SINGING ON TWO-WIRE CABLE CIRCUITS 617 



exceeded in 50 per cent of the cases is taken (i.e., Sf = + 1.8 db), 

 the degradation would evidently be too large due to the equipment 

 since the lines in which the singing margin might be expected to be 

 low, would generally be those which happen to have more than the 

 usual number of low line return losses. 



As an approximation in the usual case, it seems reasonable to 

 combine the line equipment return losses with the cable return 

 losses as the sum of their power ratios, with a value of Sf of about 

 — 1 db (i.e., where 28 per cent of the return losses are lower than 

 the value considered). One db added to the resultant will give the 

 value of the resultant for which Sf = 0. The same thing may also be 

 accomplished by combining the cable return loss for Sf = with the 

 near and far-end equipment return losses each increased by 1 db. The 

 example which is given later shows the method used. 



Taper 



The taper of a regular two-wire circuit is the amount of decrease in 

 repeater output level at a given frequency which occurs between 

 succeeding repeaters from the transmitting to the receiving end of the 

 circuit. For example, starting at the transmitting repeater, if the 

 repeater output levels at 1000 cycles were, respectively, 3.0, 2.5, 2.0, 

 1.5, etc., the 1000-cycle taper would be 0.5 db. 



An optimum taper from the standpoint of singing will be reached 

 when the singing margin around the most critical intermediate repeater 

 is equal to the singing margin around the terminal repeater. This is 

 certain to occur for some taper under the present method of dis- 

 tributing gain since an increase of the taper causes the gain of the 

 receiving terminal repeater to be increased by a multiple of the increase 

 of taper. However, the optimum taper from the standpoint of sing- 

 ing is not generally used in the field, because the optimum taper from the 

 standpoints of crosstalk and noise is zero db. A compromise is gener- 

 ally made on two-wire cable circuits; e.g., 19-gauge B and H-88-50 

 facilities use 0.5 db and 16-gauge H-44-25 facilities use 0.2 db taper. 



Active Balances 



In some cases, it may be more convenient to measure active return 

 losses or active singing points rather than singing margins. The 

 expected distribution of active return losses and active singing points 

 including the end paths when the end paths are the normal working 

 terminations, have been discussed above. 



In practice, however, the termination when such an active singing 

 point test is made will normally be a fixed one, generally consisting of 



