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From: davidsen@sixhub.UUCP (Wm E. Davidsen Jr)
Newsgroups: comp.os.msdos.misc,comp.sys.ibm.pc.misc,comp.windows.ms
Subject: Re: NEC 3D OK in interlaced 1024 x 768 ??
Keywords: NEC 3D  choice of VGA card
Message-ID: <1967@sixhub.UUCP>
Date: 29 Sep 90 21:36:22 GMT
References: <1618@mitisft.Convergent.COM> <1990Sep28.235629.15785@rick.cs.ubc.ca> <1990Sep29.200049.22028@agate.berkeley.edu>
Reply-To: davidsen@sixhub.UUCP (bill davidsen)
Followup-To: comp.os.msdos.misc
Organization: *IX Public Access UNIX, Schenectady NY
Lines: 62


  Lot of misconception on this one.

e4666881@rick.cs.ubc.ca (richard louie) writes:
:      The NEC 3D should not be ran too long in 1024 x 768 mode
: because of the feature referred to as interlaced.  When a display
: is interlaced, you will tend to see a flicker on the screen.  This
: flicker is caused by the monitor needing to refresh the display
: to get the 1024 x 768 more often than when a lower resolution 
: is displayed on the screen.  

and c60c-1gd@e260-3c.berkeley.edu (Joon Song) points out:

: This is incorrect.  Interlaced displays are not refreshed more often than
: non-interlaced displays.  In fact the opposite is closer to the truth.  When
: a display is interlaced, only every other line on the screen is refreshed
: during each vertical pass.  So it takes two vertical passes to refresh the
: entire display.  The reason the display flickers is that the display is
: being refresh at only one-half the rate of a non-interlaced display.

  Let me try to say what these people meant so it will be clear to those
who don't already understand it.

  A monitor writes horizontal dots at an almost constant frequency. When
you have more raster lines the time to rewrite the whole screen is so
long that the phosphor dims before being rewritten, so that individual
dots brighten and dim.

  When the time gets bad enough this becomes visible to even something
as insensitive as a human eye. What video card manufacturers do is to
write half the lines (every other line) in one pass vertically, then the
other half of the lines. This is called interlace. 

  If you don't interleave the screen seems to flicker as the top and
bottom alternate in being brighter. If you interleave alternate
adjacent rows become brighter. When both modes were common the
non-interleaved was called flicker and the interleaved was called
twinkle. Both are annoying.

  There are a number of solutions. One is to use a slower phosphor,
which doesn't dim as fast. This means that a moving bright spot will
leave a "comet trail" after the spot, and that when the whole screen
changes the old image will fade rather than change cleanly. A radar
screen is an example of a long persistance phosphor.

  You can also write the horizontal dots faster, so that you get back to
each dot before it fades enough to notice. That means that the monitor
and video board have to run faster, and will cost more, at least to buy.

  If you can't do either of the preceding, one thing which helps is to
view the display in a low light level with the brightness turned down.
This is because the phosphor dims faster the brighter it is.

  We used to have all these problems and solutions when we ran 80x50
back in the early PC days (about 1978) and had a choice of what
interleave and stuff to run.

-- 
bill davidsen - davidsen@sixhub.uucp (uunet!crdgw1!sixhub!davidsen)
    sysop *IX BBS and Public Access UNIX
    moderator of comp.binaries.ibm.pc and 80386 mailing list
"Stupidity, like virtue, is its own reward" -me