c. Limitations of composite circuit. The operation of the composite circuit is subject to the same limitations as
the simplex and phantom circuits. The repeating coils and lines must be balanced perfectly to prevent interference among
the various channels. Furthermore, the capacitors and coils comprising the filters must be selected carefully and installed
in order to insure proper separation of low and high frequencies in the telegraph and telephone channels.
76.
Summary.
a. Single-line telephony is a system of telephone communication in which two telephone sets are connected by a
single transmission line.
b. Single-line telephone circuits may be full-metallic, involving a two-wire line, or they may be ground-return,
in which only one wire is used, the circuit being completed through the ground.
c. Full-metallic circuits are less susceptible to inductive interference than ground-return circuits, and therefore
they are preferred for telephone communication.
d. Ground-return circuits operate well for telegraph communication, and for signaling (ringing) circuits in
telephone communication.
e. A simplex circuit is a ground-return circuit superimposed on a full-metallic circuit. It permits the addition of
either one telephone or one telegraph channel on an existing two-wire telephone channel. It is used frequently to provide
an additional telegraph channel.
f. Operation of the simplex circuit involves the use of repeating coils at each end of the metallic line. These
coils are efficient 1-to-1 iron-core transformers, consisting of two equal primary windings in series and two equal
secondary windings in series. The usual resistance of each of the four windings is 21 ohms.
g. Perfect balancing of the repeating coils and the line of the metallic circuit insures efficient operation of a
simplex circuit, without mutual interference between the simplex leg and metallic circuit.
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