Luminance/chrominance Systems in General
The primary advantages of luma/chroma systems such as Y'UV, and its relatives Y'IQ and YDbDr, are 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. 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 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 only half the resolution compared to the brightness information. This ratio is the most common form, known as 4:2:2, which is the sample ratio of Y:U:V, or Y:I:Q. The 4:x:x standard was adopted due to the very earliest color NTSC standard which used a chroma subsampling of 4:1:1 so that the picture carried only a quarter as much resolution in color than it did in brightness. 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 space conversion is lossy, particularly obvious in crosstalk from the luma to the chroma-carrying wire, and vice versa, in analogue equipment (including RCA connectors to transfer a digital signal, as all they carry is analogue composite video, which is either YUV, YIQ, or even 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 video compression algorithms such as MPEG-2. Digital television and DVDs preserve their compressed video streams in the MPEG-2 format, which uses a full Y'CbCr color space, although retaining the established process of chroma subsampling. 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 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).
Read more about this topic: YUV
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