Q. The lessons refer to the “video” signal in a TV receiver. Just what is the video signal, and why is it separate from the color signals?
A. When the standards for color TV were established, there were already many blackand-white receivers in use. To make the new signal for color compatible with these existing receivers, a black-and-white signal was included inthe color system.
It is derived from the three primary colors by mixing them in speciñc amounts: 11% blue, 30% red, and 59% green. This is properly called the “luminance” signal, since it is the brightness content of the picture. The author of the TV lessons usually calls it the “video” signal.
The color content of the picture is contained in the “chroma” signal, from which the three color difference signals are derived in the receiver. When these are mixed with the luminance signal, the original three color signals, red, blue, and green, are produced. These are then used to drive the cathodes of the three electron guns in the CRT.
A standard black-and-white receiver uses only the luminance signal in the broadcast. A color receiver basically takes the luminance signal and adds color to it.