NotTaR of Television Sets : Displaying a video signal as a picture o..  
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Displaying a video signal as a picture on an oscilloscope

I am not sure why anyone would really want to do this other than as an experiment - it would be interesting one.

If a composite video signal is the input, you will need a sync separator.

You will have to construct a vertical deflection voltage ramp generator which can be locked to your vertical sync signal.

The horizontal timebase of the scope will be fine for the horizontal deflection and should easily lock to your horizontal sync pulse or (if the scope has a TV trigger mode) directly to the video signal.

A video amplifier will be needed if your Z axis does not have an internal amplifier (you need .7 V p-p to be full brightness range.) Unless you provide automatic gain control, this will need to include offset (brightness) and gain (contrast) adjustments. Even if there is an internal amplifier, it may not have the required bandwidth for the video signal.

However, the overall brightness may be disappointing - a scope is not designed for overall high brightness. The beam focus will not be as good as that on a little TV either.