On 1/2/14 11:33 AM, Bill Hawkins wrote:
IIRC, the reason why NTSC has an almost 30 fps rate is that early vacuum tube TV sets could develop heater-cathode leakage that would put a black "hum bar" in the picture. Almost 30 allows the bar to move through the picture in a 60 Hz power distribution system. Seems like Europe would have had that problem.
And, any ripple in the power supply wouldn't cause the sync to jump around. Noise that's power line related (brush noise from universal AC motors, as used in vacuum cleaners, for instance) would be in the same place on the frame, as well.
But if the vertical retrace interval were, say, 59 Hz, and there were 60 or 120 Hz ripple on the power supply, you could see how there would be a periodic offset in the vertical sweep timing and the image would slowly drift up or down and jump every second. Which would be very annoying.
When they went to color, they kept the 60 Hz field rate, but moved it a little bit to 59.94 so that it could be generated by a divider chain from a master oscillator at the burst frequency.
And even with all this.. it's still Never The Same Color _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
