#1
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3 questions about colour television
If anyone can answer these, I would be most grateful:
The NTSC monochrome standard ran at 30 fps; when colour was introduced, it was reduced to 29.97 fps, a reduction of 0.1%. Why did it have to be reduced at all, and why by that amount? What exactly is the "colour burst"? Is it a kind of synchronizing signal? What were shadow masks made from? Presumably the material had a low coefficient of thermal expansion, but were there other considerations as well? |
#2
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Quote:
The small amounts of shift were required to make sure that existing monochrome sets could still lock onto the new color sync signals. Quote:
Quote:
http://en.wikipedia.org/wiki/Invar |
#3
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Quote:
So in choosing the slightly lower frequency in fear of upsetting some viewers with B&W sets, we got a vertical frequency of 59.94 Hz, a horizontal frequency of 15734.26 Hz. Color TV sets require a steady synchronized source of 3.579545 MHz to make the color lock to the transmitted signal. In each TV is a local oscillator to do this. The color burst is a short burst of ~9 cycles of 3.579545 MHz sent at the beginning of each horizontal line to keep the local color oscillator locked to the TV studio. Without it the screen would look like a rainbow. http://en.wikipedia.org/wiki/Colorburst The shadow mask is made usually form a metal called invar which has the property of maintaining its shape under varying temperatures. http://en.wikipedia.org/wiki/Shadow_mask Hope this helps. Cliff Last edited by cbenham; 12-11-2011 at 11:16 AM. Reason: clarification |
#4
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OK, so if I've understood this correctly, the colour burst exists to keep the local oscillator running at the correct frequency, and the oscillator exists to recreate the colour carrier wave to recover the colour information from it. Is this right?
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#5
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The osc.,when synched to the burst, also acts as a a phase reference for the phase demodulators that decode the color signal.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
Audiokarma |
#6
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It's pretty cool that the oscillator actually free-runs or "coasts" through the visible portion of each horizontal scan after being phase-locked during the brief burst period.
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