Telegraph Distortion

In most manual or Morse telegraph systems the incoming signals operate a relay which in turn closes a local circuit (Fig. 5) actuating the sounder. In teletypewriter systems a local circuit energizes the receiving magnet. The volume of the received telegraph signal is relatively unimportant; the sound produced or the letter printed will be just as satisfactory (within limits of good operation) with weak as with strong received signals, provided that the lengths of the signals are not distorted. The amount of distortion can be determined by measuring the lengths of the impulses.^18,19,20

Telegraph distortion is of two types,²¹ systematic and fortuitous. Systematic distortion may be divided into bias and characteristic distortion.

Bias. This consists of a lengthening of the marking impulses and a corresponding shortening of the spaces, or vice versa. If the signal impulses are lengthened, it is called positive bias; if the spaces are lengthened and the signals shortened, it is negative bias. This form of distortion is usually due to differences in battery potentials, improper relay adjustment, steady duplex unbalance currents, earth potential differences, and leakage currents.

Characteristic Distortion. This is distortion peculiar to a given signal combination resulting from the electrical characteristics of the particular line and relay being used. It is called characteristic distortion if it changes in neither sign nor magnitude when the functions of the spacing and marking signals are interchanged.

Fortuitous Distortion. This is an erratic (as contrasted with a systematic) lengthening and shortening of the impulses or spaces due either to chattering or sparking of relays or to interfering currents from external sources.

There are three general sources of external electrical interference. They are from paralleling telegraph circuits, called crossfire;² from paralleling power circuits; and from natural sources. Crossfire transfers telegraph signals from one circuit to another through the electric and magnetic fields. It can be controlled by transpositions in metallic circuits but is particularly bothersome in grounded circuits.²²

The aurora borealis often is accompanied by one of the worst sources of natural disturbances.²³ During such displays (as well as at other times²⁴) very large earth currents flow, and these produce high differences of potential between the ends of grounded telegraph circuits. These earth currents and resulting potentials are sometimes so severe as to prevent service, operate protective devices, and damage cables and equipment. Such disturbances often affect large sections of the country simultaneously.

Lightning also is a cause of interference with telegraph operation and by either direct strokes or induced potentials may affect transmission or damage equipment. Trouble is also caused by storms driving charged particles of snow against the line, thus causing the line to assume a potential and currents to flow. Similar interference is caused by high winds driving charged sand and dust particles against the line wires.