The Digital Fact Book - Quantel

CCITTInternational Telegraph and Telephone Consultative Committee. As the name suggests thiswas initially set up to establish standards for the telephone industry in Europe. It has now beensuperseded by ITU-T so putting both radio frequency matters (ITU-R) and telecommunicationsunder one overall United Nations body.See also: ITUCCDLSee: Color decision listChecksumA simple check value of a block of data intended to recognized when data bits are wronglypresented. It is calculated by adding all the bytes in a block. It is fairly easily fooled by typicalerrors in data transmission systems so that, for most applications, a more sophisticatedsystem such as CRC is preferred.See also: CRCChroma keyingThe process of overlaying one video signal over another, the areas of overlay being definedby a specific range of color, or chrominance, on the background signal. For this to workreliably, the chrominance must have sufficient resolution, or bandwidth. PAL or NTSC codingsystems restrict chroma bandwidth and so are of very limited use for making a chroma keywhich, for many years, was restricted to using live, RGB camera feeds.An objective of the ITU-R BT.601 and 709 digital sampling standards was to allow high qualitychroma keying in post production. The 4:2:2 sampling system allows far greater bandwidthfor chroma than PAL or NTSC and helped chroma keying, and the whole business of layering,to thrive in post production. High signal quality is still important to derive good keys so somehigh-end operations favor using RGB (4:4:4) for keying – despite the additional storagerequirements. Certainly anything but very mild compression tends to result in keying errorsappearing – especially at DCT block boundaries.Chroma keying techniques have continued to advance and use many refinements, to thepoint where totally convincing composites can be easily created. You can no longer see thejoin and it may no longer be possible to distinguish between what is real and what is keyed.See also: Color space, Digital keying, Photo-real37