ted during each repetition period sending terminal, a baseband channel is connected to the common contains a number of time slots: one is allocated to the synchroni- transmission path by means of a sampling gate which is opened for sing signal and the others to the channel samples. The complete short intervals by means of a train of pulses. In this way, samples of waveform is called a frame, by analogy with a television signal the baseband signal are sent at regular intervals by means of ampli- waveform, and the synchronising signal is called the frame align- tude modulated pulses. ment signal. Pulses with the same repetition frequency f. but staggered in time, The elementary TDM system shown in Figure 18.20 uses PAM. as shown in Figure 18.21 are applied to the sending gates of the Pulse length modulation and pulse position modulation can also be used. Pulse frequency modulation is unsuitable. If the PRF of one channel changes, its pulse will leave its allotted time slot and drift across the time slots of the other channels. Attenuation and delay distortion of a transmission path cause pulse dispersion. The pulse of each channel spreads into the time slots of adjacent channels, as shown in Figure 18.22. This causes inter-channel crosstalk (Flood and Tillman, 1951; Flood, 1952). Consequently, analogue pulse modulation is now mainly used as an Channel n+1 Channel n Channel n-1 Figure 18.21 Channel pulse trains for the TDM system of Figure 18.20: (a) (n-1)th channel; (b) nth channel; Volts (c) (n+1)th channel other channels. Thus the common transmission path receives inter- leaved trains of pulses, modulated by the signals of different chan- Crosstalk Signal Crosstalk nels. At the receiving terminal, gates are opened by pulses coincident with those received from the transmission path SO that Figure 18.22 Inter-channel crosstalk in TDM systems the demodulator of each channel is connected to the transmission 18/16 Analogue modulation intermediate stage towards pulse code modulation (PCM), in order Gardner, F.M. (1979)