[time-nuts] Measuring TV delays

[Chuck Forsberg WA7KGX]

Why didn't they make the color burst an exact multiple of 60 Hz?

On 01/02/2014 02:25 PM, Jim Lux wrote:
> 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
>
>
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-- 
      Chuck Forsberg WA7KGX   caf at omen.com   www.omen.com
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