Semi-OT - Using optoisolators with composite video signals

Hi Ian, Lets get a bit deeper into your requirement: Is it really composite NTSC, picture data or a composite digital signal (computer game etc.) I ask because perhaps just isolating digital RGB from the source by opto may be the way to go, and regenerate the composite NTSC on the other side of the opto barrier with the National LM1886 / LM1889 combination. If you really have to isolate a NTSC signal, here are your options: RF Modulator, demod with some fidelity loss Broadband transformer with possibly a gain stage on the output side (think a 75 ohm 1:1 transformer) depending on how forgiving the monitor is on signal levels, and front porch back porch, sink sag during the essentially DC time of the signal. A quick test of this idea would be to wire 2 coax to 300 ohm flatlead baluns back to back (I know they are expecting RF, not DC-4.5MHz but a cheap and quick experiment just the same) And, finally, as you suggested, a linear optocoupler. They are inherently non-linear, but avoid all the DC problems at the sync pulses. If you go this route, the topology will use 2 optocouplers, one one carying the signal, and one in a feedback path to linearize it. This was a popular circuit, and I bet we can find it in a single IC package by now... Let me know some more specifics of your two devices you are mating, and we can come up with something. It may be just simpler to add an isolation transformer to the monitor, or if its a 'hot' chasis just be sure the supplies of both systems have no difference in hot and neutral. This was the original IBM PC solution, remember the special polarized plug for the monitor, that plugged into the CPU? Cheers, Randy _________________________________________________________________ The New Busy is not the too busy. Combine all your e-mail accounts with Hotmail. http://www.windowslive.com/campaign/thenewbusy?tile=multiaccount&ocid=P…