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(1) Concept of PSK method
The PSK method expresses digital information by changes in the phase of
the carrier wave. The size of the change in the phase that occurs as a result
of modulation differs according to the PSK method adopted. By the PSK
method, information is not expressed by the actual shape of the carrier
wave itself but by changes in the phase of the carrier wave. For this reason,
"spatial diagrams" are used to show phase changes adopted by a certain
particular modulation method. A spatial diagram is a two-dimensional
diagram in which the carrier wave shows the phase of 360°. Diagram axes
shows the amplitude range of the signal at each respective phase. The num-
ber of these axes is either four or eight depending on which PSK method is
adopted.
Fig. 5-41 4-phase and 8-phase Spatial Diagram
(2) 2400 bps PSK methods
By this PSK method on a facsimile, four relative phase changes are used to
express digital signals. If we use the spatial diagram in Fig. 5-42 below, we
can fully understand that this method uses the relative phase changes at 0°,
90°, 180° and 270°, and further that each of the phase changes expresses a
combination of two digital bits called a "dibit."
Fig. 5-42 Spatial Diagram of 4-phase PSK Method
G3 FACSIMILE COMMUNICATIONS
"01"
"11"
"10"
Dibit
Phase shift (relative phase)
"00"
(Absolute phase)
1
2
3
4
5
6
ït
çi
5–49
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