Chrominance
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Chrominance (chroma for short) comprises the two components of a television signal that encode color information. It defines the difference between a color and a chosen reference color of the same luminous intensity.
The idea of transmitting a color television signal as luminance and chrominance comes from Georges Valensi, who patented it in 1938. Previous color television systems tried to transmit RGB signals in different ways and were incompatible with monochrome receivers.
In analog television, chrominance is encoded into a video signal using a special "subcarrier" frequency, which, depending on the standard, can be either quadrature-amplitude (NTSC and PAL) or frequency (SECAM) modulated. In the PAL system, the color subcarrier is 4.43 MHz above the video carrier, while in the NTSC system it is 3.58 MHz above the video carrier. SECAM uses two different frequencies, 4.250 MHz and 4.40625 MHz above the video carrier.
The presence of chrominance in a video signal is signalled by a "color burst" signal transmitted on the "front porch," just after horizontal synchronization and before each line of video starts. If the color burst signal were to be made visible on a television screen, it would look like a vertical strip of a very dark olive color. In NTSC and PAL hue is represented by a phase shift in the chrominance signal within each video line relative to the color burst, while saturation is determined by the amplitude of the subcarrier. In SECAM (R-Y) and (B-Y) signals are transmitted alternatively and phase does not matter.
Chrominance is represented by the U-V color plane in a PAL and SECAM video signals, and by the I-Q color plane in NTSC.