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BELL SYSTEM TECHNICAL JOURNAL 



attenuate those above this point, or pass all frequencies above the cut- 

 off point and attenuate all those below. The cut-off points of this 

 pair of filters are so arranged that the passing region of the low-pass 

 filter includes the three lower frequency channels and the passing range 

 of the high-pass filter includes the three upper frequency channels. 

 The location of the various filters in the frequency spectrum is shown 

 in Fig. 2. 



10 12 14 16 18 20 22 



FREQUENCY IN KILOCYCLES 



Fig. 2 — Frequency spectrum of type "C" normal system. 



If the carrier system is long enough to require repeaters, additional 

 directional filters must be employed. These filters are required 

 because repeaters must amplify frequencies going in both directions. 

 The arrangement of the repeater is shown in Fig. 3. 



LOW PASS 



DIRECTIONAL 



FILTER 



EQUALIZER 



AMPLIFIER 



HIGH PASS 

 DIRECTIONAL 

 H FILTER 



AMPLIFIER 



EQUALIZER 



Fig. 3 — Schematic of type "C" repeater. 



This paper will be concerned with the filters for the Western Electric 



Type "C" Carrier Telephone System,' since the requirements placed 



on the apparatus for this system raised certain problems in filter 



1 H. A. AfTel, C. S. Demarest and C. W. Green, "Carrier Systems on Long Distance 

 Telephone Lines," Bell Sys. Tech. Jour., July, 1928. 



