Some characteristic of the sampling pulses must be varied by the modulating signal for the
intelligence of the signal to be present in the pulsed wave. Figure 2-42 shows three typical waveforms in
which the pulse amplitude is varied by the amplitude of the modulating signal. View (A) represents a sine
wave of intelligence to be modulated on a transmitted carrier wave. View (B) shows the timing pulses
which determine the sampling interval. View (C) shows PULSE-AMPLITUDE MODULATION (pam) in
which the amplitude of each pulse is controlled by the instantaneous amplitude of the modulating signal at
the time of each pulse.
Figure 2-42A.Pulse-amplitude modulation (pam). MODULATION.
Figure 2-42B.Pulse-amplitude modulation (pam). TIMING.
Figure 2-42C.Pulse-amplitude modulation (pam). PAM.
Pulse-amplitude modulation is the simplest form of pulse modulation. It is generated in much the
same manner as analog-amplitude modulation. The timing pulses are applied to a pulse amplifier in which
the gain is controlled by the modulating waveform. Since these variations in amplitude actually represent
the signal, this type of modulation is basically a form of AM. The only difference is that the signal is now
in the form of pulses. This means that pam has the same built-in weaknesses as any other AM signal -
high susceptibility to noise and interference. The reason for susceptibility to noise is that any interference
in the transmission path will either add to or subtract from any voltage already in the circuit (signal
voltage). Thus, the amplitude of the signal will be changed. Since the amplitude of the voltage represents
the signal, any unwanted change to the signal is considered a SIGNAL DISTORTION. For this reason,
pam is not often used. When pam is used, the pulse train is used to frequency modulate a carrier for
transmission. Techniques of pulse modulation other than pam have been developed to overcome problems
of noise interference. The following sections will discuss other types of pulse modulation.
What action is necessary to impress intelligence on the pulse train in pulse modulation?