2-43Pulse-Amplitude ModulationSome characteristic of the sampling pulses must be varied by the modulating signal for theintelligence of the signal to be present in the pulsed wave. Figure 2-42 shows three typical waveforms inwhich the pulse amplitude is varied by the amplitude of the modulating signal. View (A) represents a sinewave of intelligence to be modulated on a transmitted carrier wave. View (B) shows the timing pulseswhich determine the sampling interval. View (C) shows PULSE-AMPLITUDE MODULATION (pam) inwhich the amplitude of each pulse is controlled by the instantaneous amplitude of the modulating signal atthe 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 thesame manner as analog-amplitude modulation. The timing pulses are applied to a pulse amplifier in whichthe gain is controlled by the modulating waveform. Since these variations in amplitude actually representthe signal, this type of modulation is basically a form of AM. The only difference is that the signal is nowin 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 interferencein the transmission path will either add to or subtract from any voltage already in the circuit (signalvoltage). Thus, the amplitude of the signal will be changed. Since the amplitude of the voltage representsthe 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 fortransmission. Techniques of pulse modulation other than pam have been developed to overcome problemsof noise interference. The following sections will discuss other types of pulse modulation.Q-20.What action is necessary to impress intelligence on the pulse train in pulse modulation?
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