Wednesday, 22 April 2015

An inexpensive video RF modulator

    This is a description I wrote a few years ago of my RF modulator for displaying video on older TVs that don't have a composite video input. The Grundig telly mentioned and the cheap DVD player that prompted the build are both long gone, but the modulator is still a very useful piece of kit that gets used for a variety of video sources.

    My TV set is an ancient Grundig. It's a good telly for its age but nowadays any TV without a SCART or other AV socket on the back is getting rather difficult to connect to newer kit like my DVD player that didn't come with an RF output. No problem, I thought, I'll just plug the DVD into the SCART on the back of my VCR and feed the TV through that. At which point I came up against our friends the copyright owners. I don't think they like people plugging DVD players into VCRs! To stop people copying DVDs to tape, they incorporate an alternating peak white/peak black bar into the teletext lines on the video signal encoded on the DVD. These are the lines that normally sit out of sight above the screen on the TV, you'll only see them move past if your TV loses frame hold. The effect of this peak white/peak black cycling is to play havoc with the VCR's automatic level control, resulting in a signal from the VCR that flashes on and off and is unwatchable. As someone who is just using their VCR as an RF modulator I get caught in the crossfire.

    At this point I had several options. (1)Buy a new telly with a SCART socket, (2)Return my DVD player and buy one with an RF output, (3)Buy an RF modulator, or (4)build an RF modulator of my own. I chose (4) because I didn't have the cash for the other three and since it was Christmas I wouldn't have found a shop open to sell me one. I looked on the Web to see if anyone else had built one and couldn't find any information, so here to fill the gap are the details of my RF modulator.
I decided not to build my modulator from first principles. A simple design with a UHF cavity oscillator and simple sound and vision carrier and modulation circuits is not impossible to make using parts from a scrap TV set, but when so many set top devices already have a modulator built into them why bother? Instead I lifted the RF modulator from a scrap Salora satellite receiver I picked up at a radio rally.

    RF modulator modules usually conform to a fairly generic design. I have seen almost identical modules from different manufacturers in VCRs, set-top boxes and satellite receivers with a wide variety of brand names over the years. They are usually a shiny tinplate box a bit larger than a matchbox with PCB mounting pins protruding from the bottom and at least one co-axial RF socket on the side. Mine has 2 co-axial connectors, one for the antenna and one for the TV, a small switch to enable a test signal for tuning the TV and a 5 pin PCB header for signal and power. Since it came from a device for the British market it has a 6MHz FM sound carrier and outputs on UHF channel 36. The output channel is adjustable by means of a trimmer screw. If you live somewhere else in the world your local specs may be different, however the principle should be the same.

    Before I removed the module from the donor satellite receiver an element of signal tracing was necessary to work out which pin did what on the PCB header. I was fortunate that while the receiver I was using had a dead CPU its main functions were in working order so I was able to identify the power supply pins quickly by powering it up and using a multimeter. The video and audio pins were a little more difficult to trace, while it was pretty obvious which two pins were the signal pins a little tracing of PCB tracks to the video processor and the sound chips respectively was required to be certain which was which.

    There now follows a quick description of each pin on my modulator. If you do this, there is no guarantee that yours will be the same, however the generic nature of these modules means that they are usually similar. I have numbered the pins from left to right with the RF connectors on the top and to the left.

    Pin 1 Video. I don't know the spec of this input but it is very happy with the 1V peak to peak composite video from the phono socket on the back of the DVD player.
    Pin 2 Audio. This pin takes a line level audio input. My module is not a stereo device so this is mono only. As I run the audio through a hi-fi system this doesn't matter to me but I could simply connect both the left and right audio outputs from the DVD to this pin.
    Pin 3 +5V modulator power. This pin provides power to the modulator circuit.
    Pin 4 Ground. I connected this pin to the tinplate chassis of the module, which also formed the ground for my power supply circuit.
    Pin 5 +5V antenna passthrough power. I did not use this pin. In the satellite receiver it was powered by the standby power supply to provide an amplified passthrough from the antenna socket to the TV socket. Since I did not need this function I ignored it.

    The circuit diagram of my modulator is shown below.

    To power this modulator I built a simple 5 volt regulator using the ubiquitous 7805 IC. I simply soldered a TO220 heatsink to the module case and built the circuit around it. My choice of capacitor values was based on those I had to hand. I also included an LED to serve as a pilot light to indicate that the unit was turned on.

    The 7805 circuit was powered from a surplus 7.6v adapter originally designed for an Ericsson mobile phone. There are so many pieces of electronic equipment powered by small low voltage adapters these days that it should not be difficult to find a suitable surplus power supply. Any DC source between 6 and 9 volts should be suitable, though if you do not have a suitable candidate you can buy a universal power supply.

    The video and audio pins were simply connected to trailing phono sockets by short lengths of coaxial cable and the whole unit was mounted in a small plastic box with some hot glue.

    The performance of the completed modulator is the same as you would expect for the piece of equipment the module came from. Once the TV is conencted to the modulator output and tuned to the test signal the test switch can be turned off and signal applied to the phono sockets. An RF modulated signal is unlikely to deliver picture quality equivalent to a directly fed video signal but in this case the unit performed well and the quality when viewed on my TV was not noticeably different from that of off-air analogue broadcast signals.

    In conclusion, this unit provides a quick high quality RF modulator from a selection of junkbox parts.

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