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From: dsi@unccvax.UUCP (Dataspan Inc)
Newsgroups: net.video,net.micro
Subject: Re: Computer monitors as TV sets
Message-ID: <128@unccvax.UUCP>
Date: Wed, 30-Jan-85 21:19:51 EST
Article-I.D.: unccvax.128
Posted: Wed Jan 30 21:19:51 1985
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Xref: watmath net.video:943 net.micro:9248


    Your problem will be in demodulating the NTSC composite colour video
signal if you want to derive any benefits. To get 80 * 24 characters on a
colour screen, the amplifiers driving the picture tube would have to do
13 mHz (approx). On the other hand, I doubt seriously if your TV tuner
is capable of providing any useable response at 4.2 mHz (due to the crudity
of sound traps used in such things) and only mediocre response at 3.58 mHz.

    High resolution colour monitors, when operated in the computer graphic
mode, are almost always RGB. That is, there is a separate amplification
channel for red, green, and blue primaries. The output of your tuner is
probably not RGB but composite NTSC video which encodes the colour by
varying the amplitude and phase of a multiplexed subcarrier. You would
then require a demodulator, which are available but are rather expensive,
to convert NTSC to RGB signals.

     If you were to do I/Q axis demodulation with the full bandwidth, you
would notice that NTSC really can produce beautiful colour pictures. The
fact that most "consumer" TV sets hack 66 percent of the I-channel bandwidth
(to save money) from 1.5 mc to 500 kc should disturb most people. 

     You are definitely on the right track, though. Assuming you could
get a "switchable" RGB-NTSC high res monitor, one benefit would be the
smaller dot pitch on the face of the CRT. Having smaller triads of dots
means better high frequency colour detail; virtually every consumer set
does not have enough dots even in the 19 inch size to prevent aliasing
at 4.2 mHz (the upper limit of NTSC). Even with "narrowband" NTSC decoding
(I have a professional monitor that is switchable, with fine pitch CRT)
the fine pitch CRT makes one whale of a difference.

     I have long felt that video is very low-fi (well, a Conrac 5700 pro
monitor and Tektronix demodulator makes me rather biased...) and that
real high performance VIDEO COMPONENTS are required. These are NOT the
kinds of crap you see as good stuff in Video or Sterno Review. The tuners
used in a Zenith Z-TAC cable box are atrocious, mangling all the high
frequencies in both amplitude and phase. I CAN'T STAND DIFFERENTIAL
DISTORTION in a colour picture. You are SUPPOSED to be able to see what
colour shirts people are wearing in the stands while the camera is 
set up to capture a basketball tip-off. Broadcasters go to great lengths
to transmit a distortion free image.  

     We won't even discuss colour under VCR's here. GAKKKKKKKKKKKKKKK!!!

     By the time your RF (which comes down the cable or antenna in
reasonable shape) hits the  1) tuner in your cable box (sometimes) and
2) a modulator in your cable box (sometimes) and 3) the tuner in your
VCR and 4) the modulator in your VCR and 5) the tuner in your TV set
the video is ROTTEN, ROTTEN, ROTTEN, ROTTEN, and ROTTEN.

     I have deviated somewhat. The point is, while there may be som
improvement, the high res monitor will suffer due to all the manglement
prior to the screen. It's as if you connected a pair of JBL 4311 studio
monitors to the amplifier in a Close-N-Play, thence to a pocket AM radio,
thence to your local AM station, with a compact disc player providing
the program material. 

     Now, is there interest in a product which would recover 100 % of
the colour signal ( to 4.2 mc) with both composite and RGB outputs? A
real tuner? (Seriously??) Send me some mail -- how much would you spend
to get all 5 packets of the Multiburst signal ruler flat ?

David Anthony
Senior Analog Engineer
DataSpan Corporation