A–31
1
2
3
4
5
6
A
çi
(3) Echo Canceller (Standardized in
ITU-T
G.165)
If the signal propagation delay time were not too long, if the echo return
loss was not too low, and if the speakers would not frequently interrupt
each other, then an echo suppressor would be sufficient. However, the echo
return loss varies considerably from connection to connection. The echo
canceller has been made using advances in digital circuit technology to
provide satisfactory service under such adverse conditions.
As shown in the Fig. A-31, an echo canceller contains complex signal pro-
cessing circuits. These circuits compare the signals in both directions of
the transmission line and synthesize a replica of the echo, and subtract that
from the actual echo. Equipped on every 120 telephone lines, echo cancel-
lers completely eliminate echo while allowing telephone conversations to
pass through unimpaired, thus allowing full duplex transmission.
Though the performance of echo cancellers is superior to that of echo sup-
pressors, echo cancellers are more complex and expensive. The percentage
of equipment on which echo cancellers are equipped is increasing. Cur-
rently, echo cancellers are equipped on 100% of equipment in the USA and
Japan.
Fig. A-31 Echo Canceller
Return Loss
“Return loss” is a ratio expressed in decibels (dB) of how much reflected
power that occurs due to impedance mismatching attenuates with respect
to the incidence power. In the following formula, the amount of attenuation
caused by mismatching of two impedances Z1 and Z2 is calculated. Bal-
ancing on a hybrid circuit (2-wire/4-wire conversion circuit) is also
expressed as return loss.
Fig. A-32 Return Loss
BNW
BNW
BNW: Balancing network
4-wire
4-wire
4-wire
4-wire
Long distance 4-wire
transmission line
2-wire
2-wire
2-wire/4-wire
conversion
circuit
Echo
canceller
Echo
canceller
2-wire/4-wire
conversion
circuit
L
R
(Return Loss)=10 log
10
[dB]=20 log
10
[dB]
Z
1
+Z
2
Z
1
-Z
2
[Reflection power]
[Incidence power]
