CABLE CARRIER TELEPHONE TERMINALS 107 



line. Copper oxide bridge circuits again are used for this group modu- 

 lation stage. In all cases they are connected to suppress the carrier. 

 To provide the various carriers required for modulation and demodula- 

 tion, a carrier supply system has been designed somewhat along the 

 lines of an office power distribution system using bus bars and pro- 

 tective arrangements for the various carriers. Each carrier supply 

 system is capable of supplying as many as ten carrier terminals, or a 

 total of one hundred and twenty two-way channels. 



Because of the large number of circuits involved, every effort has 

 been made to provide reliable operation of the carrier supply and 

 common terminal equipment. The terminal and carrier supply equip- 

 ment is designed to permit maintenance tests for checking the per- 

 formance of amplifier tubes and to permit switching between regular 

 and spare equipment without interruption of the large number of 

 circuits involved. 



The emphasis placed upon ease of maintenance and the necessity for 

 more careful handling of higher-frequency circuits have resulted in 

 new equipment design features. These include new cable terminals, 

 new shielded office cabling, and panels arranged for front wiring and 

 maintenance which are mounted on racks having wiring ducts at both 

 edges of the bays. In the following sections a more detailed descrip- 

 tion is given of the circuits, their performance, equipment and main- 

 tenance features. 



Circuits 



The frequency allocation for one direction of transmission and a 

 block schematic of one terminal are shown in Figs. 1 and 2, which sup- 

 plement each other and need little explanation. The twelve voice 

 bands shown at the left in Fig. 1 are modulated individually in the 

 channel modems.* This forms a 12-channel block lying between 60 

 and 108 kc. which is then modulated in the group modulator by a 120 

 kc. carrier to move the block down in the range from 12 to 60 kc. for 

 transmission to the distant terminal. On the receiving side the 

 processes are reversed. One of the channels, as well as the group 

 modem of Fig. 2, is presented in more circuit detail in Fig. 3. This 

 shows the circuit from the point where the voice comes into the carrier 

 system to the point where the twelve carrier sidebands go out onto the 

 cable and vice versa. 



At the left the four-wire terminating circuit serves, not only as a 

 device to transform from a two-wire to a four-wire circuit, but also as a 



* The term "modem" has been coined to mean a panel or equipment unit in which 

 there is both a modulator and a demodulator to take care of both the outgoing and 

 the incoming signal. 



