Canon fax basic 2000 Manuale Di Servizio
A–29
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time, such phenomenon occur. That is, one’s own voice is reflected at the
other party, and is heard after 0.6 sec and 0.2 sec, respectively.
The time for the echo to return may reach 1.2 sec in the case of satellite
communications.
Fig. A-28 Transmission Lines for International Communications
7.3.2 Echo control devices on telephone line networks
(1) Echo suppressor (Standardized in
G.164)
Since echo is annoying to the person making the telephone call, a device
called an “echo suppressor” is attached on international telephone lines
and domestic long-distance transmission lines where delay exceeds 45
msec. Echo suppressors are equipped with special switches for eliminating
voice signals that are reflected on the other party while the speaker is still
talking. Each suppressors are equipped on each channel. The Fig. A-29
shows the operation of switches when the voice of the speaker on the right
reaches the other party on the left.
Fig. A-29 Echo Suppressor
Communication
satellite
International
exchange
Submarine
coaxial cable
BNW
SW
BNW
SW
Path of echo
4-wire
Long distance 4-wire
transmission line
transmission line
4-wire
2-wire
2-wire
/4-wire
conversion
circuit
Echo
suppressor
suppressor
Echo
suppressor
suppressor
2-wire
4-wire
4-wire
Voice signal
BNW: Balancing network
2-wire
/4-wire
conversion
circuit