R 601 calls for uniformly spaced quantisation levels and 8 bit colour component signals, it is common practice when sampling code words. The nominal black and white levels are represented by composite colour signals to increase the sampling rate to an exact 16 and 235 respectively. The nominal range of the colour dif- integer multiple of the colour subcarrier frequency fsc. This mini- ferences is from 16 to 240 with 128 representing a zero difference. mises the visibility of beat frequencies between harmonics of the subcarrier and the sampling clock arising from non-linearities in the The resulting bit rate of the 4:2:2 version of CCIR 601 is digital to analogue and analogue to digital converters. Sampling 216Mbit/s. CCIR Recommendation 656 covers serial and parallel rates of 3xfsc have been used in transmission equipments, where the interface arrangements (CCIR, 1986). Some reduction of the bit rate emphasis is on minimising the bit rate and 4xfsc, with its 90 degree of PCM encoded video can be obtained by omitting the blanking subcarrier phase shift between samples, has attractions when sub- intervals and synchronising pulses as these can be regenerated at the sequent digital signal processing will be employed to modify the decoder. CCIR 601 4:2:2 specifies a digital active line as containing input signal in certain ways. 720 luminance samples. Taking into account the vertical blanking Because of the scanning process, the energy spectrum of video period, the net rate representing visible picture information is of the signals tends to be concentrated in the vicinities of harmonics of the order of 166Mbit/s. line scanning rate. By careful choice of a sub-Nyquist sampling A PCM representation is invariably a prerequisite for video com- frequency it is possible to arrange for the alias spectrum to inter- pression coding. leave with the original and yet be separable at the decoder using comb filter techniques. Some loss of diagonal resolution is intro- 57.4.2 Bit rate reduction duced but using this technique, compos