The main disadvantages of the FM telemetry system is that the capacity of channels offered is less. So the basic need is to have higher capacity of channel than offered by the FM telemetry system within a limited bandwidth. To fulfil such requirement, the pulse amplitude modulation (PAM) telemetry system is used extensively which empolyes time division multiplexing technique.

Consider a basic time division multiplexing system as shown in the below figure. All the channels use same frequency band of the alloted frequency spectrum but at different time. The commutator is used to sample signal of each channel in sequence. The amplitude of each sample represents the instantaneous value of the signal at that point. The sequence starts from first channel and the samples from other channels are interleaved from the first one by using bypass intervals.

As none of channel is monitored continuously, it is necessary that the rate of sampling must be higher so that in between sampling intervals, the amplitude of the channel does not vary much. And the information in the original signal is preserved.

as 250 times per second. Hence for 8 channels in the system. the samples per second that must be taken by commutator are 2000.

At the receiving end, the decommutator operates at exactly same frequency at which the commutator at the transmitting end operates. The main function of the decommutator is to distribute the multiplexed signal to proper output channel. For the precise timing, the commutator and the decommutator are synchronized properly using the synchronizing signal.

The PAM telemetry system uses same time division multiplexing system. It is the simplest form as the samples are transmitted with no modification. The data at the receiving end is demodulated only when the channels are properly recogonized. A simple PAM telemetry system is as shown in the below Figure.

Fig. PAM telemetry system