Editing Y′UV (section)

== Luminance/chrominance systems in general ==
The primary advantage of luma/chroma systems such as Y′UV, and its relatives [[YIQ|Y′IQ]] and [[YDbDr]], is that they remain compatible with black and white [[analog television]] (largely due to the work of [[Georges Valensi]]). The Y′ channel saves all the data recorded by black and white cameras, so it produces a signal suitable for reception on old monochrome displays. In this case, the U and V are simply discarded. If displaying color, all three channels are used, and the original RGB information can be decoded.

Another advantage of Y′UV is that some of the information can be discarded in order to reduce [[Bandwidth (signal processing)|bandwidth]]. The human eye has fairly little spatial sensitivity to color: the accuracy of the brightness information of the luminance channel has far more impact on the image detail discerned than that of the other two. Understanding this human shortcoming, standards such as [[NTSC]] and [[PAL]] reduce the bandwidth of the chrominance channels considerably. (Bandwidth is in the temporal domain, but this translates into the spatial domain as the image is scanned out.)

Therefore, the resulting U and V signals can be substantially "compressed". In the NTSC (Y′IQ) and PAL systems, the chrominance signals had significantly narrower bandwidth than that for the luminance. Early versions of NTSC rapidly alternated between particular colors in identical image areas to make them appear adding up to each other to the human eye, while all modern analogue and even most digital video standards use [[chroma subsampling]] by recording a picture's color information at reduced resolution. Only half the horizontal resolution compared to the brightness information is kept (termed 4:2:2 chroma subsampling), and often the vertical resolution is also halved (giving 4:2:0). The 4:x:x standard was adopted due to the very earliest color NTSC standard which used a chroma subsampling of 4:1:1 (where the horizontal color resolution is quartered while the vertical is full resolution) so that the picture carried only a quarter as much color resolution compared to brightness resolution. Today, only high-end equipment processing uncompressed signals uses a chroma subsampling of 4:4:4 with identical resolution for both brightness and color information.

The I and Q axes were chosen according to bandwidth needed by human vision, one axis being that requiring the most bandwidth, and the other (fortuitously at 90 degrees) the minimum. However, true I and Q demodulation was relatively more complex, requiring two analog delay lines, and NTSC receivers rarely used it.

However, this color modulation strategy is [[lossy compression|lossy]], particularly because of [[crosstalk]] from the luma to the chroma-carrying wire, and vice versa, in analogue equipment (including [[RCA connector]]s to transfer a digital signal, as all they carry is analogue [[composite video]], which is either YUV, YIQ, or even [[Composite video|CVBS]]). Furthermore, NTSC and PAL encoded color signals in a manner that causes high bandwidth chroma and luma signals to mix with each other in a bid to maintain backward compatibility with black and white television equipment, which results in [[dot crawl]] and [[cross color]] artifacts. When the NTSC standard was created in the 1950s, this was not a real concern since the quality of the image was limited by the monitor equipment, not the limited-bandwidth signal being received. However today's modern television is capable of displaying more information than is contained in these lossy signals. To keep pace with the abilities of new display technologies, attempts were made since the late 1970s to preserve more of the Y′UV signal while transferring images, such as [[SCART]] (1977) and [[S-Video]] (1987) connectors.

Instead of Y′UV, Y′CbCr was used as the standard format for (digital) common [[Data compression#Video|video compression]] algorithms such as [[MPEG-2]]. Digital television and DVDs preserve their [[video compression|compressed video]] streams in the MPEG-2 format, which uses a fully defined Y′CbCr color space, although retaining the established process of chroma subsampling. [[Cinepak]], a video codec from 1991, used a modified YUV 4:2:0 colorspace. The professional [[CCIR 601]] digital video format also uses Y′CbCr at the common chroma subsampling rate of 4:2:2, primarily for compatibility with previous analog video standards. This stream can be easily mixed into any output format needed.

Y′UV is not an [[Color space#Absolute color space|absolute color space]]. It is a way of encoding RGB information, and the actual color displayed depends on the actual RGB colorants used to display the signal. Therefore, a value expressed as Y′UV is only predictable if standard RGB colorants are used (i.e. a fixed set of primary chromaticities, or particular set of red, green, and blue).

Furthermore, the range of colors and brightnesses (known as the color [[gamut]] and color volume) of RGB (whether it be BT.601 or Rec.&nbsp;709) is far smaller than the range of colors and brightnesses allowed by Y′UV. This can be very important when converting from Y′UV (or Y′CbCr) to RGB, since the formulas above can produce "invalid" RGB values – i.e., values below 0% or very far above 100% of the range (e.g., outside the standard 16–235 luma range (and 16–240 chroma range) for TVs and HD content, or outside 0–255 for standard definition on PCs). Unless these values are dealt with they will usually be "clipped" (i.e., limited) to the valid range of the channel affected. This changes the hue of the color, which is very undesirable, so it is therefore often considered better to desaturate the offending colors such that they fall within the RGB gamut.<ref>Limiting of YUV digital video signals (BBC publication)
Authors: V.G. Devereux
http://downloads.bbc.co.uk/rd/pubs/reports/1987-22.pdf</ref>

Likewise, when RGB at a given bit depth is converted to YUV at the same bit depth, several RGB colors can become the same Y′UV color, resulting in information loss.