needed for telephony and the per- Report 629-2). missible degradation is much smaller. The CCIR has set a standard More complex coding techniques have been developed to achieve of 32kHz sampling and 14-bit coding for sound channels of 14kHz further bandwidth reductions. (Jain, 1981). For example, the appli- bandwidth. cation of conditional replenishment and motion prediction enables The CCITT has recommended two alternative companding the digit rate to be reduced to about 384kbit/s (Wallace et al., 1991). schemes: an 11-bit A-law, giving a reduction from 14 bits to 11 bits The performance obtained is adequate for colour video conferenc- per sample, and a five range 'near instantaneous' companding ing applications (CCITT Recommendation H.261). scheme (NIC), giving a reduction from 14 bits to 10 bits per sample (Wallace et al., 1991). 19.9 Acknowledgements Coding video signals This chapter contains material reproduced by permission of Messrs. L.D. Humphrey, M.J. Sexton and A.D. Wallace and the Institution Composite video signals for colour television contain a luminance of Electrical Engineers from Chapter 6 of Flood, J.E. and Cochrane, signal, together with colour components modulated onto quadrature P. (1991) Transmission Systems, Peter Peregrinus. sub carriers at 4.43MHz. Since the total bandwidth exceeds 5MHz, it might appear that the sampling frequency must exceed 10MHz. However, the spectrum about the colour sub carrier frequency is 19.10 References concentrated in lines at intervals equal to half the line repetition frequency. This enabled a lower sampling rate, fs, to be chosen, such that the aliases from those lines above fs/2 fall into gaps in the Alexander, A.A., Gryb, R.M. and Nast, D.W. (1960) Capabilities of spectrum below fs/2 and can be removed by comb filtering. the telephone network for data transmission, Bell Syst. Tech. A sampling frequency of 8.86MHz and 8-bit encoding gives a Jour., 39, p.431. digit rate of about 70Mbit/s. In practice, 68Mbit/s is used, but Arthurs, E. and Dym, H. (1962) On th