e, however, pulses have finite rise and fall times. As shown in Figure 18.19 These equations show that the demodulated output voltage varies interference or noise produces a displacement of the time at which inversely with the slope (dv, dt) of the input pulse. This slope is the input pulse crosses the slicing level and thus of the time at which proportional to the bandwidth of the transmission path. Thus, the the output pulse appears from the slicer. This results in noise or improvement in output signal to noise power ratio is proportional to interference in the demodulated output signal. This time displace- the square of the bandwidth (Jelonek, 1947; Kretzmer, 1950; Black, ment is given by Equations 18.58 and 18.59. 1953). It is assumed above that the noise voltage itself does not cross Pulse modulation 18/15 Sending Receiving Demodulating channel gates gates filters channel Pn - 1 channel channel channel channel Pn + 1 Pn+ 1 Figure 18.20 Elementary TDM system with one direction of transmission only the slicing level; if it does, misoperation of the slicing circuit occurs path for its allotted interval and disconnected throughout the re- and there is a severe deterioration of the output signal to noise ratio. mainder of the pulse repetition period. The combination of a multi- Thus, there is a threshold phenomenon similar to that shown in plexer and a demultiplexer at a TDM terminal is sometimes referred Figure 18.15. to as a muldex. The pulse generator at the receiving terminal must be syn- 18.4.4 Time division multiplexing chronised with that at the sending terminal to ensure that each incoming pulse train is gated to the correct outgoing baseband Pulse modulation is used for time division multiplexing (TDM). It channel. A distinctive synchronising pulse signal is therefore trans- enables channels to share a wideband transmission path by using it mitted in every repetition period in addition to the channel pulses. at different times. The principle is shown in Figure 18.20. At the The complete waveform transmit