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BELL SYSTEM TECHNICAL JOURNAL 



The following table shows the same information for circulating 

 current margins, computed on the assumption that Sn — 1.25 = Sn 

 — 1.25 = (6'2i — 1) = (522 — 1). The design requirements in this 

 case are also that 12 db, 10 db, 8 db or 6 db singing margin (not cir- 

 culating current margin) shall be exceeded in 90 per cent of the cases 

 under average conditions. 



It is estimated that the reduction in circulating current margin due 

 to regulation, variation in circuit net loss, and temporary troubles 

 will seldom exceed 4| db by very much but about this amount of 

 reduction will occur fairly frequently. Similarly, the reduction in 

 singing margin due to these causes and also to the removal of a ter- 

 mination while a circuit is being set up will seldom exceed 5| db. It is 

 believed that about 4 db circulating current margin at the critical 

 frequency is as small as can be obtained without very objectionable 

 quality distortion. 



When 5| db reduction in singing margin is obtained on the average, 

 we may read from the first table above at 5^ db margin, the percentage 

 of cases in which actual singing will take place. For example, with a 

 design requirement of 8 db, interpolating between the vertical columns 

 headed 6 and 4 db, respectively, we find that about 1| per cent of 

 cases will result in actual singing. Similarly, by reading at 4 -|- 4^ 

 = 8| db circulating current margin in the second table, we find the 

 percentage of cases in which the quality will be objectionably distorted 

 during the period in which a 4| db reduction in circulating current 

 margin is obtained. The following table shows these two different 

 percentages for the design limits given above. From this table it 



Percentage of Cases With 



Less Than 4 Db Circulat- Percentage of Cases 



Design Limit ing Current Margin Which Will Sing Under 



in Db Under Extreme Conditions Extreme Conditions 



12 0.97 0.015 



10 2.8 0.18 



8 6.8 1.45 



6 13.1 7.1 



seems reasonable to require a 10 db singing margin under average 



