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?

IIRC, the frame rate was shifted slightly to avoid an "aliasing" or "beat" effect with other components in the signal, particularly the 4.5 MHz sound carrier. The horizontal scan rate was similarly shifted from 15.750 KHz to 15.734 KHz.

The small amounts of shift were required to make sure that existing monochrome sets could still lock onto the new color sync signals.

The burst is a short pulse of 3.579 MHz located on the "back porch" of each horizintal sync pulse. It provides a phase reference to lock the set's 3.58 oscillator to the transmitted signal, keeping the reproduced colors the same as they were at the transmitter.

Typical material was an alloy called invar.

http://en.wikipedia.org/wiki/Invar

The NTSC monochrome standard of 1941 remained unchanged until 1953 when the additional color information was added to it. The color signal rides on a color subcarrier of 3.579545 MHz. This is interleaved into the original B&W signal and co-exists peacefully with it because the sync frequencies were reduced as you stated above. The choice of 3.579545 MHz for the color subcarrier frequency was the wrong one, both in 1953 and now. The reason it was chosen is because RCA feared that several million existing B&W sets with intercarrier sound in use in 1953 would display a moire pattern during a color broadcast due to the frequency first chosen: 3.583125 MHz.

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

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?

The osc.,when synched to the burst, also acts as a a phase reference for the phase demodulators that decode the color signal.

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.:cool: