band and it is impossible to separate them. This is earlier that the minimum bandwidth required to transmit pulses is called aliasing. It can be avoided by passing the input signal through half the pulse rate. Thus, a bandwidth of at least 32kHz is required a low pass filter before sampling it. It is essential to include such an to transmit a single telephone channel. The advantages of digital anti-aliasing filter at the input of a sampling system. Since the transmission are won at the expense of a much greater bandwidth nominal band of telephone signals is from 300Hz to 3.4kHz and the requirement. internationally agreed sampling frequency is 8kHz, there is a guard band of 1.2kHz between the top of the baseband and the bottom of 19.4.1 Sampling the lower sideband of the sampling frequency. This must accommo- date the transition between the pass band and the stop band of the In order to produce a digital signal, an analogue signal must first be anti-aliasing filter at the sending end and of the demodulating filter sampled by a periodic pulse train to convert it from continuous to at the receive end. Vc coswct V C coswc t c sinwc t Vc sinwc t Figure 19.13 Quadrature amplitude modulation (QAM) system Pulse code modulation 19/13 Baseband input Transmission Baseband output Coder Decoder Figure 19.14 Block diagram of a PCM system (one direction of transmission only) Anti-aliasing filters can be implemented in many ways. High dual slope conversion, successive approximation and flash conver- order passive LC filters have often been used, offering low distor- sion (Wallace et al., 1991). tion and low noise, but with the physical drawbacks of large size and The successive approximation ADC is built from a digital to weight, and inevitable insertion loss. A more up to date approach analogue converter, a comparator, a storage register and controlling might be the use of a Sallen-and-Key active filter (1955), which is logic, as shown in Figure 19.16. This type of converter essentially smaller, lighter and loss free, but adds 