ite colour signals have been sampled at 2xf (Ratliff, 1983). Video signals contain several sources of redundancy which can be For 525 and 625 colour signals in component form, CCIR Recom- used to reduce the bit rate. Some techniques operate on the statisti- mendation 601 specifies a family of related sampling frequencies cal redundancy and are fully reversible, ie the original can be (CCIR, 1990a). The most widely used is the 4:2:2 version in which recovered exactly. Other techniques exploit the characteristics of the luminance component is sampled at 13.5MHz and each of the the human visual system to conceal the distortion they introduce or two colour difference components at 6.75MHz. The latter lower rate minimise its visibility. Practical coders often incorporate both tech- reflects the reduced acuity of the human eye to colour detail. Unlike niques. f based ones, the CCIR 601 sampling frequencies result in ortho- gonal arrays of sampling points and they remain stationary from one 57.4.3 picture to the next. Conversion of the analogue samples to digital form is normally performed with a uniform quantisation law and 8 Differential Pulse Code Modulation (DPCM) exploits the facts that bits of resolution. For the highest quality applications where adjacent pels are likely to have similar values and that the human multiple tandem encoding and decoding or subsequent processing eye is more tolerant of distortion in areas containing higher spatial will be encountered an extra 1 or 2 bits may be used. Some images, frequencies, i.e. edges and detail. As shown in Figure 57.24, a PCM PCM video in DPCM out Quantiser Inverse quantiser Delay PCM video out Inverse quantiser DPCM in Delay Figure 57.24 DPCM coder (top) and decoder (bottom) Digital video coding 57/25 input sample is applied to one input of a subtractor. A prediction values, will have a positional error. To limit this error extension value, in this case a previously coded pel, is applied to the other effect it is prudent to include some resynchronisation code wor