CA RRIER SYSTE.\f FOR PROGRA M TRA .\S\f/SSIOX 167 



a single pair of wires and one set of amplifiers for transmission in either 

 direction as required. The carrier system over which the carrier program 

 channel is transmitted is constantly in operation in both directions simul- 

 taneously, and therefore requires no reversal. The program terminal 

 itself, however, must be switched between transmitting and receiving lines 

 if equipment is not to be needlessly duplicated for transmitting and re- 

 ceiving. In any case, a control signal must be carried through the carrier 

 circuit and delivered to connecting audio-frequency circuits at the receiving 

 end as a d-c. signal. This is accomplished by means of a 78-kilocycle con- 

 trol signal (42 kilocycles at K line frequencies) which is transmitted along 



PROGRAM CHANNEL 

 BAND FILTER 

 79.6KC 88KC 



CHANNEL FREQUENCIES 



INPUT TO 



GROUP MODULATOR t^\^c°'" 



I lONc 



PILOTS 64KC 76KC 92 KC 



CHANNELS 12 3 4 5 PROGRAMI 9 10 11 12 ' """^^ FREQUENCIES 



I I r '^°" CABLE CARRIER OUTPUT 



TONE 11'-"^ GROUP MODULATOR 



PILOTS 12 KC 28 KC 42 KC 56KC60KC| 



BLOCKING FILTER AT 

 BRANCHING POINTS 



|32KC 



10 20 30 40 50 60 70 80 90 100 HO 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 1 — Frequency allocation for one program channel and nine message channels in 

 cable carrier systems. 



with the program channel outside of its frequency band. This signal is 

 generated in the transmitting program terminal whenever the d-c. signal 

 is impressed from the transmitting audio-frequency circuit. At the re- 

 ceiving program terminal, the tone is converted into a d-c. signal which is 

 impressed on the receiving voice-frequency facility. When there is no 

 transmitted d-c. signal, there is no high-frequency signal and no received 

 d-c. signal. Each program terminal, then, is ready either to receive d-c. 

 from the voice circuit and send out 78 kilocycles to the carrier circuit or to 

 receive 78 kilocycles from the carrier circuit and send out d-c. to the voice 

 circuit. The program transmission path is maintained in the last estab- 

 lished direction, regardless of the presence or absence of control, until a 

 reversing signal is received. 



The arrangement of the circuit elements in a carrier program terminal is 

 shown in the block schematic of Fig. 2. The transmission circuit wiring is 



