The main issue at the inception of color TV broadcasting was again one of compatibility. The requirement that the color signal be usable by black-andwhite TV sets led to a system in which additional information was broadcast along with the previously used signals. This additional information was simply not processed by monochrome receivers. Compatibility also requires that a color set be able to receive a monochrome picture properly.

To understand the characteristics that the color signal must have, it will help to digress and take a look at the color cathode-ray tube (CRT).

The color CRT

Even though the predominant color reproduction method used today is pretty much a “brute force” solution, it is at the same time, a testimonial to modern manufacturing techniques that it can be mass produced with the high degree of precision required to make it work.

In the back of the neck of the CRT, arranged as shown in Fig. 7.1, are three electron guns instead of the single on-axis gun used in a B/W set. Even though these guns obviously don't shoot electrons of various colors, they are identified by the color they will cause on the screen. Instead of having a single (white) phosphor as in monochrome TV, the color CRT has three different phosphor colors on the inside face of the CRT. They are, in most cases, arranged in a dot pattern as shown in Fig. 7.2. The sets of dots enclosed in the triangular outlines include one of each color and are called triads. On a 19” (diagonal measurement) color CRT, the dots are about .38 mm in diameter. Between the dots and the gun assembly and close to the dots lies a sheet of metal called the shadow mask. There are holes in the shadow mask, positioned over the center of each triad of dots. The function of this mask is illustrated by Fig. 7.3, which shows only two guns and two dots. It can be readily perceived how the concept could be extended to three of each if the third spatial dimension were added.

Note that although the vertical scale shown is the same for the dots and the guns, it is not the same as the horizontal scale.