Telephone transmission over long cable circuits

The application of telephone repeaters has made it possible to use small gage cable circuits to handle long distance telephone service over distances up to and exceeding 1000 miles. A general picture of the long toll cable system which is being projected for use in the northeastern section of the United States was presented recently by Mr. Pilliod before this Institute.^∗ Many of the circuits in these toll cables are so long electrically that a number of effects, which are comparatively unimportant in ordinary telephone circuits, become of large and sometimes controlling importance. For example, the time required for voice energy to traverse the circuits becomes very appreciable so that reflections of the energy may produce “echo” effects very similar to echoes of sound. The behavior of the circuits under transient impulses, even when two-way operation is not involved so that “echoes” are not experienced, is very important. In order to keep within proper limits of variation of efficiency with frequency over the telephone range special corrective measures are necessary. Owing to the small sizes of the conductors, the attenuations in the longer circuits are very large. Special methods are, therefore, required to maintain the necessary stability of the transmission, including automatic means for adjustment of the repeater gains to compensate for changes in the resistance of the conductors caused by temperature changes.