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From: rpw3@fortune.UUCP
Newsgroups: net.micro
Subject: Re: mono monitor on PC -- question - (nf)
Message-ID: <3064@fortune.UUCP>
Date: Sat, 14-Apr-84 04:06:19 EST
Article-I.D.: fortune.3064
Posted: Sat Apr 14 04:06:19 1984
Date-Received: Sun, 15-Apr-84 01:54:01 EST
Sender: notes@fortune.UUCP
Organization: Fortune Systems, Redwood City, CA
Lines: 37

#R:sri-arpa:-1214200:fortune:28000026:000:1735
fortune!rpw3    Apr 13 20:54:00 1984

I think there is not a simple fix for your problem. I have been told that
the color monitor uses NTSC-compatible (almost) composite video. If this
is true, the color information is double-sideband modulated (surpressed
carrier) on a 3.58 Mhz sub-carrier. Now, 3.58 Mhz is right in the middle
of the response range of your black and white monitor, and the DSB modulation
spreads out over a megahertz on either side of that (depending on the
amount of I & Q channel for that particluar color). A strongly saturated
color will appear to be almost pure 3.58 Mhz (of some phase). Your monitor
displays this as "stripes".

If you tried to "filter out" the "stripes", you could, but the resulting
characters will be low-res smears (just like they are on the color
monitor ;-} ). NTSC-color is not compatible with high resolution.

Unforunately, most NTSC display systems use the 3.58 (or some multiple)
as master clock for the card. Otherwise, you could simply short out
the sub-carrier modulator, and the "colors" would show up as various
shades of grey. (The "phase" would still be there, so some very bright
colors might display as very dim grey; other bright colors display as
nearly white. Light pastels would all display as half-intensity white
[or "medium grey"], regardless of the color.) Still, there may be a
spot in the circuit where you can apply a suitable "defeat". If you
are a real hardware hacker with some color TV experience, try it.
If not, don't.

[ If IBM PC's don't use composite video, somebody flame at me quitely
  through the mail, huh? ]

Rob Warnock

UUCP:	{ihnp4,ucbvax!amd70,hpda,harpo,sri-unix,allegra}!fortune!rpw3
DDD:	(415)595-8444
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