Table of Contents
Advertisement
"011"
"111"
Fig. 5-44 Spatial Diagram of 8-phase PSK Method
Coded image data is grouped into three consecutive digital bits, and the
carrier wave is modulated every 1/1600 seconds on each group. In other
words, there are 1600 phase changes every second. Accordingly, the mod-
ulation speed of this system is 1600 baud. If the modulation speed is 1600,
then the bit rate is 4800 bps.
4800 bps PSK methods are also called ITU-T recommendation V.27ter in
the same way as 2400 bps PSK methods.
(4) Limits of PSK methods
Logically, PSK methods can be used to achieve transmission speeds of
7200 bps or higher. If we fully consider the 4800 bps 8-phase system
described above, we can understand how this can be achieved. For exam-
ple, if the modulation speed is increased to 2400 baud, then a transmission
speed of 7200 bps can be obtained. However, transmission speeds this fast
are not used in facsimile transmission. There are two reasons for this.
• As the bandwidth of telephone lines is limited, modulation speeds
exceeding 2400 baud result in signal errors.
• By PSK methods, the number of phase changes cannot be set to eight
or higher.
To achieve transmission speeds of 7200 bps or higher, a modulation
method called QAM is used.
G3 FACSIMILE COMMUNICATIONS
"010"
"000"
"001"
"101"
"110"
"100"
(Absolute phase)
Tribit
Phase shift (relative phase)
5–51
1
2
3
4
5
6
ït
çi
Advertisement
Table of Contents
