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G3 FACSIMILE COMMUNICATIONS
5.3.3 Quadrature amplitude modulation method
Quadrature Amplitude Modulation (QAM) is a modulation method that
expresses digital information using the changes in both the amplitude and
phase. From this, we can understand the following:
AM + PSK = QAM
(1) 9600 bps QAM method
This QAM method groups digital image data into 4-bit groups called
"quad bits." The initial bit of the quad bit determines the amplitude of the
modulated carrier wave. The remaining three bits determine the relative
phase changes to be applied to the carrier wave in the same way as in the
PSK method.
Fig. 5-45 shows the spatial diagram of the QAM method. Here, too, we
can see that the relative changes at 0°, 45°, 90°, 135°, 180° , 225°, 270°,
and 315° are used, and further that each of the phase changes indicates a
tribit. The figure also shows the fact that the carrier wave has two relative
amplitudes.
These amplitudes are each identified as large amplitudes whose initial bit
indicates digital "1" and small amplitudes whose initial bit indicates digital
"0".
5–52
"010"
5
"011"
3
3 2
1
2
1
"111"
"110"
"100"
Fig. 5-45 Spatial Diagram of QAM Method
The relative amplitude is
determined by the amplitude
determination bit (initial bit).
"0"
"000"
"1"
"001"
(Absolute phase)
3
5
"101"
Tribit
Phase shift
(relative phase)
Small amplitude
Large
amplitude
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