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BELL SYSTEM TECHNICAL JOURNAL 



apparatus and the high-pass filter which has a low end cutofif around 

 9,000 cycles directs the carrier transmission to its associated carrier 

 equipment. Attenuation frequency characteristics of these filters are 

 shown on Fig. 14. The low pass filter is of unusual design and is 

 described at some length in a companion paper.* 



70 



O 50 



8 10 IE 14 16 18 20 22 



FREQUENCY IN KILOCYCLES PER SECOND 



24 26 



Fig. 14 — Attenuation characteristics of line filter set. 



Monitoring Features 



A very important factor in the satisfactory operation of a program 

 system is the provision of monitoring arrangements by which the 

 operating forces are enabled to observe the quality of transmission, 

 listen for extraneous interferences and observe indicating devices in 

 order to make certain that the program is maintained at its proper 

 volume. 



Three types of aural monitoring facilities were provided on a trial 

 basis for the new program system. The first type consists of a single 

 unit loudspeaker operated by a suitable amplifier. With this loud- 

 speaker system a good response characteristic from approximately 100 

 to 5,000 cycles is obtained, the low-frequency response depending, of 

 course, on the size of the baffle used with the loudspeaker. 



The second type of monitoring consists of two headset receivers 



arranged with a proper equalizing network circuit. This type of 



monitoring provides good response characteristics from approxi- 



3 A. W. Clement, "Line Filter for Wide-Band Open-Wire Program System," 

 published in this issue of the Bell Sys. Tech. Jour. 



