Table of Contents
Advertisement
G3 FACSIMILE COMMUNICATIONS
(3) Additional explanation relating to QAM methods
The purpose of QAM methods is not to increase the transmission speed by
generally increasing the number of phases in modulation processing. The
reason for this is that as a distortion occurs due to the characteristics of
telephone line which results in the position of the modulation signal shift-
ing and amplitude change. The mutual interval between phase changes can
be narrowed by increasing the phase changes. When this is performed,
demodulation can no longer be executed due to the influence of line distor-
tion. Fig. 5-47 illustrates this.
For this reason, only eight phase changes are used in QAM methods. The
QAM method provides a "target" for each phase change and amplitude
change. So that these change points are regarded as valid QAM signals,
they must be positioned within the target range.
As shown in the figure below, demodulation will not be affected by influ-
ence of line distortion even if these points move within each target range.
5–54
Influence of line distortion
Fig. 5-47 QAM Method and Influence of Line Distortion
Influence of line
distortion prevents
these points from
being recognized.
?
Advertisement
Table of Contents
