay less than the
refresh rate is impossible to perceive because the refresh rate delay
dominates.  Most people cannot perceive delays of up to 50 msec.
(Fast refresh rates *do* affect the perceived image quality, so the
difference between 15 and 30 msec is still significant.)

Flicker perception is triggered by phosphors that decay too quickly
when compared to refresh rates.  Here, some people will perceive
flicker down even to 15 msec, although after-image effects on the
retina must be considered in the experimental design for measuring
these effects.  

For those who do not know what I mean by rate timing, this is the
timing used in race events where human eye plus stopwatch match the
electronic race timers to within 10-20 msec reliably.  A part of this
is matching the motion of the thing being timed with the finger
pushing the stopwatch.  Unpredictable event timing is much less sensitive.
Rate timing effects are significant when performing full screen
updates.  People will perceive pauses that would go unnoticed during
single character updates.
-- 
				Rob  Horn
	UUCP:	...harvard!adelie!infinet!rhorn
		...ulowell!infinet!rhorn, ..decvax!infinet!rhorn
	Snail:	Infinet,  40 High St., North Andover, MA