Phantom telephone circuits, and combined telegraph and telephone circuits, worked at audio frequencies

The superposing of additional circuits on telegraph and telephone conductors, so as to obtain two or more channels of independent communication from the same physical circuit, now occupies a very important place in the design of such circuits. The loading of telephone circuits, combined with the use of thermionic amplifiers in them, now renders it possible to provide efficient long-distance telephonic communication¿including the provision of phantom circuits¿on small-gauge conductors in underground cables carrying a large number of circuits. The provision of these cables involves the gradual replacement of overhead open telephone circuits in a large measure, and constitutes a revolution in modern circuit provision. The object of this paper is to review the present position of the art as applied to superposed circuits worked at audio frequencies. The theory of simultaneous telegraph and telephone working on the same circuit is first dealt with, and the limitations of this method of working are stated. The comparative efficiency of side and phantom circuits is then dealt with, and the principal problems arising from the loading and relaying by amplifiers of phantom circuits are outlined. The solution of the problems is indicated. Finally an appendix is given, dealing with the derivation of formulae for the calculation of electric wave filters, for application to the circuits dealt with.