Chrominance

Chrominance

Chrominance (chroma or C for short) is the signal used in video systems to convey the color information of the picture, 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 signals, the U and V signals modulate a color subcarrier 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 color spaces 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.

Contents

History

The idea of transmitting a color television signal with distinct luma and chrominance components originated with Georges Valensi, who patented the idea in 1938.[1] Valensi's patent application described:

(t)he 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 signals in various ways.

Television standards

In analog television, chrominance is encoded into a video signal using 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 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, and 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 and PAL, hue is represented by a phase shift of the chrominance signal 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.

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 RGB colorspace is first converted (by a rotation matrix) to a 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

References

  1. ^ French patent 841335, issued Feb. 6, 1939; cited in U.S. Patent 2375966 "System of Television in Colors", issued May 15, 1945.

Wikimedia Foundation. 2010.

Игры ⚽ Поможем написать курсовую

Look at other dictionaries:

  • chrominance — [ krɔminɑ̃s ] n. f. • 1957; mot angl. 1952, du gr. khrôma « couleur », d apr. luminance ♦ Audiovis. Représentation des informations relatives à la couleur d une image de télévision. Signal de chrominance. ● chrominance nom féminin Signal de… …   Encyclopédie Universelle

  • chrominance — [krō′mə nəns] n. [ CHROM(O) + (LUM)INANCE] that attribute of light which produces the sensation of color apart from luminance; specif., the color of an object as measured quantitatively in terms of a reference color …   English World dictionary

  • Chrominance — La chrominance désigne la partie de l image vidéo correspondant à l information de couleur. Il a été indiqué, pour la luminance, que l information de couleur est fournie à partir de 3 couleurs primaires : R, G et B, pour respectivement, le… …   Wikipédia en Français

  • Chrominance — A&V That portion of the video signal which contains the color information (hue and saturation). Video picture information contains two components: luminance (brightness and contrast) and chrominance (hue and saturation). VP The colour level or… …   Audio and video glossary

  • chrominance — spalvis statusas T sritis fizika atitikmenys: angl. chrominance vok. Chrominanz, f rus. цветность, f pranc. chrominance, m …   Fizikos terminų žodynas

  • chrominance — F/A/V the color portion of a composite or S Video signal. TV Portion of video signal that carries color information (hue and saturation, but not brightness); frequently abbreviated as C, as in “Y/C” for luminance/chrominance.[See luminance.] …   Audio and video glossary

  • chrominance channel — spalvių kanalas statusas T sritis radioelektronika atitikmenys: angl. chromatic channel; chrominance channel; color channel vok. Farbartchrominanzkanal, m; Farbartkanal, m; Farbkanal, m rus. канал цветности, m pranc. canal de la chrominance, m;… …   Radioelektronikos terminų žodynas

  • chrominance signal band — spalvio signalo dažnių juosta statusas T sritis radioelektronika atitikmenys: angl. chrominance signal band vok. Chrominanzband, n; Farbartband, n rus. полоса частот сигнала цветности, f pranc. bande occupée par le signal de chrominance, f …   Radioelektronikos terminų žodynas

  • chrominance demodulation — spalvių signalo demoduliavimas statusas T sritis radioelektronika atitikmenys: angl. chrominance demodulation vok. Chrominanzdemodulation, f rus. демодуляция сигнала цветности, f pranc. démodulation de la chrominance, f …   Radioelektronikos terminų žodynas

  • chrominance signal — spalvių signalas statusas T sritis radioelektronika atitikmenys: angl. chroma signal; chrominance signal; color signal vok. Chrominanzsignal, n; Farbsignal, n rus. сигнал цветности, m; цветовой сигнал, m pranc. signal de chrominance, m; signal de …   Radioelektronikos terminų žodynas

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”