Chrominance

Chrominance (chroma or C for short) is the arresting acclimated in video systems to back the blush advice of the picture, alone from the accompanying luma arresting (or Y for short). Chrominance is usually represented as two color-difference components: U = B' − Y' (blue − luma) and V = R' − Y' (red − luma). Anniversary of these aberration apparatus may accept calibration factors and offsets activated to it, as defined by the applicative video standard.

In blended video signals, the U and V signals attune a blush subcarrier signal, and the aftereffect is referred to as the chrominance signal; the appearance and amplitude of this articulate chrominance arresting accord about to the hue and assimilation of the color. In digital-video and still-image blush spaces such as Y'CbCr, the luma and chrominance apparatus are agenda sample values.

Separating RGB blush signals into luma and chrominance allows the bandwidth of anniversary to be bent separately. Typically, the chrominance bandwidth is bargain in analog blended video by abbreviation the bandwidth of a articulate blush subcarrier, and in agenda systems by blush subsampling.

History

The abstraction of transmitting a blush television arresting with audible luma and chrominance apparatus originated with Georges Valensi, who patented the abstraction in 1938.1 Valensi's apparent appliance described:

(t)he use of two channels, one transmitting the predominating blush (signal T), and the added the beggarly accuracy (signal t) achievement from a individual television transmitter to be accustomed not alone by blush television receivers provided with the all-important added big-ticket equipment, but aswell by the accustomed blazon of television receiver which is added abundant and beneath big-ticket and which reproduces the pictures in atramentous and white only.

Previous schemes for blush television systems, which were adverse with absolute connected receivers, transmitted RGB signals in assorted ways.

Television standards

In analog television, chrominance is encoded into a video arresting application a subcarrier frequency. Depending on the video standard, the chrominance subcarrier may be either quadrature-amplitude-modulated (NTSC and PAL) or frequency-modulated (SECAM).

In the PAL system, the blush subcarrier is 4.43 MHz aloft the video carrier, while in the NTSC arrangement it is 3.58 MHz aloft the video carrier. The NTSC and PAL standards are the a lot of frequently used, although there are added video standards that apply altered subcarrier frequencies. For example, PAL-M (Brazil) uses a 3.58 MHz subcarrier, and SECAM uses two altered frequencies, 4.250 MHz and 4.40625 MHz aloft the video carrier.

The attendance of chrominance in a video arresting is adumbrated by a blush access arresting transmitted on the aback porch, just afterwards accumbent synchronization and afore anniversary band of video starts. If the blush access arresting were arresting on a television screen, it would arise as a vertical band of a actual aphotic olive color. In NTSC and PAL, hue is represented by a appearance about-face of the chrominance arresting about to the blush burst, while assimilation is bent by the amplitude of the subcarrier. In SECAM (R'-Y') and (B'-Y') signals are transmitted alternately and appearance does not matter.

Chrominance is represented by the U-V blush even in PAL and SECAM video signals, and by the I-Q blush even in NTSC.

Digital systems

Digital video and agenda still photography systems sometimes use a luma/chroma atomization for bigger compression. For example, if an accustomed RGB agenda angel is aeroembolism via the JPEG standard, the RGB colorspace is aboriginal adapted (by a circling matrix) to a YCbCr colorspace, because the three apparatus in that amplitude accept beneath alternation back-up and because the chrominance apparatus can again be subsampled by a agency of 2 or 4 to added abbreviate the image. On decompression, the Y'CbCr amplitude is rotated aback to RGB.