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G3 FACSIMILE COMMUNICATIONS
5.3.2 Phase shift keying
Phase Shift Keying (PSK) is a general-purpose modulation method that is
used on facsimiles to transmit image signals. This system expresses digital
data by changing the phase of the carrier wave. Here, let's describe PSK by
starting with a description of the meaning of "phase change."
"Phase" refers to the relative position in time of the signal of a fixed fre-
quency. A complete cycle of these signals contains a phase of 360°. Fig. 5-
39 shows an example of the relative position between the carrier wave and
the phase.
As can be seen from this figure, the same start point is returned to if the
360° phase is advanced along the signal whichever point is taken as the
start point. In other words, the position of 360° indicates exactly the same
point as the position of 0° on the signal.
"Phase change" shows the size of the phase by which the signal is shifted.
Fig. 5-40 shows a simple example to illustrate this. This example shows
instances where the phase change is 0° (no phase change), 90° and 180°.
5–48
(cycle)
Fig. 5-39 Carrier Wave and Phase
Amplitude
Analog signal
Phase shift
Fig. 5-40 Carrier Wave and Phase Change
Carrier wave of fixed frequency
Modulated carrier wave
Phase shift
Phase shift
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