It is to be noted that the carrier circuits, like the four-wire cable 

 circuits, use different channels of transmission in the two directions, 

 thus avoiding the difficulties of balance which would otherwise be 

 encountered. In the case of the carrier systems now in general use, 

 however, the two channels are channels of different frequencies oper- 

 ating in opposite directions on the same pair of wires rather than using 

 two separate pairs of wires. As a result the Type C carrier system 

 providing three two-way telephone channels transmits over the line 

 six bands of carrier frequencies, one for each direction for each of the 

 channels. The maximum frequency used by the Type C system is 

 about 30,000 cycles. 



In addition to the Type C system just described, there is used in 

 the Bell System a simpler single-circuit carrier system (Type D) 

 providing one carrier circuit in addition to the voice-frequency tele- 

 phone circuit. 



In the case of carrier systems as in the case of telephone repeaters, 

 their application to the telephone plant involved not only the develop- 

 ment of the system itself but the development and application of new 

 practices to the telephone plant. This came about from the fact that 

 the plant, heretofore designed primarily for the transmission of 

 ordinary voice frequencies, that is, currents up to about 3,000 cycles 

 per second in frequency, was now called upon to transmit currents up 

 to 30,000 cycles successfully and without interference. In order to 

 do this, it was necessary in the open-wire circuits to use non-loaded 

 pairs and where loading was necessary in short sections of incidental 

 cable in such circuits, to design new loading systems with loading coils 

 of small inductance placed at frequent intervals which would transmit 

 these higher frequency currents. A major problem of adapting the 

 plant to the use of these currents arose from the increasing tendency 

 with higher frequencies for currents flowing in one circuit to induce 

 currents into other circuits in the vicinity. The transposition systems 

 used to prevent crosstalk between voice-frequency telephone circuits 

 on the same pole line were wholly inadequate to prevent crosstalk of 

 the carrier currents and without extensive changes such crosstalk 

 would have been far too great to make possible the satisfactory use of 

 carrier systems. New systems of transpositions involving a large 

 increase in the number of transpositions used in a given section of line 

 were designed for this purpose. Also, it was found that for the 

 largest use of carrier systems it would be necessary to give up the use 

 of phantoms on the circuits involved and also to rearrange the con- 

 ductors to provide less space between the two wires of the pair and 

 greater amounts of space between the pairs on the same crossarm. 



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