Transmitted Frequency Range for Circuits 

 In Broad-Band Systems 



By H. A. AFFEL 



IN 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 ob- 

 tain 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 em- 

 ployed 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. The transmission frequency characteristic of a single link 

 of such systems, in accordance with present designs, is shown on Fig. 1. 

 A curve for five similar links connected in tandem is also indicated. 

 Based on a 10 db cutoff as compared with 1000-cycle transmission, 

 a single-link band extends from approximately 150 to 3600 cycles, and 

 a five-link band extends from about 200 to 3300 cycles. 



There is, of course, no fixed relationship between the channel spacing 

 and the frequency range of the derived voice-frequency circuit. This 

 is largely a matter of economics in the design of a particular system. 



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