294 



BELL SYSTEM TECHNICAL JOURNAL 



97 



FREQUENCY IN PER CENT 

 99 100 101 



25 



20 



102 



103 



dl5 



o 



38.8 39.2 39.6 40.0 40.4 40.8 41.2 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 5 — ^Transmission characteristic of carrier supply crystal filter. 



Filters 



The transmission characteristics of the carrier channels are deter- 

 mined largely by the filters and associated equalizers. The filters 

 principally affecting transmission are the band filters. Identical units 

 are employed at the sending and receiving ends. The transmission 

 and phase shift characteristics of one of these units are shown in Fig. 6. 

 These band filters are equalized to produce the desired squared band 

 characteristic. 



The characteristics in the frequency region near the carrier (i.e., at 

 40,000 cycles) are shown on a large scale. This region is of particular 

 interest because it is here that the degree of success in the application 

 of the vestigial sideband method, for the purpose of insuring the satis- 

 factory transmission of the low music frequencies, is determined. If 

 for a given frequency interval above the carrier the phase change is 

 arranged to be equal and opposite to that of the same frequency 

 interval below the carrier, then in the action of demodulation the 

 demodulated current produced by the action of one sideband adds 



