606 



BELL SYSTEM TECHNICAL JOURNAL 



if their powers added directly; e.g., Su X 521 = Szi means that 



\{)-Snnfi _j_ \Q-S21no — \{)-Sz\ 



/lO 



The circulating current margin {Mc) around this repeater at a given 

 frequency will therefore be ^si + ^32 + Sp, as derived in Appendix II. 

 Written in the more general form, the circulating current margin is 



M, = IS, - F{N„ T) - Gc] X IE, + 7] 



p 



+ [5i - F{N,, T) - gc] X [£2 + 7] + Sp. (7) 



p 



Singing Margins 



The objectionable effects of too low circulating current margin are 

 to cause poor quality in transmission over the circuit and to cause the 

 circuit to " ring " or sound " hollow " to the talker or listener. When 

 a circuit oscillates or " sings," conversations over it become difficult 

 or impossible, voice-operated devices on connecting circuits are locked 

 up, parts common with other circuits such as common " C " batteries 

 may be adversely affected, and other circuits are made noisy through 

 crosstalk in the cable or in the repeater station. It is therefore im- 

 portant that the percentage of cases in which singing occurs shall be 

 very small. 



In general, the tendency will be for most of the loaded cable circuits 

 to sing within a fairly limited frequency range which is usually near 

 the upper frequency point at which the overall circuit loss begins to 

 be appreciably greater than the loss at 1000 cycles. Figure 4 shows a 



1200 1600 2000 2400 2800 



FREQUENCY IN CYCLES PER SECOND 



3600 4000 



Fig. 4 — Singing frequency in 22 tests on 46 19-gauge B & H-88-50 two-wire cable 



circuits. 



cumulative plot of the singing frequencies during 22 tests ^ on 46 

 19-gauge B & H-88-50 two-wire cable circuits. It may be seen that 



^ A 22 test is a singing test made by increasing the gain of a normal working 

 repeater in a two-wire circuit until singing begins. 



