2-44
Q-21.
To ensure the accuracy of a transmission, what is the minimum number of times a modulating
wave should be sampled in pulse modulation?
Q-22.
What, if any, noise susceptibility advantage exists for pulse-amplitude modulation over analog-
amplitude modulation?
Pulse-Time Modulation
In pulse-modulated systems, as in an analog system, the intelligence may be impressed on the carrier
by varying any of its characteristics. In the preceding paragraphs the method of modulating a pulse train
by varying its amplitude was discussed. Time characteristics of pulses may also be modulated with
intelligence information. Two time characteristics may be affected: (1) the time duration of the pulses,
referred to as PULSE-DURATION MODULATION (pdm) or PULSE-WIDTH MODULATION (pwm);
and (2) the time of occurrence of the pulses, referred to as PULSE-POSITION MODULATION (ppm),
and a special type of PULSE-TIME MODULATION (ptm) referred to as PULSE-FREQUENCY
MODULATION (pfm). Figure 2-43 shows these types of ptm in views (C), (D), and (E). Views (A) and
(B) show the modulating signal and timing, respectively.
Figure 2-43A.—Pulse-time modulation (ptm). MODULATION.
Figure 2-43B.—Pulse-time modulation (ptm). TIMING.
Figure 2-43C.—Pulse-time modulation (ptm). PDM.
Figure 2-43D.—Pulse-time modulation (ptm). PPM.
