Lesson 1/ Learning Event 2
d. Balance.
Balance of the hybrid is a critical matter.
When
properly balanced, the two directions of transmission on the loop remain
entirely independent of each other, and there is no signal path leading
from the W-E line to the E-W line.
In the even either hybrid becomes
unbalanced, energy may feed through the hybrids from one line to the other
and become enlarged by the amplifiers.
This may cause oscillation
(singing) in the feedback path. Imbalance of a hybrid unit can be caused
either by maladjustment of the balance network or, more often, by the
change in line impedance due to changing weather conditions. A 4W circuit
has less tendency to sing than a 2W circuit because the 4W circuit
contains no hybrids.
A 4W circuit therefore has greater stability of
operating characteristics than a 2W circuit.
e. Equalizer.
Each receive line amplifier has an equalizer.
The
equalizer
contains
circuit
elements
and
resistance) to develop resistance equal and opposite to the line
reactances.
The result is a relatively flat frequency response over the
voice band. In the absence of an equalizers, the amplifier line response
together may greatly distort the response of the voice band passing over
the lien.
The equalizers and amplifier are almost always placed at the
receive end of the line. A send line amplifier may be used if the loss in
the line is too great for the receive line amplifier to overcome, or if
line noise becomes excessive.
f. Echo.
In the case of a very long line, the lines themselves may
become unbalanced.
When that happens, the hybrids are unbalanced by the
amount of the line imbalance. Energy may therefore feed through a hybrid
unit and return to the ear of the talker some time after a short delay.
The result is an echo in the ear of the speaker, reflecting his own voice.
An echo suppressor may therefore become necessary in each telephone loop
to disable a party's receive line circuit while he is talking, and to
disable his send lien circuit when he is listening. It is, in effect, a
voice-controlled line switching and lien terminating unit. Each disabled
line will be terminated in its characteristic impedance during the time of
its disablement.
g. Patching.
Equal-level patching cannot be achieved on a 2W line,
because changes made in the level for one direction of transmission
automatically do the same for the other direction.
Since the level is
different for each direction of transmission, equal-level patching is
impractical.
However, 4W circuits are easily patched at equal level,
since the level in either line can be raised or lowered at will without
affecting the other line.
Pads and attenuators lower the signal level,
and amplifiers raise the signal level when necessary.
7
Previous Page
