The attenuation of the telegraph impulses is lower than in the case of the simplex circuit, since the wires of the two side
circuits are effectively in parallel and thus offer less resistance to current flow. (An interesting feature of the operation of
a simplexed-phantom circuit is the fact that the telegraph channel operates even if three of the four lines are broken, since
only one wire is needed to provide a complete path in a ground-return circuit.)
75.
Composite Circuit.
a. Description. Another type of circuit that provides simultaneous telephone and telegraph service is the
composite circuit shown in figure 72. This circuit permits the simultaneous operation of three telephone channels over a
phantom group, and four additional telegraph channels, two in each side circuit. The telegraph channels operate as
ground-return circuits. They are indicated on the left side of the circuit by the designations I, II, III, and IV.
b. Operation. In addition to the two metallic lines and the six repeating coils required for phantom-group
operation, the composite circuit requires eight capacitors and eight coils, indicated by C and L. These elements act as
high-pass and low-pass filters, respectively. The turns ratio of the six repeating coils is not always 1-to-1.
(1) In order to comprehend the need for these capacitors and coils, it must be understood that a telegraph channel
can operate over a path that passes frequencies up to 100 hertz, whereas a telephone channel can operate over a band of
from 200 to 3,000 hertz, approximately. It must be remembered, also, that a capacitor offers low impedance to high
frequencies, but relatively high impedance to low frequencies, and that a coil, inversely, offers low impedance to low
frequencies, but relatively high impedance to high frequencies. Because of this, telephone impulses originating in the
telegraph set on the left side of telegraph channel I pass through coil L, through the central portion of the upper wire of
side circuit No. 1, through the second coil and the set on the right-hand side of channel I, and back to its origin through the
ground-return path. This telegraph current is blocked by the four capacitors in the line of side circuit No. 1, and thus is
prevented from reaching any of the six telephone sets. Operation is similar to each of the other three telegraph channels.
(2) Similarly, a telephone conversation originating in telephone set T1 is passed by the capacitors in the line
of side circuit No. 1, and, by the transformer action of the repeating coils, is heard in set T2 (fig. 72). Since
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