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Chrominance (''chroma'' or ''C'' for short) is the signal used in
video Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) ...

video
systems to convey the
color Color (American English) or colour (British English) is the visual perception, visual perceptual Physical property, property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physica ...

color
information of the picture (see YUV color model), separately from the accompanying luma 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 difference components may have scale factors and offsets applied to it, as specified by the applicable video standard. In
composite video Composite video is an Video#Analog video, analog video signal format that carries Standard-definition television, standard-definition video (typically at 525 lines or 625 lines) as a single channel. Video information is Video coding format, e ...

composite video
signals, the U and V signals modulate a color
subcarrier A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio broa ...
signal, and the result is referred to as the chrominance signal; the phase and
amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible s ...

amplitude
of this modulated chrominance signal correspond approximately to the
hue
hue
and saturation of the color. In digital-video and still-image
color space A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of colorwhether such representation entails an analog signal, analog or a Dig ...

color space
s such as Y′CbCr, the luma and chrominance components are digital sample values. Separating
RGB The RGB color model is an additive color, additive color model in which the red, green and blue primary colors of light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of t ...

RGB
color signals into luma and chrominance allows the
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
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 Chroma subsampling is the practice of encoding images by implementing less resolution for Chrominance, chroma information than for luma (video), luma information, taking advantage of the human visual system's lower acuity for color differences tha ...
.


History

The idea of transmitting a
color television Color television or Colour television is a television transmission technology that includes color information for the picture, so the video image can be displayed in color on the television set. It improves on the monochrome or black-and-white t ...
signal with distinct luma and chrominance components originated with
Georges Valensi M. Georges Valensi (1889–1980) was a French telecommunications engineer who, in 1938, invented and patented a method of transmitting color images via luma (video), luma and chrominance so that they could be received on both color and black & white ...
, who patented the idea in 1938.French patent 841335, issued Feb. 6, 1939; cited in U.S. Patent 237596
"System of Television in Colors"
issued May 15, 1945.
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, transmitted
RGB The RGB color model is an additive color, additive color model in which the red, green and blue primary colors of light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of t ...

RGB
signals in various ways.


Television standards

In
analog television Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, instantaneous phase and frequency, ...
, chrominance is encoded into a
video signal Video is an electronic medium for the recording, copying, playback, broadcasting Broadcasting is the distribution (business), distribution of sound, audio or video content to a dispersed audience via any electronic medium (communicatio ...
using a
subcarrier A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio broa ...
frequency. Depending on the video standard, the chrominance subcarrier may be either quadrature-amplitude-modulated (
NTSC The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplementa ...
and
PAL Phase Alternating Line (PAL) is a colour encoding system for analogue television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
) or
frequency-modulated
frequency-modulated
(
SECAM SECAM, also written SÉCAM (, ''Séquentiel de couleur à mémoire'', French for ''color sequential with memory''), is an analog television, analog color television system that was used in France, some parts of Europe and Africa, and Russia ...

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 PAL-M is the analogue TV system used in Brazil since 19 February 1972. At that time, Brazil was the first South American country to broadcast in Color, colour. Colour TV broadcast began on 19 February 1972, when the TV networks Rede Globo, Globo ...
(Brazil) uses a 3.58 MHz subcarrier, and
SECAM SECAM, also written SÉCAM (, ''Séquentiel de couleur à mémoire'', French for ''color sequential with memory''), is an analog television, analog color television system that was used in France, some parts of Europe and Africa, and Russia ...

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 indicated by a color burst signal transmitted on the back 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. In
NTSC The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplementa ...
and
PAL Phase Alternating Line (PAL) is a colour encoding system for analogue television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
, hue is represented by a
phase shift In physics and mathematics, the phase of a periodic function F of some real number, real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is denoted \phi(t) and expressed in such a sca ...

phase shift
of the chrominance signal relative to the color burst, while saturation is determined by the amplitude of the subcarrier. In
SECAM SECAM, also written SÉCAM (, ''Séquentiel de couleur à mémoire'', French for ''color sequential with memory''), is an analog television, analog color television system that was used in France, some parts of Europe and Africa, and Russia ...

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.


Digital systems

Digital video and digital still photography systems sometimes use a luma/chroma decomposition for improved compression. For example, when an ordinary
RGB The RGB color model is an additive color, additive color model in which the red, green and blue primary colors of light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of t ...

RGB
digital image is compressed via the
JPEG JPEG ( ) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image ...

JPEG
standard, the RGB colorspace is first converted (by a rotation matrix) to a
YCbCr YCbCr, Y′CbCr, or Y Pb/Cb Pr/Cr, also written as YCBCR or Y′CBCR, is a family of color spaces used as a part of the color image pipeline in video and digital photography systems. Y′ is the Luma (video), luma component and CB and CR are t ...

YCbCr
colorspace, 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.


See also

*
Luma (video) In video, luma represents the brightness in an image (the "black-and-white" or achromatic portion of the image). Luma is typically paired with chrominance. Luma represents the grey, achromatic image, while the chroma components represent the color ...
*
Chroma subsampling Chroma subsampling is the practice of encoding images by implementing less resolution for Chrominance, chroma information than for luma (video), luma information, taking advantage of the human visual system's lower acuity for color differences tha ...


References

{{Color topics Film and video technology Color