Table of Contents
Advertisement
G3 FACSIMILE COMMUNICATIONS
In this example, the digital signals change the amplitude of the carrier
wave. For this reason, this modulation method is called "amplitude shift
keying modulation." As can be seen from the figure, the digital informa-
tion (0, 1) is expressed by the size of the amplitude of the analog carrier
wave.
Alternatively, the process of restoring the original digital signal from the
modulated carrier wave is called "demodulation."
Accordingly, the MODEM built into the facsimile unit has two operation
modes. In other words, modulation is performed when data is transmitted,
and demodulation is performed when data is received.
• Signals transmitted by MODEM
On a facsimile, coded image data can be mutually transmitted and received
by the MODEM. However, a MODEM transmits and receivers not only
image data. Control signals such as facsimile control procedure signals are
also transmitted and received by the MODEM.
Bandwidth of Telephone Line
There is also another important point in data transmissions using a public
switched telephone line. That is the problem of the bandwidth of the tele-
phone line. The actual bandwidth of a public switched telephone line is
between 300 Hz and 3400 Hz. MODEMs assure facsimile data transmis-
sion within this bandwidth by using the carrier wave of frequencies of
3400 Hz or less.
5.2
Shift and Modulation Methods
"Shift" refers to changing of the carrier wave into a different form in
response to the logical values of "1" and "0" of a binary digital signal. The
carrier wave can be changed in various ways depending on which modula-
tion method is adopted. The amplitude, frequency or phase of the carrier
wave can be changed independently, or phase and amplitude can also be
changed together.
In the amplitude shift keying modulation example on the previous page,
the amplitude of the carrier wave is changed. For this reason, ASK is also
called Amplitude Modulation (AM). Note, however, that this modulation
method is currently not used on facsimiles.
5–46
Advertisement
Table of Contents
