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G3 FACSIMILE COMMUNICATIONS
Coded image data is grouped into two consecutive digital bits, and the car-
rier wave is modulated every 1/1200 seconds on each group. In other
words, there are 1200 phase changes every second.
• Modulation speed
The number of changes that occur in the carrier wave per second is called
the modulation speed (baud rate).
The modulation speed of this method is 1200 baud. If the modulation
speed is 1200, then the number of bits of digital data that corresponds to
each relative phase changes is two, and so the bit rate (actual transmission
speed) is 2400 bps.
•
ITU-T
ITU-T recognizes PSK methods that transmit data at a speed of 2400 bps,
and related standards are called ITU-T recommendation V.27 ter.
(3) 4800 bps PSK methods
4800 bps modulation PSK methods are also used in facsimile data trans-
missions.
By this PSK methods, eight relative phase changes are used to express dig-
ital signals. If we use the spatial diagram in Fig. 5-44 below, we can fully
understand that this method uses the relative phase changes at 0°, 45°, 90°,
135°, 180° , 225°, 270°, and 315°, and further that each of the phase
changes expresses a combination of three digital bits called a "tribit."
5–50
1/1200 sec
Coded image data
PSK-modulated
image data
Advances 90 from
the waveform on
the left
Fig. 5-43 4-Phase PSK Method Modulation Carrier Wave Image
recommendation V.27 ter
Modulation speed (baud rate) = 1200 baud
Transmission speed = 2400 bps
Time
01
11
00
10
00
Advances
Same
Advances
phase
180
270
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