2-48
Figure 2-47B.—Pulse-frequency modulation (pfm). TIMING.
Figure 2-47C.—Pulse-frequency modulation (pfm). PFM.
Q-23.
What characteristics of a pulse can be changed in pulse-time modulation?
Q-24.
Which edges of the pulse can be modulated in pulse-duration modulation?
Q-25.
What is the main disadvantage of pulse-position modulation?
Q-26.
What is pulse-frequency modulation?
Pulse-Code Modulation
PULSE-CODE MODULATION (pcm) refers to a system in which the standard values of a
QUANTIZED WAVE (explained in the following paragraphs) are indicated by a series of coded pulses.
When these pulses are decoded, they indicate the standard values of the original quantized wave. These
codes may be binary, in which the symbol for each quantized element will consist of pulses and spaces:
ternary, where the code for each element consists of any one of three distinct kinds of values (such as
positive pulses, negative pulses, and spaces); or n-ary, in which the code for each element consists of nay
number (n) of distinct values. This discussion will be based on the binary pcm system.
All of the pulse-modulation systems discussed previously provide methods of converting analog
wave shapes to digital wave shapes (pulses occurring at discrete intervals, some characteristic of which is
varied as a continuous function of the analog wave). The entire range of amplitude (frequency or phase)
values of the analog wave can be arbitrarily divided into a series of standard values. Each pulse of a pulse
train [figure 2-48, view (B)] takes the standard value nearest its actual value when modulated. The
modulating wave can be faithfully reproduced, as shown in views (C) and (D). The amplitude range has
been divided into 5 standard values in view (C). Each pulse is given whatever standard value is nearest its
actual instantaneous value. In view (D), the same amplitude range has been divided into 10 standard
levels. The curve of view (D) is a much closer approximation of the modulating wave, view (A), than is
the 5-level quantized curve in view (C). From this you should see that the greater the number of standard
levels used, the more closely the quantized wave approximates the original. This is also made evident by
the fact that an infinite number of standard levels exactly duplicates the conditions of nonquantization
(the original analog waveform).
