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branch which includes a modulator, a receiving branch which includes 

 a demodulator, and a hybrid coil to combine the two branches into a 

 two-wire voice-frequency circuit. The carrier is generated by a 

 vacuum tube oscillator which supplies both the modulator and demodu- 

 lator. The output of the modulator, which is of the copper oxide 

 type, consists principally of the two sidebands. The desired sideband 

 is selected by the modulator output filter and an amplifier raises the 

 level to that desired for transmission over the line. The demodulator 

 is also of the copper oxide type; its output consists principally of the 

 two sidebands, one of which is a reproduction of the original voice- 

 frequency input. This is selected by means of a low-pass filter and 

 applied to a voice-frequency amplifier which provides the necessary 

 receiving gain. Adjustable pads serve as a means for adjusting the 

 transmitting and receiving gains. The characteristics and functions 

 of the various filters are described later. 



8 10 . 12 14 16 18 20 22 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 3 — Frequency allocation. 



For signaling over the carrier circuit, a 1000-cycle signaling system 

 is employed. The method is similar to that used on other types of 

 circuits, but includes a number of simplifications. The number of 

 tubes has been reduced, and the vibrating relays used in the older type 

 circuits have been replaced by copper oxide rectifiers and simple d-c. 

 relays requiring little maintenance. When 20-cycle ringing current is 

 received from the switchboard, the frequency of the carrier supply 

 oscillator is shifted by 1000 cycles and its output is interrupted at a 

 20-cycle rate, and applied to the input of the transmitting amplifier. 

 At the receiving end this appears at the input of the signal receiving 

 circuit as a 1000-cycle current interrupted at a 20-cycle rate. It is 

 then demodulated in a copper oxide rectifier and the resulting 20-cycle 

 current is amplified and applied to a second rectifier the output of 

 which is connected to a d-c. relay. Operation of this relay causes 

 20-cycle current from a local source to be sent toward the switchboard. 



