CARRIER-CURRENT COMMUNICATION 639 



the same carrier frequency may be used for transmission in both direc- 

 tions so that there may be as many channels as there are separate 

 carrier frequencies. On the other hand, the wire circuit must be very 

 uniform throughout so that the impedance will be very regular over 

 the entire carrier-frequency range, and may be simulated by an 

 artificial line. The line must also be very stable so that the impedance 

 balance, once having been secured, will not be disturbed. Further- 

 more, as transmission with the same carrier takes place in the two 

 directions, the efifect of the cross-talk between systems of the same 

 type is very severe, so that it is usually impracticable to operate two 

 of these over wires which are in close proximity for any considerable 

 distance. The grouped system has the advantage that a balancing 

 line is not required and hence small circuit irregularities are relatively 

 unimportant. Furthermore, the effect of cross-talk is much less 

 severe, so that a number of systems may often be operated over 

 adjacent circuits. One disadvantage is that two carrier frequencies 

 are required for each channel so that fewer circuits can be secured 

 with one system. 



Carrier systems may also be divided into two classes depending 

 upon the manner in which the carrier current is provided at the 

 receiving end. In the carrier transmission system, the carrier cur- 

 rent is supplied by the oscillator at the sending end and is trans- 

 mitted over the circuit along with one or both of the side bands. In 

 the carrier suppression system, the carrier current itself is not trans- 

 mitted but is introduced into the receiving equipment from a local 

 source. This latter system is proving to be superior for general carrier 

 purposes because of the advantages which accrue from relieving 

 the line and apparatus from the load of the carrier current. 



Turning now to the electrical characteristics of the cables, we 

 find that each one provides a circuit having a transmission equiva- 

 lent which increases throughout the carrier range but is moderate in 

 magnitude. The impedance, as is to be expected with a uniform, non- 

 loaded cable, is very smooth, and since there is no opportunity for 

 any change in the cable constants, the impedance has practically no 

 variation. The transmission equivalent and the impedance of one 

 of the cables are shown in Figs. 2 and 3, respectively. The cross-talk 

 between the cables is small enough to be entirely negligible, regard- 

 less of the type of carrier systems employed. 



In view of all the conditions outlined, a balanced system of the 

 carrier suppression type was decided upon. Such a system provides 

 the maximum number of channels per cable, while the usual ditft- 

 culties of impedance balance and inter-system cross-talk are largely 



