Neutralization of Telegraph Crossfire 



By R. B. SHANCK 



Synopsis: With the simple means here described for neutralizing 

 mutual interference between parallel telegraph circuits, it has been found 

 practicable to effect a reduction to 10 or 20 per cent of the original values. 

 This has improved considerably the operation of some circuits and made 

 available others which were previously unsuitable. The resulting improve- 

 ment in transmission has made possible the elimination of certain inter- 

 mediate telegraph repeaters with material savings. The neutralizing 

 apparatus has no material effect when crossfire is not present, that is, 

 when the paralleling wires are idle. It has been found that the use of 

 arrangements here described on certain long open wire circuits makes pos- 

 sible fast manual full-duplex operation where only medium-speed half-duplex 

 operation was possible before. Furthermore, in the case of some cable 

 circuits where it was impossible to operate more than two telegraph circuits 

 per quad, it is now practicable to obtain four telegraph circuits. 



Introductory 



MANY ground-return telegraph circuits are subject to serious 

 mutual interference due to their proximity to one another on 

 pole lines or in cable and in certain cases due to interconnection in 

 office apparatus. The interfering currents, commonly referred to 

 as "crossfire", in one telegraph circuit, caused by the transmission 

 of signals on paralleling telegraph circuits, have caused consider- 

 able difficulty in the operation of such circuits. Crossfire has either 

 limited the speed of operation or seriously impaired the quality of 

 transmission in many cases. 



In the following there are described methods which have been 

 successfully applied to a number of ground-return polar-duplex 

 telegraph circuits in the Bell System for the purpose of neutralizing 

 crossfire. These arrangements are comparatively inexpensive and 

 afford a marked improvement in transmission. This paper deals 

 specifically with methods for use on wires which are either used 

 simultaneously for telephone purposes or at least are grouped and 

 transposed so as to be suitable for telephone operation; there is, 

 however, no reason why the principles may not be profitably applied 

 in many cases where wires are intended exclusively for telegraph use. 



Nature of Crossfire 



When mutual admittance, or coupling, exists between two tele- 

 graph circuits, operation of one, of course, occasions extraneous 

 current impulses in the other circuit. The presence of such impulses 

 in the receiving apparatus at the terminals of the disturbed circuit 

 results in ad\erse effects on the telegraph signals. In the case of 



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