filter to the line filter. Similarly, the three receiving circuits are 

 brought together by a common receiving amplifier with which is as- 

 sociated a "band" filter, which in turn is connected to the line filter. 

 The line filter consists of two parts, one of which permits the carrier 

 currents of Channels A, B, and C, to pass but excludes the voice- 

 frequency currents and through this operation of the line filter the 

 carrier currents are transmitted to the line. The voice-frequency 

 circuit V is connected to the line through the other part of the line 

 filter which permits voice-frequency currents to pass but excludes all 

 of the carrier-frequency currents. These currents of dififerent fre- 

 quencies from four channels are then transmitted together over the 

 line and at the receiving end are separated by apparatus similar to 

 that indicated in this sketch. 



RECEIVING- 



COMMON 



TRANSMITTING 



EQUIPMENT 



-TRANSMITTING 



COMMON 

 RECEIVING 

 EQUIPMENT 



LINE 

 FILTER 



LINE 

 FILTER 



Fig. 17 — Schematic arrangement showing the association of a type "C" carrier 

 telephone terminal with the telephone line. 



At intermediate points along the circuit, it is necessary to install 

 amplifiers for the carrier-frequency currents as well as for the voice- 

 frequency circuits. At these points, the carrier-frequency currents are 

 separated as a whole from the voice-frequency circuit, a telephone 

 repeater being used for the voice-frequency circuit and a carrier 

 repeater consisting of two amplifiers with electrical filters to separate 

 the two directions of transmission, being used to amplify the three 

 carrier circuits as a group. After amplification, the carrier and voice- 

 frequency currents are again brought together for transmission over 

 another section of line. 



32 



