Line Filter for Program System * 



By A. W. CLEMENT 



Open wire circuits recently have been developed for transmitting radio 

 broadcast programs with greater naturalness and over greater distances 

 than heretofore.^ The simultaneous utilization of these circuits for the 

 transmission of broadcast programs and carrier telephone messages requires 

 the use of line filters to restrict the program and carrier currents to the 

 proper circuits. The low pass line filter developed for the program circuits 

 and its contribution to the maintenance of good quality in the programs 

 transmitted are described in this paper. 



PROGRAM transmission systems operated on open wire telephone 

 lines ordinarily are not assigned the exclusive use of the lines, 

 but usually share them with other communication facilities. The 

 wide-band system described in an accompanying paper ^ transmits 

 currents in the frequency band extending from 35 to 8,000 cycles per 

 second, while the lines over which it is routed possess useful transmis- 

 sion ranges extending from 35 to considerably above 30,000 cycles. 

 In order that the range above 8,000 cycles shall not be wasted, carrier 

 telephone systems utilizing these frequencies usually are operated on 

 the same wires with the program systems. 



Line filters are used at each terminal and repeater point in the pro- 

 gram system to separate the program currents from the carrier currents 

 and to guide each to the proper channel. They are operated in sets 

 consisting of a low-pass filter and a high-pass filter connected in parallel 

 at one end, the end that faces the line. The low-pass filter transmits 

 the program currents freely while effectively excluding the carrier 

 currents, and the high-pass filter transmits the carrier currents while 

 excluding the program currents.^ 



The line filters are located in the open-wire program systems as 

 shown in Figs. 1, 15, 16, 17, 18, and 20 of the accompanying paper by 

 H. S. Hamilton.^ The low-pass filter is in the direct path of the 

 program currents and therefore has a number of features of special 

 interest. It is the object of this paper to describe this filter and its 

 contribution to the maintenance of good quality in the programs trans- 

 mitted over the system. 



* Published in April, 1934 issue of Electrical Engineering, Scheduled for presen- 

 tation at Pacific Coast Convention of A. I. E. E., Salt Lake City, Utah, September, 

 1934. 



^ "Wide-Band Open- Wire Program System" by H. S. Hamilton, published in this 

 issue of the Bell. Sys. Tech. Jour. 



^ "Telephone Transmission Networks" by T. E. Shea and C. E. Lane, published 

 in ^. /. E. E. Transactions, Vol. 48, 1929, pages 1031-1034. 



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