START-STOP RECEIVERS 223 



In any figure Lb — Le = s —b 

 and U B — U E = —s—b 



Adding and subtracting, we find that: 



Internal bias = ^-— — " 



2 2 



oi Ue — Lb Ub — Lb 



Skew = — 



2 2 



U -\- L 

 Any — - — is the center of an orientation range. Hence it may be stated 



that the internal bias is equal to the difference between the centers of toler- 

 ance to "end distortion" and bias. It will also be noted that any — - — is half 

 of an orientation range. When the test signal displacements determining the 

 range limits are equal, the amount of tolerance equals — - — + D (assuming 



no curvature in the distortion parallelogram). Hence the skew is equal to 



the difference between the amounts of tolerance to "end distortion" and bias. 



For example, the receiver cited previously has the following characteristics 



Internal bias = 48 - 52 = -4% 

 Skew = 43 - 41 = +2% 



Incidentally, this means that internal bias does not reduce the total bias 

 tolerance of a receiver, but merely shifts the center of bias tolerance with 

 relation to the center of "end distortion" tolerance. Hence the effects of 

 internal bias may be compensated for, as far as the bias tolerance of the 

 receiver is concerned, by setting the orientation at the center of bias tol- 

 erance. However, internal bias does reduce the minimum "end distortion" 

 tolerance of a receiver whose orientation is adjusted to the center of bias 

 tolerance. 



"Switched" Bias 



When biased signals are produced by the action of a biasing current on a 

 relay driven by a symmetrical wave, and the sign of bias is suddenly reversed 

 during the transmission of a teletypewriter character, all the succeeding 

 transitions of that character are affected, not by bias, but by "end dis- 

 tortion." This is shown in Fig. 7, of which (A) shows the original unbiased 

 signals, (B) shows the signals affected by bias which changes from positive to 

 negative at time T, and (C) shows the effect on the same signals when the bias 

 is changed from negative to positive. 



Signals such as these, in which the sign of bias is changed at intervals, 

 are said to be affected by "switched bias." Since all four types of displace- 



