CARRIER SYSTEMS ON TELEPHONE LINES 



573 



overall transmission afforded by a carrier system may be noted by 

 the curve on Figure 4, wfiich shows the relative speech frequency 

 transmission characteristics of a typical channel. Where the carrier 



FREQUENCY CYCLES 



Figure 4 — Representative overall transmission-frequency characteristic — -type "C" 



carrier telephone sy^stem 



channel is employed for terminal to terminal business the overall 

 equivalent at 1,000 cycles is ordinarily adjusted to about 10 TU. The 

 channels not infrequently form sections of much longer overall circuits, 

 being connected to cable or perhaps open-wire circuits, in which case 

 it is rather common to adjust the carrier section to a zero equivalent 

 or even a gain of several TU. 



Line Considerations. The passage of the carrier currents from the 

 terminal apparatus over the line circuit which serves to connect 

 the two terminals, or a terminal and repeater station, gives rise to 

 several problems: the line loss or attenuation, the stability of trans- 

 mission, the possibilities of crosstalk from other carrier systems on 

 the same pole line and interference from currents from external 

 sources. These factors must be considered not only in connection 

 with the arrangement of the wires themselves but also in conjunction 

 with the design of the terminal apparatus, repeaters, etc., so that 

 satisfactory overall speech transmission may result. 



As was brought out in the Colpitts-Blackwell paper, the line 

 attenuation at the high frequencies is in accord with the recognized 

 transmission theory. Because of skin effect in the wires and rising 



