the original baseband, together with upper and lower side- lowest possible rate at which a signal may be sampled, 2F is often bands about the PRF and its harmonics, as shown in Figure 18.17(f). known as the Nyquist rate. If the sampling frequency is less than the The PAM signal can be demodulated by means of a low pass filter Nyquist rate, the lower sideband of the PRF overlaps the baseband which passes the baseband and stops the lower sideband of the PRF 18/14 Analogue modulation and it is impossible to separate them. The output from the low pass filter then contains unwanted frequency components; this situation vi(t) is known as aliasing. To prevent aliasing, it is essential to limit the bandwidth of the Slicing level signal before sampling. Thus, practical systems pass the input signal through an anti-aliasing low pass filter of bandwidth 1/2f before sampling as shown in Figure 18.16. Practical filters are non-ideal; it is therefore necessary to have in order that the anti-alias- ing filter and demodulating filter can have both very low attenuation at frequencies up to F and very high attenuation at frequencies down to - F For telephony, a baseband from 300Hz to 3.4kHz is provided and a sampling frequency of 8kHz is used. Thus = 4.6kHz and there is a guardband of 1.2kHz to accommodate the transition of the filters between their pass band and their stop band. 18.4.3 Pulse time modulation Pulse frequency modulation, pulse phase modulation and pulse length modulation all vary the times of occurrence of the individual pulses of a pulse train. They can therefore be collectively called pulse time modulation. Since the modulating signal is not carried by Figure 18.18 Use of slicer in pulse time modulation: the amplitude of the pulses, pulse time modulation can be almost (a) input pulse; (b) output pulse immune to amplitude variations caused by interference and noise and so give a better output signal to noise ratio than PAM. When pulse time modulation is used, the sidebands about the PRF and its harmonics contain sid