(2) When the voice of the first talker passes over the send line the
amplified signal enters the send channel of the control circuit to
develop a control voltage for closing of the receive gate. As long
as the talker continues to speak, the receive line is locked up and
no signal is received by him.
When he stops talking, the send
channel opens the receive gate.
If a voice signal enters the
receive line from the distant station, a sample of the signal is
amplified for use by the control circuit.
The receive channel
closes the send gate, thus preventing a signal from passing in the
talk direction.
Only one person at a time can talk on such a
circuit, and the talker cannot hear echoes of his own voice.
(3) Since the activating circuits require a short period of time for
operation, the first impulse of signal will be lost. This can be
detrimental to data signals. For this reason, the usual practice is
to switch the echo suppressor out of the telephone network during
data transmission.
c. Trunk Circuits. One effective method for minimizing the development of
echo without resorting to echo suppressors is to use four-wire trunks. This
change eliminates the hybrid coils at repeaters and, with them, lessens the
need for coil and line balance. Level compensation becomes simpler and circuit
patching is accomplished with less difficulty.
3-4. COMPANDOR
To be useful in telephone communication, a compandor must have a path for
each direction of transmission, as shown in figure 3-4. Four-wire operation is
needed to establish the two independent directions of transmission. A hybrid
coil is therefore necessary to convert the 2-W switchboard loop to 4-W
operation on the line between terminals.
a. Voice Distortion. Distortion of voice is noticeable when the expandor
and compressor units are improperly adjusted, giving rise to incompatible
compression and expansion ratios.
Distortion will also be noticed if attack
and recovery times are unsynchronized.
Quality of voice transmission may
suffer, but still be understandable for all normal voice communication.
Trouble develops, however, when signals other than normal voice are transmitted
over the circuit containing compandors.
b. Restrictions. Compandors are beneficial when used with long-distance
voice or facsimile (analog) communication, because of their ability to improve
the signal-to-noise ratio.
They are usually detrimental, however, when
digitized signals are passed over the circuit. The time delay in attack and
recovery circuits may exceed the tolerance for envelope delay.
When data
signals are transmitted over a voice facility containing compandors, the
compandors should be switched out of the telephone network unless the technical
characteristics indicate freedom from digital distortion.
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