Transmitted Frequency Range for Circuits in Broad-Band Systems

N utilizing the broad frequency ranges which the newer carrier systems can transmit the telephone engineer has a problem of choice in band width per channel to be allotted to speech currents. A sufficient width is vital to faithful speech reproduction; and desire for better telephone service always recommends an increase in band width over past practice. A reasonable balance, however, must obtain between various economic factors; and there must always be considered the relation between a proposed system and the other parts of the telephone plant, and also the trend of the art. The message band widths and the channel spacing which have been chosen by the Bell System for various new systems are summarized and discussed in this paper. These systems are expected to play a large part in the future growth of its long distance plant; and the reasons underlying this choice may therefore be of general interest. Different broad band systems are under development: A 12-channel system for use on open-wire lines employing frequencies up to 140,000 cycles, a 12-channel system for use on 19-gauge pairs in existing toll cables using frequencies up to 60,000 cycles, and a coaxial system capable of transmitting frequencies up to a million cycles or more, from which it is proposed to obtain 240 or more channels. In the different systems noted above, terminal apparatus is employed which has many common features: The different channels are uniformly spaced at 4000-cycle intervals; the same band filters are used in the ultimate channel selecting circuits; and the derived voice circuit band widths are substantially identical for all channels of all systems.