Composite video to TV

I have tried this twice with some luck in both cases. In both cases the computer was an Atari 800XL. One monitor started as a Black and white TV able to operate from 12 volts or line power. It was easy because it generated 12 volts with a transformer when running from the line. The signal went into the contrast control, not the wiper terminal but the "hot" stationary terminal. If you have enough signal level this will work directly but you can try getting in before the video amplifier or simply put in a small amplifier between the new video input and the contrast control (trivial with today's fast opamps) You can steal the power from somewhere in the TV. The other case was more interesting because it involved one of those nasty TV s running without a proper line isolating transformer. Here I went to an optical coupler based on a fast HP model coupler for analog signals (I believe these reach 10 MHz with some effort.) I needed a tiny transformer to provide drive for the LED on the computer side of the optocoupler (the computer output is not enough). This was setup to use a slight bias current at zero input voltage from the computer and increasing light level (more drive to the LED) with increasing video level. The main trick in all this is some extra frequency compensation to delay the fall-off of the signal at the useful higher frequencies (you should preserve as much as possible at least to 6 MHz and even a bit more if possible). The receiving end of the optocoupler was configured to serve as an amplifier to get enough signal for the contrast control. The B&W version gave a quite good picture containing most of the sharpness you might expect from a proper monochrome monitor and I liked it much better than the green things I was using at the time with other computers. The color version with the optocoupler was fairly satisfactory and I was happy to use it when a color monitor was simply too expensive to justify. Text was adequately sharp at 40 characters per line. Don't forget, the color still had to go through a PAL decoder with loss of sharpness. Incidentally, PAL is inferior to NTSC for this kind of job because the fundamental idea of PAL is the mixing of two adjacent lines to correct for level dependent phase shifts in transmitted pictures. The idea is great for TV but poor for computer work. Narrow horizontal lines simply suffer from this mixing. Still, the overall result was "good enough". HOWEVER; PLEASE OBSERVE ALL WARNINGS IN EARLIER MESSAGES. These transformerless sets kill the foolish. If you live in 230 volt land, the risk is still worse. Bob