Voice-frequency signalling and dialling in long-distance telephony

During the 21 years between the two great European wars telephone systems of the majority of the cities and large towns of the world have been converted from manual to automatic operation. During the same period notable improvements have also been made to the long-distance telephone service. The introduction of the use of voice-frequency currents for signalling and dialling represents a stage in a long process of development, and the main portion of this paper describes the problems encountered in the design of a system and the various methods which may be employed for avoiding interference between signals and other currents in the voice-frequency range. Reference is also made to the recommendations of the C.C.I.F. regarding the means of avoiding interference between different signalling systems on international connections. The interruption in development caused by the present war provides an opportunity for a review of the progress made and for a consideration of future lines of development necessary to keep pace with changes in automatic switching and speech transmission. Therefore, while the paper starts by setting forth the reasons for modern methods of signalling and dialling over long-distance circuits and for preferring the use of voice-frequency currents for these purposes, it concludes by considering the possible trend of future development.