START-STOP RECEIVERS 231 



operate. The line A-A indicates the boundary between the marking and 

 spacing positions of the selectors. In this type of receiver, as mentioned 

 previously, when the selector crosses to the marking or upper side of line 

 A-A it becomes locked and cannot again go to spacing even though the sig- 

 nal should subsequently become spacing during the selective period T. 



It will be noted that the limits of end displacement tolerance occur at 

 time t after the beginning of the selective period. This instant is sometimes 

 called the "instant of decision for end displacement." On the other hand, 

 the limiting tolerances to bias are determined at a time / before the end of 

 the selective period, sometimes known as the "instant of decision for bias." 

 If the selective periods were advanced relative to the start transition by 

 lowering the orientation until the bias tolerances were equal, the instants of 

 decision for bias would correspond with the center of bias tolerance. If, 

 then, the selective periods were delayed, by raising the orientation, by an 

 amount T — 2t, the instants of decision for end displacement would corre- 

 spond with the center of end displacement tolerance. Since the difference 

 between the center of end displacement tolerance and the center of bias 

 tolerance is equal to the internal bias of the receiver, it will be obvious that 

 the internal bias is also equal to the difference between the instant of deci- 

 sion for bias and the instant of decision for end displacement. In this type 

 of receiver the internal bias is T — 2/, and will be positive, zero, or nega- 

 tive according as 2/ is less than, equal to, or greater than T. 



Figure 10 (B) shows the action of a type S selector. Here the instant 

 of decision for bias occurs at time t before the end of the selective period and 

 that for end displacement at time t after' the beginning of the selective 

 period. Hence, the internal bias is equal to 2/ — T. 



The action of a type P receiver is illustrated in Fig. 10 (C). It is assumed 

 in this figure that the selector operates toward marking at the same rate as 

 toward spacing, since the effect of unequal rates of operation has been de- 

 scribed previously. In a selector of this type, both instants of decision 

 occur at time / before the end of the selective period and hence the internal 

 bias is not dependent upon the relation between T and t. If, however, / 

 is so long that the selector cannot pass from one extreme of travel to the 

 other, attain a steady state, and return to the center position within time T, 

 a sort of characteristic distortion occurs, in which the instant of decision de- 

 pends upon whether the selector began the selective period in the same or 

 the opposite condition from that finally selected. In measurements of 

 miscellaneous signals this appears similar to a fortuitous effect, since it 

 decreases all tolerances equally. Hence it is usually considered as internal 

 fortuitous distortion. 



Receivers equipped with holding magnet selectors are of Type S, since 

 the armature may be released, but not operated, by the magnet. In this 



