TRANSMISSION LINES 291 



tance coils and condensers (which normally make up such a filter 

 structure) by quartz crystals to provide the sharp selectivity required 

 on the sides of the band. The other alternative consisted in providing 

 a filter of moderate selectivity so that in the neighborhood of the 

 carrier frequency the unwanted sideband is not completely suppressed, 

 and in arranging that the resultant reproduced music at the receiving 

 terminal is obtained by the proper coordination of the desired and 

 the vestige of the unwanted sideband. The "vestigial" sideband 

 method was decided upon. Although this does not require filters 

 having particularly sharp selectivity on the sides of the band, it does, 

 however, impose more severe requirements upon the control of the 

 phase characteristics of the filters in the neighborhood of the carrier 

 frequency. It makes it necessary also to have the carrier frequencies 

 at the sending and receiving ends not only synchronized, but phase 

 controlled as described later. 



For the modulating elements in the system at both the sending and 

 receiving terminals, copper oxide rectifying disks were chosen. These 

 elements can be made very simple. In stability, with respect to 

 transmission loss and the ability to suppress the unwanted carrier 

 frequency by balanced circuits, this arrangement is superior to the 

 usual vacuum tube circuits. 



In Fig. 4 is shown schematically the arrangements of the carrier 

 circuit at the transmitting and receiving ends. At the transmitting 

 terminal the circuit from the microphones is led first through low- and 

 high-pass filters to limit the bands to the desired width; i.e., 40 c.p.s. 

 to 15,000 c.p.s. The 40-cycle limiting filter was included because 

 tests had demonstrated that lower frequencies are not required for 

 the satisfactory transmission of music of symphony character, and 

 because it was feared that occasional high energy pulses of subaudible 

 frequency might cause overloading. When these 40-cycle filters are 

 omitted, as was done in tests, the carrier channels are capable of trans- 

 mitting frequencies down to and including zero frequency, a charac- 

 teristic which could not possibly be obtained in a single sideband 

 system by other than such a vestigial sideband technique. 



As may be noted further in Fig. 4, carrier current is supplied to the 

 rectifying disks of the modulator along with the incoming music fre- 

 quencies. The balancing connection of the four rectifying disks mak- 

 ing up the modulator is arranged to suppress the carrier frequency, the 

 final degree of suppression being adjusted by means of the variable 

 condenser and resistance shown, which were included to make up for 

 slight dissimilarities in the characteristics of the individual copper 

 disks. A ver\' high degree of carrier suppression can be achieved by 



