CARRIER SYSTEMS ON TELEPHONE LINES 



579 



from each end, and the meter indications are measurements of this 

 current at the output of repeater ampHfiers, and at the receiving 

 terminal amphfiers (see Figure 2). A separate pilot frequency is 

 utilized for each direction of transmission. Because no communication 

 is carried on over this pilot carrier current, the band provided is 

 extremely narrow, and no appreciable portion of the frequency 

 spectrum is sacrificed. 



The frequency selected for the pilot channel must coordinate with 

 the other carrier system frequencies. The two frequency allocations 

 of the type "C" system require different pilot channel frequencies, 

 because their speech channels occupy different frequency bands. The 

 apparatus has, therefore, been made so that the frequency of the pilot 

 current can be adjusted to any value desired in the carrier range. 

 The frequency selected for a given system may be determined by 

 local conditions of crosstalk or interference, although in general the 

 preferable location is between the channel bands as noted in Figure 5. 

 The amount of current which is used is limited by its interfering effect 

 into adjacent channels or into other carrier systems on the same line, 

 and it is ordinarily of a low value, of the order of 2 to 6 milliamperes 

 on the line. 



Figure 8 — Schematic of pilot channel circuits. (The alarm circuit is used with 



terminals only) 



Figure 8 shows schematically the principal features of the terminal 

 pilot-channel circuit as a whole. The oscillator at each transmitting 

 terminal which produces the pilot current is connected to the carrier 

 circuit at the input to the transmitting amplifier, in parallel with the 

 band filters. This current is amplified with the speech currents and 



