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Is it possible to bypass the RF modulator on an old TV?
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Is it possible to bypass the RF modulator on an old TV?

by shobley on Wed Dec 15, 2010 3:41 pm

I've seen projects where people have used uControllers to create composite video signals fed to TVs - what I'm wondering is, is it possible to either

(a) Create an RF modulated version that can be plugged into a coax input on a TV
(b) Bypass the RF demodulating circuits and tap into the TV circuits to create a display.

I just picked up an old 5" BW TV from Goodwill for a few bucks and I would love to add it to my vacuum tube Theremin as a "visualizer" for the output signal.

I've been successful in converting it into a basic oscilloscope by tapping the deflection coils with analog signals, but would like to take it a step further and create a rudimentary display driven by a uController.

It only has an antenna input, no coax :(

Steve
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Re: Is it possible to bypass the RF modulator on an old TV?

by neutron spin on Thu Dec 16, 2010 12:09 am

I am assuming the theremin is putting out audio frequencies. Televisions are really poor at displaying waveforms when modified and there are safety concerns when modifying the deflection circuitry. If you decide to use the TV you will have to add a sawtooth generator and connect it to the horizontal deflection coil to get the image to sweep properly. Also the deflection coils will have to be either moved or modified to remove the vertical sweep signal from the television circuitry. I would rather just get an small electrostatic CRT and make a simple deflection circuit with a sawtooth generator. Of course you would also need a power supply which would add to the complexity and cost. You can purchase old oscilloscopes for next to nothing that also would work.

If you desire to take the audio from the device and feed it into the video circuitry you have to convert the audio to a composite video signal and would require going from audio to digital and back to a video signal with the appropriate sync signals added back in....really alot of work for so little reward but it could be done...
Last edited by neutron spin on Thu Dec 16, 2010 12:23 am, edited 2 times in total.
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Re: Is it possible to bypass the RF modulator on an old TV?

by ImaginaryAxis on Thu Dec 16, 2010 12:15 am

I can only offer a few suggestions, because I know enough about analog video to be ignorant.

You can create an RF signal - I believe it is analog QAM or I-Q data that you will need to create in order for the demodulator with the coax input to accept. I guess the chain would look like:

uC --> RGB data out --> QAM modulator --> Coax TV input (demodulator) --> CVBS output --> signal conditioning circuits (DC restoration, etc).

If you bypass the demodulator then you would have to provide CVBS (composite signals) yourself. So the chain may look like:

uc --> RGB data out --> video encoder --> signal conditioning circuits (DC restoration, etc.)

Does this help? Analog video is a bitch...for that matter video in general is a bitch

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Re: Is it possible to bypass the RF modulator on an old TV?

by westfw on Thu Dec 16, 2010 12:56 am

(a) Create an RF modulated version that can be plugged into a coax input on a TV

Yes. There is a device called (strangely enough) an "RF Modulator." These were common back when video games first came out (and pre-VCR), when TVs with composite video inputs were quite rare. The "Color Graphics Adaptor" on the original IBM PC was designed to operate through an RF Modulator. These required FCC approval (after all, it's essentially a small RF transmitter, especially if you connect it and your TV antenna to the TV at the same time) and were originally rather expensive. They got cheaper as they become more common and assorted hacks were published. Then as TVs got more complex there were a lot on the surplus market. I haven't seen them so much recently, but a web search turns up a bunch of fairly inexpensive units. You tended to get decreased resolution through such a setup. "32 to 40 characters wide" was the recommendation for terminal-like applications (about 320x240 dots resolution.)

(b) Bypass the RF demodulating circuits and tap into the TV circuits to create a display.

This is possible too, and was also a popular hack back in the days when video monitors were hard to find and expensive. It wasn't recommended on "modern" TVs without an isolating transformer power supply (nearly all of them), and generally required tracking down a service manual containing a schematic of the TV in question. Higher resolution; you might have been able to get 64 characters across your screen...

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Re: Is it possible to bypass the RF modulator on an old TV?

by neutron spin on Thu Dec 16, 2010 9:14 pm

Taking raw analog audio from the Theremin and feeding it to the RF modulator will not work...you must convert it first to a composite video signal the television can understand...regards...
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Re: Is it possible to bypass the RF modulator on an old TV?

by westfw on Fri Dec 17, 2010 12:23 pm

I should add that middle-aged VCRs, of the sort being thrown out these days in vast quantities because they lack a digital tuner, frequently contain an rf modulator in a nice little modular box inside. If the VCR has video input, you may be able to use the whole unit as a modulator. (that might be a bit ridiculous connected to a 5inch TV, but ... cheap and easy.)

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Re: Is it possible to bypass the RF modulator on an old TV?

by richms on Sun Dec 19, 2010 4:37 am

There was a time when standalone modulators were sold alongside DVD players since you couldnt run them thru most DVD players, I have seen them on ebay etc.

The issue with tvs having live chassis is pretty rare on anything that should still be running, but looking on the videokarma forums it seems that the USA has a lot of people have really old TVs still in use.

If the TV has a DC input on it like most small ones, then it probably isnt live chassis, but you are not guarenteed that the -ve of the DC input is connected to the chassis either..

RF is easy to isolate - use a capacitor. baseband signals are less easy.

IMO find a VCR from junk, and on the back there is usually a couple of metal cans connected together, one is a tuner, one is a modulator. but some need strange voltages to operate, most are just 5v.

Otherwise find an old RF lead for a console like a snes or whatever - they have a modulator in them. On a 5" screen any loss of resolution will be way less than the dotpitch of the tube.

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