us input adaptive A-law or A-law or Haw coding quantiser PCM input adjustment Signal estimate Adaptive predictor Figure 19.23 ADPCM decoder 19.6 Delta modulation Alternatively, instead of transmitting a bipolar pulse train, a uni- polar pulse train can be used (i.e. '1'= + and '0' = zero volts). The integrators are then discharged at a constant rate, SO that their output 19.6.1 Basic delta modulation voltages decrease by one quantum step during a sample period not containing '1'. The limiting case of DPCM is when the number of bits in the code As shown in Figure 19.24, a delta modulator is very simple and is reduced to only one. This is possible if over sampling is used, i.e. therefore cheap. It also has the useful property that all transmitted the sampling frequency is made much greater than the Nyquist rate. digits have equal weight, corresponding to one quantum step. Thus, The difference that can occur between adjacent samples is then a one bit error in transmission only causes an error of one quantum reduced, so that it rarely exceeds one quantum step. The method is in the output signal. In PCM, an error in the most significant digit called delta modulation (DM) (Steele, 1975). causes an error of half the maximum signal amplitude, with disas- trous results if this occurs frequently. Thus, DM is very robust in the A basic DM system is shown in Figure 19.24. At the sending end, presence of frequent transmission errors. It has therefore been used the comparator causes the pulse generator to transmit +V when the in military systems. input voltage exceeds the output voltage from the integrator and -V In theory, there is no limit to the amplitude of baseband signal when it is less. As well as being transmitted, each of these pulses which the system can transmit. It can handle a signal of any ampli- causes the output voltage from the integrator to rise or fall by one tude, providing that it is not changing SO rapidly that it causes slope quantum step, so that it closely tracks the input voltage. At the overload. To 