Chrominance
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Chrominance (chroma for short), is the signal used in many video systems to carry the color information of the picture separately from the accompanying luma signal. Chrominance is usually represented as two color difference components: B'–Y' (blue – luma) and R'–Y' (red – luma). Both of these components have scale factors and offsets applied to them, which differ depending on the video signal scheme in question.
In composite video signals, the U and V signals modulate a color carrier signal, and the result is referred to as the chrominance signal; the phase and amplitude of this modulated chrominance signal correspond approximately to the hue and saturation of the color. In digital-video and still-image colorspaces such as Y'CbCr, the luma and chrominance components are digital sample values.
Separating RGB color signals into luma and chrominance allows the bandwidth 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 by chroma subsampling.
The idea of transmitting a color television signal as luma 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.
[edit] Chrominance in television standards
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 alternately and phase does not matter.
Chrominance is represented by the U-V color plane in PAL and SECAM video signals, and by the I-Q color plane in NTSC.
[edit] Chrominance in digital systems
Digital video and digital still photography systems sometimes use a luma/chroma decomposition for improved compression. For example, when an ordinary RGB digital image is compressed via the JPEG standard, the R'G'B' colorspace is first converted (by a rotation matrix) to a Y'CbCr colorspace, because the three components in that space have less corrrelation 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 R'G'B'.
