OPEN-WIRE CROSSTALK 223 



would be prohibitively great. Consequently, carrier circuits are now 

 designed to operate on a two-path basis. Two separate bands of 

 frequencies are set aside, each being restricted, by means of one-way 

 ampUfiers and electrical filters, to transmission in one direction only. 

 Each telephone circuit is then made up of two oppositely directed 

 channels, one in each frequency band. Thus, direct "near-end" 

 crosstalk is kept from passing to the telephone subscribers. Conse- 

 quently, the "near-end" type of crosstalk needs to be considered 

 only with respect to that portion which arises from electrical waves 

 reflected at discontinuities in the circuits, which effects have already 

 been mentioned. 



In practice a pole line may have some of the pairs very frequently 

 transposed to make them suitable for carrier frequency operation and 

 other pairs less frequently transposed and suitable only for voice- 

 frequency operation. A system of transpositions must permit any 

 arrangement of the two types of pairs which may be found economical 

 for a given line and layout of circuits. Each pair must meet limits 

 for near-end and far-end crosstalk to any other pair which may 

 crosstalk into it in its frequency range. Pairs used only for voice 

 frequencies are usually phantomed and transpositions must, of course, 

 be designed for the phantom circuits as well as the side circuits. The 

 design of a transposition system is, therefore, extraordinarily compli- 

 cated and tedious and, to paraphrase the Gilbert and Sullivan police- 

 man, "A transposer's lot is not a happy one." 



Bibliography 



The published material on the matter of open-wire crosstalk and 

 transposition design appears to be very limited. The following papers 

 are of interest: 



The Design of Transpositions for Parallel Power and Telephone Circuits, H. S. 



Osborne. Trans, of A. I. E. E., Vol. XXXVII, Part II, 1918. 

 Telephone Circuits with Zero Mutual Induction, Wm. W. Crawford, Trans, of A. I. 



E. E., Vol. XXXVIII, Part I, 1919. 

 Measurement of Direct Capacities, G. A. Campbell. Bell System Technical Journal, 



July, 1922. 

 Propagation of Periodic Currents Over a System of Parallel Wires, John R. Carson 



and Ray S. Hoyt. Bell System Technical Journal, July, 1927. 

 On Crosstalk Between Telephone Lines, M. Vos. L. M. Ericsson Review, English 



Edition, 1930, Vol. 7. 

 Application of High F"requencies to Telephone Lines, M. K. KupfmuUer. Presented 



Before International Electrical Congress, July, 1932. 

 Probability Theory and Telephone Transmission Engineering, Ray S. Hoyt. Bell 



System Technical Journal, Jan., 1933. 



