3 questions about colour television

[cbenham]

Originally Posted by Rinehart

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?

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