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
Figure 2-43E. – Pulse-time modulation (ptm). PFM
PULSE-DURATION MODULATION. – Pdm and pwm are designations for a single type of modulation. The width of each pulse in a train is made proportional to the instantaneous value of the modulating signal at the instant of the pulse. Either the leading edges, the trailing edges, or both edges of the pulses may be modulated to produce the variation in pulse width.