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BELL SYSTEM TECHNICAL JOURNAL 



latter being the tolerance of a perfect receiver) is due to internal fortuitous 

 effects, even though part of it may be due to the effects of mternal char- 

 acteristic distortion. Hence the internal fortuitous distortion is usually 

 defined as the difference between 50 and the tolerance to bias or end dis- 

 tortion, whichever of the latter may be the larger. 



For example, in the sample receiver considered on page 220, the internal 

 fortuitous distortion is: 



50 - 43 = 7 per cent 



+20 +40 



BIAS OF TRANSMITTED SIGNALS 

 PERCENT OF A UNIT PULSE 



■WITHOUT CHARACTERISTIC DISTORTION 



■WITH NEGATIVE CHARACTERISTIC DISTORTION 



Fig. 9 — Effect of negative characteristic distortion on bias parallelogram. 



Internal Characteristic Distortion 



In practice it is found that the relation between displacement and reduc- 

 tion of margin is sometimes not strictly linear. Especially at large values of 

 displacement, the reduction in margin is often greater than the displacement 

 causing it. This effect is due to internal characteristic distortion, which 

 causes an increase in the distortion of shortened pulses. Internal char- 

 acteristic distortion, like any other form of characteristic distortion, is caused 

 by the failure of some circuit or mechanical element to attain steady state 

 before the occurrence of a succeeding transition. Figure 9 shows an example 



