Video Synchronization 817of the red, green, and blue components of a color picture. Provided that these images are
displayed frequently enough, the impression of a full color scene is indeed gained. Identical
reasoning led to the development of the fi rst color television demonstrations by Baird in 1928
and the fi rst public color television transmissions in America by CBS in 1951. Known as a
fi eld-sequential system, in essence the apparatus consisted of a high fi eld-rate monochrome
television system with optical red, green, and blue fi lters presented in front of the camera
lens and the receiver screen, which, when synchronized together, produced a color picture.
Such an electromechanical system was not only unreliable and cumbersome but also
required three times the bandwidth of a monochrome system (because three fi elds had to
be reproduced in the period previously taken by one). In fact, even with the high fi eld rate
adopted by CBS, the system suffered from color fl icker on saturated colors and was soon
abandoned after transmissions started. Undeterred, the engineers took the next most obvious
logical step for producing colored images. They argued that instead of presenting sequential
fi elds of primary colors, they would present sequential dots of each primary. Such a (dot
sequential) system using the secondary primaries of yellow, magenta, cyan, and black forms
the basis of color printing. In a television system, individual phosphor dots of red, green, and
blue, provided they are displayed with suffi cient spatial frequency, provide the impression of
a color image when viewed from a suitable distance.
Consider the video signal designed to excite such a dot-sequential tube face. When a
monochrome scene is being displayed, the television signal does not differ from its black
and white counterpart. Each pixel (of red, green, and blue) is equally excited, depending
on the overall luminosity (or luminance) of a region of the screen. Only when a color
is reproduced does the signal start to manifest a high-frequency component, related to
the spatial frequency of the phosphor it is designed successively to stimulate. The exact
phase of the high-frequency component depends, of course, on which phosphors are to
be stimulated. The more saturated the color (i.e., the more it departs from gray), the more
high-frequency “ colorizing ” signal is added. This signal is mathematically identical to a
black and white television signal whereupon a high-frequency color-information carrier-
signal (now known as a color subcarrier) is superimposed—a single frequency carrier
whose instantaneous value of amplitude and phase, respectively, determines the saturation
and hue of any particular region of the picture. This is the essence of the NTSC^1 color
(^1) NTSC stands for National Television Standards Committee, the government body charged with choosing the
American color system.