526 BELL SYSTEM TECHNICAL JOURNAL 



is then determined in a similar manner. The sum of these two active 

 return losses minus the two-way gain of the repeater in question is 

 approximately the singing margin around that repeater. 



Whatever singing margin is obtained under average conditions, there 

 will be certain factors tending to reduce this margin while the circuit 

 is in normal operation. These factors include net loss variations, 

 transmission-frequency characteristic deviations, removal of one of the 

 normal terminations for short intervals, gain lumping due to pilot 

 wire regulation, and slight troubles which have not yet been corrected. 

 Because of those factors, and because of the disadvantages of near 

 singing, 10 db singing margin under average conditions (8 db for short 

 circuits) is believed desirable in the Bell System. 



Crosstalk 



Net losses may also be limited by the danger of excessive crosstalk. 

 Far-end crosstalk from circuit 1 to circuit 2, each extending from A 

 to B, is crosstalk which manifests itself at the B end of circuit 2 from 

 the speech of the subscriber at A on circuit 1. Near-end crosstalk 

 from the same talker may manifest itself at A on circuit 2. 



From a general standpoint, the crosstalk volume should be so low 

 that no subscriber can understand what any other subscriber says on 

 another circuit. This is desirable from the standpoint of preserving 

 secrecy and also from the standpoint of the annoyance which may be 

 caused by unwanted speech currents. 



The assumed limitation on circuits from a crosstalk volume stand- 

 point is that a subscriber shall have only a very small chance of hearing 

 understandable crosstalk. This chance is determined by the distri- 

 butions of the crosstalk couplings, the room noise and circuit noise, 

 the terminal losses, the talker volumes on other circuits, and the 

 natures of the talkers and listeners. Present data indicate that the 

 chance of a subscriber hearing understandable crosstalk is very small 

 in the case of two-wire cable circuits if the crosstalk conditions are 

 such that there is not more than about one chance in 100 that any 

 one or more of the couplings between the disturbed circuit and the 

 various disturbing circuits shall exceed 1,000 crosstalk units (60 db 

 loss). Further investigations of this matter and other questions in 

 connection with crosstalk are being made. 



Crosstalk in cable circuits may be either within-quad or between- 

 quad crosstalk. Crosstalk within the quad may be phantom-to-side, 

 side-to-phantom or side-to-side and may be divided into office cross- 

 talk and cable crosstalk."^ The office crosstalk is due to capacitance 



'* Specific values of the various sources of crosstalk are given in a paper entitled 

 "Long Distance Telephone Circuits in Cable," by A. B. Clark and H. S. Osborne, 

 B. S. T. /., Vol. XI, Oct., 1932. 



