Quote:
Originally Posted by russell_w_b
For tellies it had to be open more than 1/25 second shutter speed or you'd get part of the image blanked out.
It's an interesting picture, that.
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Back in the early '80's I was tasked with getting a film camera (which had a DC motor) to film a CRT/monitor picture without a rolling bar.
Firstly I found I could reduce the width of the bar by making the shutter exactly 180 degrees. But it was still just visible. (bigger or smaller in angle the bar was grey or white and wider due to under or over exposure effects) . I then drilled a hole in one of the rotating shafts in the camera and fitted an optical sensor. Also I added a current sensing resistor in sensor in series with the motor.
I designed a servo, similar to those in early VCR's, used the motor current to give a little + feedback, to control the speed of the motor for drag and friction, and used the optical sensor output to phase lock the shaft angle to the vertical sync pulse of the video signal driving the monitor with a sample hold arrangement. This phase locked the "rolling bar" and I could get it into the vertical interval where it could not be seen.
(ps: that raster image is from Manfred Von Ardenne's pre war TV book 1936)