[old_tv_nut]

Details continued:

The color burst had to be present for color programs, of course, but there's a gray area for monochrome. If the monochome broadcast was at the old monochrome scan rates (as often happened in the early days) then the color burst had to be omitted because it would become a color broadcast at illegal scan rates for color. Early on, there could be a lot of hot switching between different sources.

If the local station was equipped for color and always operated at color scan rates, then I suppose they could leave the burst on and argue that they were sending a color broadcast of a monochrome image, but I believe they always followed the practice of turning off the burst for monochrome programs even if they used the color scan rate throughout their plant. This allowed color sets to turn off the color circuits on monochrome programs and eliminate any color noise as well as cross-color (high frequency monochrome info being interpreted as color "twinkles").

Things really got messed up in analog cable TV systems. Regulations required that a clean color burst and sync be inserted into all incoming signals before they went out on the cable. Headend equipment, as far as I know, typically did not include the capability to turn the outgoing burst on and off, so that monochrome programs were sent to the home with the burst on.

Regarding the use of burst for a high precision frequency reference, this started prior to the use of frame synchronizers at local stations. The three major networks each installed a master atomic clock, and the local stations would lock to the network, so that the scan and burst frequencies had accuracy approaching that of the National Bureau of standards. You could actually sync the video from one network with the video of another except for a constant offset in time delay. NBS then published a paper on using the color burst as a tight-tolerance frequency reference. The networks' purpose had been to prevent vertical flipping due to hot-switching sources, but that also created the precise reference frequency.

After the development and installation of frame synchronizers in local studios, the locals went back to their own crystal references, so the use of the burst for precise frequency reference was lost.

Today, digital broadcasts are all locked to GPS time, even more precise than the original network atomic clocks. This allows precision offset of stations' carrier frequencies and phase for minimum co-channel interference, and even the implemention of single-frequency networks where multiple transmitters cooperate to cover a given service area.