Editing Shadow mask (section)

===Color television===

Color television had been studied even before commercial broadcasting became common, but it was not until the late 1940s that the problem was seriously considered. At the time, a number of systems were being proposed that used separate red, green and blue signals ([[RGB]]), broadcast in succession. Most experimental systems broadcast entire frames in sequence, with a colored filter (or "[[Color gel|gel]]") that rotated in front of an otherwise conventional black and white television tube. Each frame encoded one color of the picture, and the wheel spun in sync with the signal so the correct gel was in front of the screen when that colored frame was being displayed. Because they broadcast separate signals for the different colors, all of these systems were incompatible with existing black and white sets. Another problem was that the mechanical filter made them flicker unless very high refresh rates were used.<ref name=seq>Ed Reitan, [http://novia.net/~ereitan/Color_Sys_CBS.html "CBS Field Sequential Color System"] {{webarchive|url=https://web.archive.org/web/20100105183213/http://novia.net/~ereitan/Color_Sys_CBS.html |date=5 January 2010 }}, 24 August 1997.</ref> (This is conceptually similar to a [[Digital Light Processing|DLP]] based projection display where a single DLP device is used for all three color channels.)

[[RCA]] worked along different lines entirely, using the luminance-chrominance system first introduced by [[Georges Valensi]] in 1938. This system did not directly encode or transmit the RGB signals; instead it combined these colors into one overall brightness figure, called the "[[luminance]]". This closely matched the black and white signal of existing broadcasts, allowing the picture to be displayed on black and white televisions. The remaining color information was separately encoded into the signal as a high-frequency [[modulation]] to produce a [[composite video]] signal. On a black and white television this extra information would be seen as a slight randomization of the image intensity, but the limited resolution of existing sets made this invisible in practice. On color sets the extra information would be detected, filtered out and added to the luminance to re-create the original RGB for display.<ref name=dot>Ed Reitan, [http://novia.net/~ereitan/Color_Sys_RCA.html "RCA Dot Sequential Color System"] {{webarchive|url=https://web.archive.org/web/20100107142004/http://novia.net/~ereitan/Color_Sys_RCA.html |date=7 January 2010 }}, 28 August 1997.</ref>

Although RCA's system had enormous benefits, it had not been successfully developed because it was difficult to produce the display tubes. Black and white TVs used a continuous signal and the tube could be coated with an even painting of phosphor. With RCA's system, the color was changing continually along the line, which was far too fast for any sort of mechanical filter to follow. Instead, the phosphor had to be broken down into a discrete pattern of colored spots. Focusing the right signal on each of these tiny spots was beyond the capability of [[electron gun]]s of the era.<ref name=dot/>