Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 11/08/85; site unccvax.unccvax.UUCP
Path: utzoo!watmath!clyde!bonnie!akgua!mcnc!unccvax!dsi
From: dsi@unccvax.UUCP (Dataspan Inc)
Newsgroups: net.video,net.analog
Subject: Re: Adding Video Input to a TV Receiver
Message-ID: <373@unccvax.unccvax.UUCP>
Date: Fri, 22-Nov-85 08:45:01 EST
Article-I.D.: unccvax.373
Posted: Fri Nov 22 08:45:01 1985
Date-Received: Sun, 24-Nov-85 06:18:19 EST
References: <6263@amdcad.UUCP> <743@hou2e.UUCP> <590@ttrdc.UUCP>
Organization: UNC-Charlotte
Lines: 72
Xref: watmath net.video:1687 net.analog:537

> Mightn't it be possible to isolate the video connection, rather than
     Why youse guys insist on trying to do this is mindblowing.  There
are isolation amplifiers available that can do this, but they are far
more expensive than any decent monitor.  

     The original question was from someone who wanted to (evidently)
bypass the RF modulator and tuner/IF amplifier/2nd detector in their
TV set for better VCR quality.  I agree that there is probably enough
group delay and differential gain/phase distortion to make this method
seem attractive.  MOST modulators, including the "professional" B-T
junk used by SMATV operators, are too noisy and introduce too many
artifacts into the image.  Virtually ALL demodulators (source of video)
are pure rubbish, even when put into context.

     If an analysis of the latest el-cheapo circuits (this includes such
fine consumer junk as Proton) used for driving CRT's is any indication,
forget bypassing your RF amplifier/tuner/IF/2nd det and instead buy
a real VTR and monitor.  Consumer junk is designed by people who still
have not discovered that there is a very big difference between differential
frequency response (or small-signal response) and watching some bipolar
transistor try to swing +/- 60 v p-p
whilst driving a 150 pfd load (namely, the picture tube and its wiring).
You still can't get a I-Q demodulated set with wide bandwidth I-channel
response (RCA excepted) that anyone will guarantee; I have yet to see
a multiburst response flat out of any VCR, and besides, your VCR can't
do anything with luminance above about 2.0 mHz (Whooopieee-Doooo that Super
Beta shifted the FM carrier frequency a whole, whopping tens of kilohertz)

     My advice is to forget the whole thing.  Many of the consumer sets
have rather elaborate engineering into the problem of "what happens when
the picture tube arcs internally." This used to be the numero uno killer
of solid state TV sets.  Believe it or not, robbing power from a section
of a set and/or changing the internal wiring in any way can seriously
shorten the life of your receiver by changing the way in which arcbacks
are handled.  

     Second, the power budget inside the receiver is extremely lean. There
are (albeit boring) guys who sit around all day in Japan and figure out ways
to get 0.005 cents out of the cost of winding a HT/LT transformer for next
year's Sony. (This also has the unhappy consequence of shooting high 
voltage regulation all to hell, with attendant blooming and loss of highlight
detail).  Your circuit may reduce the MTTF by a power of 10 or more just
by stressing the power "supply" that much more. There are also scumbags
who don't even put a freakin' zener diode and pass transistor to regulate
the high B+ to a safe value for IF amplifier use; some of the really cheap
sets like GoldStar use "bulletproof" transistors which don't care if the
power supply regulation is 40%.  Adding your circuit may cause failure of
some other component, thereby roasting some other part in the set.

     I used to hack TV sets as a young lad (this was when analog hacking
was all there was) for better video performance, and can speak from
experience that modern sets are basically not worth messing with. Get a real
VTR and monitor; write the manufacturers and demand cheaper Y I Q Betacart
type vcr's, etc.  (Yes, they do now have 1/2 inch VCR's with honest to God
4.0 mHz luminance and 2.0 mHz - yes - 2.0 mHz chrominance (presumably in
preparation for enhanced NTSC such as the MAC systems now in the works)
and greater than 50 dB video S+N/N.  Until then, you and your TV set will
be much happier (plural) if modifications aren't made.

     It seems ironic that I've a $1100 tuner at home which is better than
any commercial FM station's signal; and yet I live within the 90 dBu contour
of several TV stations (all of which transmit incredibly clean video) and
have to resort to occasional glimpses form a $11,000 Tek demodulator . . .
the consumer junk not being able to do diddly squat in comparison.

Keeping NTSC the best television system in the world
David Anthony
Chief Engineer
DataSpan, Inc 

P.S. At home? I have a 9 inch RCA ColorTrak so that the imperfections
     aren't visible from the BarcaLounger.