Table of Contents
Advertisement
2.3
MODEM
Facsimiles must transmit image signals (digital signals) of scanned docu-
ments to other parties via telephone lines (analog lines). For this reason,
the transmitting facsimile must convert these digital signals to analog sig-
nals and transmit them out onto the telephone line. This conversion is
called "modulation." Alternatively, the receiving facsimile must convert
the analog signals (modulated signals) that arrive over the telephone line to
digital signals. This conversion is called "demodulation" due to the fact
that "modulated signals are restored to their original state." A device that
modulates signals is called a "modulator," and a device that demodulates
signals is called a "demodulator." A device that can perform both modula-
tion and demodulation is called a "MODEM" (MOdulator and DEModula-
tor). As the frequency bandwidth of a telephone line is between 300 to
3,400 Hz, the modulation methods indicated below recommended by
T
and at high speed to the other party within that frequency bandwidth.
FSK (Frequency Shift Keying) is used as a modulation method for proce-
dure signals required for transmitting image signals.
are used as the G3 facsimile modem to transmit image signals efficiently
• PSK: Phase Shift Keying
• QAM: Quadrature Amplitude Modulation
T1
T2
Telephone line
L1
L2
Fig. 2-19 Modem Location
GENERAL DESCRIPTION OF A FACSIMILE
MODEM
NCU
board
Facsimile
1
2
3
4
ITU-
5
6
ït
çi
System
control
setion
2–21
Advertisement
Table of Contents
