of these frequencies lie in the transmitted voice-frequency range, between 200 and 2,700 hertz. When power lines and
telephone circuits exist side by side, voltages at these frequencies may be induced in the telephone lines by exactly the
same process discussed in a above. These voltages cause corresponding currents to flow in the lines, and result in the
production of noise in the receivers. Noise produced in this manner usually takes the form of a hum of varying pitch. It
can be as objectionable and disturbing to the listener as crosstalk.
64.
Reduction of Interference on Transmission Lines.
Various methods have been developed to reduce interference on telephone lines.
a. Maintenance of lines. One obvious way of minimizing interference is by keeping telephone lines in good
repair. This requires periodic inspection of splices and joints, as well as insulators and other equipment. Careful initial
installation of lines also helps to prevent causes of interference.
b. Transposition of wires. Transposition of the wires of a telephone line, as shown in figure 63, is an effective
method of reducing crosstalk or noise produced by inductive coupling between lines. Note that the wires of circuit 3-4
have been transposed, or made to cross over. As explained previously, the closer the wire is to the center of the magnetic
field of the adjacent circuit, the greater is the emf induced in it. By transposition of the wires, however, a greater emf is
induced in parts of wires E3A and E4B. Similarly, a smaller emf is induced in the other parts of wires E3B and E4A.
This makes the resultant induced emf in wires 3 and 4 nearly equal. Since very little unbalanced emf now exists between
the wires, the crosstalk current produced in circuit 3-4 is at a minimum. The same effect can be obtained by transposing
the wires of circuit 1-2.
c. Capacity balance. Cables, since they often contain hundreds of pairs of wires, are particularly
susceptible to crosstalk caused by capacity coupling between adjacent pairs. An important reason for this
is the fact that the various wires exhibit different capacities to ground, making the system unbalanced
to ground.
One method of overcoming this is to equalize the capacities by transposing the various
wires in the cable at points where one length of cable is spliced to an adjacent length of cable.
Another method consists of equalizing the capacities by adding capacity to those pairs of wires that show
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