A technique by which we may take advantage of the sampling principle for the purpose of time division multiplexing is illustrated in the idealized representation of Fig. a.

At the transmitting end on the left,

a number of bandlimited signals are connected to the contact point of a rotary switch.

We assume that the signals are similarly bandlimited.

For example, they may all be voice signals, limited to 3.3 kHz.

As the rotary arm of the switch swings around, it samples each signal sequentially.

The rotary switch at the receiving end is in synchronism with the switch at the sending end.

The two switches make contact simultaneously at similarly numbered contacts.

With each revolution of the switch, one sample is taken of each input signal and presented to the correspondingly numbered contact of the receiving-end switch.

The train of samples at, say, terminal 1 in the receiver, pass through low-pass filter 1,

and, at the filter output, the original signal m1(t) appears reconstructed.

Of course, if fM is the highest-frequency spectral component present in any of the input signals,

the switches must make at least 2fM revolutions per second.

When the signals to be multiplexed vary slowly with time,

so that the sampling rate is correspondingly slow, mechanical switches, indicated in Fig. a, may be employed.

When the switching speed required is outside the range of mechanical switches, electronic switching systems

may be employed. In either event, the switching mechanism,

corresponding to the switch at the left in Fig. a, which samples the signals, is called the commutator.

The switching mechanism which performs the function of the switch at the right in Fig. (a ) is called the decommutator.

The commutator samples and combines samples,

while the decommutator separates samples belonging to individual signals so that these signals may be reconstructed.

The interlacing of the samples that allows multiplexing is shown in Fig. (b).

Here, for simplicity, we have considered the case of the multiplexing of just two signals m1(t) and m2(t).

The signal m1(t) is sampled regularly at intervals of Ts, and at the times indicated in the figure.

The sampling of m2(t) is similarly regular, but the samples are taken at a time different from the sampling time of m1(t).

The input waveform to the filter numbered 1 in Fig. (a) is the train of samples of m1(t),

and the input to the filter numbered 2 is the train of samples of m2(t).

The timing in Fig. (b) has been deliberately drawn to suggest that there is room to multiplex more than two signals.

We shall see shortly, in principle, how many signals may be multiplexed.

We observe that the train of pulses corresponding to the samples of each signal are modulated in amplitude in accordance with the signal itself.

Accordingly, the scheme of sampling is called pulse-amplitude modulation and abbreviated PAM.

Multiplexing of several PAM signals is possible because the various signals are kept distinct and are separately recoverable by virtue of the fact that they are sampled at different times.

Hence, this system is an example of a time-division multiplex (TDM) system.

Such systems are the counterparts in the time domain of the systems of Chapter 2.

There, the signals were kept separable by virtue of their translation to different portions of the frequency domain,

and those systems are called frequency-division multiplex (FDM) systems.

If the multiplexed signals are to be transmitted directly, say, over a pair of wires, no further signal processing need be undertaken.

Suppose, however, we require to transmit the TDM-PAM signal from one antenna to another.

It would then be necessary to amplitude-modulate or frequency-modulate a high-frequency carrier with the TDM-PAM signal;

in such a case the overall system would be referred to, respectively, as PAM-AM or PAM-FM.

Note that the same terminology is used whether a single signal or many signals (TDM) are transmitted.

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