M, the analogue modulation is AM; for DM, it is PM; but there is no companding to maintain the SQNR at lower signal for DSM, it is FM. levels. It follows that one type of digital modulation can be converted to another. If DM is generated with a sampling frequency n times the 19.6.2 Adaptive delta modulation Nyquist rate and the output of the delta modulator feeds an up-down binary counter, then output readings from the counter at intervals of Simple DM is susceptible to slope overload. Moreover, since it n digits give PCM code words at the Nyquist rate. (Goodman, provides uniform quantisation, the SQNR decreases when the level 1969). If DSM is used, the number of 1's in the digit stream should of the input signal is reduced. Several schemes of adaptive delta be counted over a period of n digits and the counter then reset modulation (ADM) have been introduced to minimise these disad- (Wallace et al., 1991). Coding programme quality sound 19/21 Delta Integrator modulator Demodulator Differentiator Transmission Baseband Baseband output input Delta Integrator modulator Transmission Baseband Baseband output input Figure 19.25 Delta sigma modulation: (a) derivation of delta sigma modulation; (b) delta sigma modulation system Counters used in this way are called conversion filters. Use of a order to provide a higher quality than that provided by analogue delta modulator or a delta sigma modulator, together with a conver- transmission (CCIR Report 962). sion filter, provides an alternative to A/D converters. A number of techniques have been developed to reduce the band- width required for a 70Mbit/s video signal. Differential PCM can be used instead of conventional PCM. With non uniform quantisation, 19.7 Coding programme quality sound 4 to 6 bits are sufficient for the luminance signal and 2 to 4 bits for the colour information. This enables 34Mbit/s to be used for pro- For broadcast programmes, the bandwidth and dynamic range re- gramme quality signals or 8Mbit/s for video telephony (CCIR quired are greater than those 