START-STOP RECEIVERS 221 



The shift of orientation has increased the minimum tolerance (spacing 

 bias) from 38 to 40. Any further shift would make the tolerance to spacing 

 "end distortion" less than the tolerance to spacing bias. This setting is 

 called the "center of fortuitous distortion tolerance," since at this point the 

 receiver will tolerate the maximum amount of fortuitous distortion. 



If, on the other hand, bias is considered more probable than distortions 

 which produce "end distortion" effects, the orientation might be adjusted to 

 the point at which the tolerances to marking and spacing bias are equal. 

 For example, suppose the orientation setting of the receiver under con- 

 sideration were raised 1 per cent to 52. The tolerances would then be 



MB 41 



SB 41 



SE 39 



ME 47 



This setting is called the "center of bias tolerance," since at this point the 

 receiver will tolerate the maximum amount of bias regardless of the sign of 

 the bias. 



There is one more setting that is of interest. It is that at which the 

 tolerances to marking and spacing "end distortion" are equal. Suppose the 

 orientation of the receiver were lowered 4 per cent to 48. The tolerances 

 would then be 



MB 45 



SB 37 



SE 43 



ME 43 



This setting is called the "center of end distortion tolerance," since at this 

 point the receiver will tolerate the maximum amount of "end distortion" 

 regardless of its sign. 



Calculation of Components of Internal Distortion 



Figure 6 illustrates how the components of internal distortion are deter- 

 mined from measurements of distorted signals. Each diagram shows a 

 portion of a teletypewriter character consisting of a start pulse, a marking 

 selective pulse and a spacing selective pulse. The solid lines show an undis- 

 torted signal. The dashed lines show the displacement of a transition due to 

 internal bias. The shaded area defines the fortuitous effect which is skew; 

 that is, the transition in question may fall anywhere within the shaded area 

 during repeated transmission of the signal. The arrows below the figure 

 show the extent of the displacement occurring on each transition due to the 

 presence of a given displacement of the transmitted signals. The four types 

 of displacement are of equal magnitude D. The arrows above the diagram 

 designated Lb and Le show the lower limits of orientation with, respectively, 



