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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.micro.amiga
Subject: Re: Interlaced monitor
Message-ID: <5973@utzoo.UUCP>
Date: Mon, 16-Sep-85 19:37:17 EDT
Article-I.D.: utzoo.5973
Posted: Mon Sep 16 19:37:17 1985
Date-Received: Mon, 16-Sep-85 19:37:17 EDT
References: <6789@ucla-cs.ARPA> <204@cirl.UUCP>, <30637@lanl.ARPA>
Organization: U of Toronto Zoology
Lines: 19

The reason why interlace is a more serious compromise for a computer
display than for normal video is that normal video doesn't usually have
a lot of one-pixel detail in it.  The picture on any interlaced display
is made up of pixels flashing at 30 Hz.  If a whole area of pixels are
(roughly) the same color, then the eye will average out the alternating
30-Hz flashing into a 60-Hz flash frequency for the whole area.  When
color changes drastically from one pixel to the next, this averaging
can't happen.  TV pictures generally are composed of substantial areas
of continuous color.  Computer displays often have one-pixel-wide lines
and characters with one-pixel-wide strokes.

Consider a pattern of well-separated white one-pixel dots on a black
background.  If the dots are all in even-numbered scan lines, then
in the first 60th of a second all the dots are refreshed, and in the
second 60th of a second nothing happens.  So the net refresh rate is
30 Hz, and the interlace is useless.  This is an extreme case, mind you.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry