ramesh@amdcad.UUCP (N.Ramesh) (11/14/85)
I would like to add a Video input to my Sears 19 inch TV receiver, to obtain better pictures from my VCR. Is there anyone out there who knows if this is possible, and if so how difficult is it? Is it worth the trouble in terms of picture quality ? Thanking you in advance for your suggestions ----Ramesh
gv@hou2e.UUCP (A.VANNUCCI) (11/19/85)
> I would like to add a Video input to my Sears 19 inch TV receiver, > to obtain better pictures from my VCR. Is there anyone out there > who knows if this is possible, and if so how difficult is it? Is > it worth the trouble in terms of picture quality ? > Thanking you in advance for your suggestions > > ----Ramesh The main reason why TV sets don't have a video input is that they don't have an AC transformer in the power supply. In order to save money, weight, etc. TV sets rectify the AC from the outlet directly and, therefore, have a hot chassis. If you connect a coax cable to the video circuit input the shield of the cable must go to the chassis; however, the chassis may be connected to the 117 V live wire, so that you will get an electric shock when you touch it, you will blow fuses, damage other equipment, etc. etc. From what I understand, you just need an isolation transformer to power the TV set and then you can add a video input with no problems. An isolation transformer is just a normal transformer with a 117 V primary and a 117 V secondary, so that the load is not directly connected to the power grid. Giovanni Vannucci AT&T Bell Laboratories HOH R-207 Holmdel, NJ 07733 hou2e!gv
levy@ttrdc.UUCP (Daniel R. Levy) (11/20/85)
In article <743@hou2e.UUCP>, gv@hou2e.UUCP (A.VANNUCCI) writes: >> I would like to add a Video input to my Sears 19 inch TV receiver, >> to obtain better pictures from my VCR. Is there anyone out there >> who knows if this is possible, and if so how difficult is it? Is >> it worth the trouble in terms of picture quality ? >> Thanking you in advance for your suggestions >> ----Ramesh > The main reason why TV sets don't have a video input is that they >don't have an AC transformer in the power supply. In order to save >money, weight, etc. TV sets rectify the AC from the outlet directly >and, therefore, have a hot chassis. If you connect a coax cable to >the video circuit input the shield of the cable must go to the chassis; >however, the chassis may be connected to the 117 V live wire, so that >you will get an electric shock when you touch it, you will blow fuses, >damage other equipment, etc. etc. > From what I understand, you just need an isolation transformer to >power the TV set and then you can add a video input with no problems. > An isolation transformer is just a normal transformer with a 117 V >primary and a 117 V secondary, so that the load is not directly >connected to the power grid. > Giovanni Vannucci Mightn't it be possible to isolate the video connection, rather than the entire set, as through, say, an opto-isolator? (I do not know offhand if the bandwidth is high enough.) Then it wouldn't matter whether the set were "hot-chassis" or not, and would perhaps be less clumsy than a big bulky transformer (and there would be no chance for harm such as what would occur if someone plugged the set directly into the wall outlet by mistake). The issue of powering the phototransistor on the set side of the optoisolator needs to be taken into consideration--does the set possess a suitable internal supply voltage? (It should probably be well filtered between where it is "stolen" from the set and where it is used to power the phototransistor circuit, to avoid unwanted feedback.) The wiring from the phototransistor (and switch, if you use one to switch between normal reception and the VCR input) to the video circuit should be both as short as possible and well shielded. (Note that your video detector circuit is probably inside or under a little metal can on the circuit board.) Impedance matching (anti-ringing) may be a bit of a problem. Watch the video polarity, too, if you don't want negative pictures. [Like others have mentioned, if you can't get the set schematic from the manufacturer, (or even if you can) Howard Sams puts out a nicely done, if a bit pricey, set of documentation called "Photofact" (available from many electronic parts dealers who sell to radio and television service firms) on most television sets sold in America since WAY back. The parts dealer has the index, and you just bring your set's model number (and other identifying information just in case) and he/she will look it up. If it is not in stock you can send directly to Sams for it.] I would not suggest any video hookup not already provided by the manufac- turer, however, for in-warranty television sets; it would likely void the warranty. -- ------------------------------- Disclaimer: The views contained herein are | dan levy | yvel nad | my own and are not at all those of my em- | an engihacker @ | ployer or the administrator of any computer | at&t computer systems division | upon which I may hack. | skokie, illinois | -------------------------------- Path: ..!ihnp4!ttrdc!levy
dsi@unccvax.UUCP (Dataspan Inc) (11/22/85)
> 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.collinge@uvicctr.UUCP (Doug Collinge) (11/23/85)
>>> I would like to add a Video input to my Sears 19 inch TV receiver, >>> to obtain better pictures from my VCR. >>> ----Ramesh >> The main reason why TV sets don't have a video input is that they >>don't have an AC transformer in the power supply. >> Giovanni Vannucci > >Mightn't it be possible to isolate the video connection, rather than >the entire set? > dan levy | yvel nad So we'll isolate the video with a transformer - whoops the necessary transformer will be too big, bulky, and expensive. Hmmm. Aha! We'll use a cheap little transformer and run it at high frequency! So we need a modulator opstream and a demodulator downstream... Wait! My TV already has a demodulator and my VCR already has a modulator so we can do it for FREE!!! Huh? Why are we doing this again?!? Doug !-) -- Doug Collinge School of Music, University of Victoria, PO Box 1700, Victoria, B.C., Canada, V8W 2Y2 decvax!nrl-css!uvicctr!collinge decvax!uw-beaver!uvicctr!collinge ubc-vision!uvicctr!collinge