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BELL SYSTEM TECHNICAL JOURNAL 



ment are present in equal magnitude in switched bias signals, the effect on a 

 start-stop receiver resembles that of fortuitous distortion. Thus the center 

 of switched bias tolerance is the center of fortuitous distortion tolerance and 

 the amount of switched bias tolerance is the amount of fortuitous distortion 

 tolerance. This center is also the center of orientation in a receiver having no 

 curvature or symmetrical curvature of the displacement-vs.-orientation- 

 limit characteristic. The switched bias tolerance is, of course, one-half the 

 orientation range in a receiver having no curvature of the characteristic. 



In actual field practice, switched bias signals, applied at a central oflSce, 

 are used as a test of tolerance of the teletypewriter at a subscriber station in 

 combination with the subscriber loop. They provide a more accurate 

 measure of transmission capabilities than an orientation range measurement 

 with undistorted signals from the central office, since not only is the curva- 

 ture of the distortion parallelogram taken into account, but the character 



(A) I START I 



CB) I START I [mB| 2 |_ 



(C) START I ;56 2 



3 I 4 I 5 I STOP 



DECREASED 

 CHARACTER LENGTH 

 \ 

 W\ 5 I STO^ [j 



INCREASED 

 CHARACTER LENGTH 



3 I 4 [m I| 5 I STOP j^ 



Fig. 7 — Switched bias. 



length changes in much the same manner as in signals affected with char- 

 acteristic or fortuitous distortion. 



The components of internal distortion of a receiver may be estimated from 

 bias and switched bias measurements, but they cannot be accurately specified 

 thereby. Figure 8 illustrates the difficulty in separating bias and skew by 

 means of measurements of the difference between the amounts and centers 

 of tolerance to steady bias and switched bias. Figure 8(A) shows the bias 

 and end displacement parallelograms of a receiver having -}-24 per cent bias 

 and + 16 per cent skew. The center of tolerance to switched bias is 4 per 

 cent above the center of steady bias tolerance and the steady bias tolerance 

 is 4 per cent greater than the switched bias tolerance. Figure 8(B) shows 

 the parallelograms of a receiver having +4 per cent bias and —4 per cent 

 skew. Again, the center of tolerance to switched bias is 4 per cent above the 

 center of steady bias tolerance and the steady bias tolerance is 4 per cent 

 greater than the switched bias tolerance. 



Of course, the components of internal distortion can be measured by 



