414 BELL SYSTEM TECHNICAL JOURNAL 



Such filters have received a wide variety of uses. Very narrow band 

 filters have been used in carriei systems as pilot channel filters for separating 

 out the pilot or control frequency from the other frequencies present; in 

 radio systems for separating the carrier frequency from the sideband fre- 

 quencies; and in heterodyne sound analyzing devices for analyzing the fre- 

 quencies present in industrial noises, speech, and music. Wider band 

 filters employing coils as well as crystals have provided very selective de- 

 vices which are able to separate one band of speech frequencies from another 

 band different by only a small percentage frequency range from the desired 

 band. This property makes it possible to space channels close together 

 with only a small frequency separation up to a high frequency, and such 

 filters have had a wide use in the high frequency carrier systems and in the 

 coaxial system which transmits up to 480 conversations over one pair 

 of conductors. In radio systems such filters have been used extensively 

 in separating one sideband from the other in single sideband systems. 



