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Chrominance (chroma orC for short) is the signal used invideo systems to convey thecolor information of the picture (seeYUV color model), separately from the accompanyingluma signal (or Y' for short). Chrominance is usually represented as two color-difference components: U = B′ − Y′ (blue − luma) and V = R′ − Y′ (red − luma). Each of these different components may have scale factors and offsets applied to it, as specified by the applicable video standard.
Incomposite video signals, the U and V signals modulate a colorsubcarrier signal, and the result is referred to as the chrominance signal; thephase andamplitude of this modulated chrominance signal correspond approximately to thehue andsaturation of the color. In digital-video and still-imagecolor spaces such asY′CbCr, the luma and chrominance components are digital sample values.
SeparatingRGB color signals into luma and chrominance allows thebandwidth of each to be determined separately. Typically, the chrominance bandwidth is reduced in analog composite video by reducing the bandwidth of a modulated color subcarrier, and in digital systems bychroma subsampling.
The idea of transmitting acolor television signal with distinctluma and chrominance components originated withGeorges Valensi, who patented the idea in 1938.[1] Valensi's patent application described:
The use of two channels, one transmitting the predominating color (signal T), and the other the mean brilliance (signal t) output from a single television transmitter to be received not only by color television receivers provided with the necessary more expensive equipment, but also by the ordinary type of television receiver which is more numerous and less expensive and which reproduces the pictures in black and white only.
Previous schemes for color television systems, which were incompatible with existing monochrome receivers, transmittedRGB signals in various ways.
Inanalog television, chrominance is encoded into avideo signal using asubcarrier frequency. Depending on the video standard, the chrominance subcarrier may be eitherquadrature-amplitude-modulated (NTSC andPAL) orfrequency-modulated (SECAM).
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. The NTSC and PAL standards are the most commonly used, although there are other video standards that employ different subcarrier frequencies. For example,PAL-M (Brazil) uses a 3.58 MHz subcarrier, andSECAM uses two different frequencies, 4.250 MHz and 4.40625 MHz above the video carrier.
The presence of chrominance in a video signal is indicated by acolor burst signal transmitted on theback porch, just after horizontal synchronization and before each line of video starts. If the color burst signal were visible on a television screen, it would appear as a vertical strip of a very dark olive color. InNTSC andPAL, hue is represented by aphase shift of the chrominance signal relative to the color burst, while saturation is determined by the amplitude of the subcarrier. InSECAM (R′ − Y′) and (B′ − Y′) signals are transmitted alternately and phase does not matter.
Chrominance is represented by theU-V color plane in PAL and SECAM video signals, and by theI-Q color plane in NTSC.
Digital video and digital still photography systems sometimes use a luma/chroma decomposition for improved compression. For example, when an ordinaryRGB digital image is compressed via theJPEG standard, the RGB color space is first converted (by arotation matrix) to aYCbCr color space, because the three components in that space have less correlation redundancy and because the chrominance components can then be subsampled by a factor of 2 or 4 to further compress the image. On decompression, the Y′CbCr space is rotated back to RGB.
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