524 BELL SYSTEM TECHNICAL JOURNAL 



for two-wire circuits without echo suppressors, except that the delay 

 weighting terms for all echo paths which are acted upon by the echo 

 suppressor are determined from the curve for the proper zero level 

 sensitivity. The paths which are not affected by the echo suppressor 

 are all paths which do not pass through the suppressor and any paths 

 .with enough delay beyond the suppressor so that the hangover ^^ is 

 insufficient to suppress the echo. (Echoes in this latter class are nor- 

 mally not obtained, since the hangover is made large enough to sup- 

 press all echoes beyond the suppressor.) 



Singing and Circulating Currents 



Another effect which is important principally on two-wire circuits 

 is that of singing and circulating currents. In a two-wire circuit, if 

 the total gain around a repeater is increased sufficiently, it will become 

 greater than the losses across the hybrid coils and singing will occur 

 if the phase relations are right. When this occurs, the subscriber may 

 hear the singing tone, repeaters may be overloaded, voice-operated 

 devices on connecting circuits may be falsely operated and other cir- 

 cuits in the same cable may be made noisy by cross-induction. 



Even when actual singing does not occur, if the loss minus the gain 

 around a circulating path is small, the voice currents may be con- 

 siderably distorted due to the feedback currents around the repeater. 

 If the singing margin ^^ becomes small, the circulating current or "near- 

 singing" effect is quite objectionable. 



In order to provide against this possibility, it has seemed desirable 

 in the Bell System to require a 10 db singing margin " around the 

 most critical repeater in any long circuit, under average conditions of 

 temperature, regulation, net loss, etc., and with 5 db terminal return 

 losses. (For circuits equipped with only one or two repeaters, 8 db 

 margin is considered sufficient.) In a similar manner to that outlined 

 above for echoes on two-wire circuits, the quantity S-L, the taper, 

 and the terminal return loss are the important things in determining 

 this singing margin. In this case, of course, the delay does not have 

 any large eff^ect. The value of S-L which is usually of the most im- 

 portance is the one at about the highest frequency efficiently trans- 

 mitted, since this usually tends to be the lowest value of S-L within 

 that range. 



The process of computation of the singing margin around a given 



'^ This is the same as the "releasing time" discussed in the paper entitled "Echo 

 Suppressors for Long Telephone Circuits" mentioned above. 



'^ The singing margin is the sum of the additional gains in the two directions 

 which may be inserted at the most critical repeater in the circuit before singing starts, 

 under specified conditions as to the terminations, etc. 



