(1) Hopefully, the circuits connected to the hybrid coil will balance
correctly.
No singing will be noticed and no echo will appear.
Since this is rarely the case in practice, long-distance telephone
networks are often provided with echo suppressors.
(2) Some of the major causes of unbalance of hybrid coils include wrong
values of balancing resistors, temperature change of the wires or
cables, leakage, variation in line matching impedance, and
Secondary causes include changes in traffic routing,
improperly adjusted amplifiers, and changes in types of terminal
equipment.
(3) A long-distance telephone system may exhibit many echoes, one echo
resulting from each impedance mismatch or unbalanced hybrid. Echoes
from the greatest distance are usually more noticeable because of
greatest time delay, but fortunately they are usually weakest since
the line loss is greatest between the echo source to the talker.
b. Echo Suppressor Principles. The simplest form of echo suppressor is
shown in figure 3-3. It is essentially a voice-controlled circuit wherein the
voice passing through the circuit locks up the remaining path.
Figure 3-3.
Echo suppressor circuit, block diagram.
(1) One side of the echo suppressor circuit connects to the hybrid coil
by a four-wire line and the other side terminates in a four-wire
line to the distant station. Each direction of transmission has its
own amplifier to raise the level to the value needed to operate the
control circuit.
Control lines cross, so that the send channel
controls the receive gate and the receive channel controls the send
gate.
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