START-STOP RECEIVERS 217 



In the case of what we have termed "the usual type of positive char- 

 acteristic distortion," the displacements {SB and ME) which affect the lower 

 end of the orientation range are those in which the start transition suffers a 

 short delay and a selective transition suffers a long delay. The delay of the 

 start transition can be quite short, since it may be preceded by a long mark- 

 ing signal. Moreover, the delay of the selective transition may be very 

 long, since the pulse which precedes the transition can be of unit length and 

 this, in turn, may be preceded by a signal of the opposite type which may be 

 four or more units in length. Hence these displacements, being the differ- 

 ence between a short and a long delay, are large. 



On the other hand, the displacements {MB and SE) which affect the upper 

 end of the range are, in this type of distortion, those in which the start 

 transition suffers only a fairly long delay and a selective transition suffers a 

 short delay. The delay of the start transition cannot be very short for the 

 two reasons mentioned previously. The delay of the selective transition 

 can be short, as when the transition is preceded by a pulse four to six units in 

 length. Hence the MB and SE displacements, being the difference between 

 a short selective transition delay and only a fairly long start transition delay, 

 are smaller than the SB and ME displacements. 



For this reason positive characteristic distortion of this type affects the 

 lower end of the range more than it does the upper. 



In the case of a wave which oscillates, causing positive characteristic 

 distortion on some transitions and negative on others, no such general 

 statements as are made above are applicable. In practice, cases have been 

 observed in which one end of the orientation range was cut and the other 

 was actually extended. 



Due to the fact that characteristic distortion delays the start transition by 

 different amounts from character to character, it causes the character length 

 to vary during continuous automatic transmission. The maximum varia- 

 tion in character length is roughly of the same magnitude as the maximum 

 displacement affecting the selective pulses. 



Effect of Fortuitous Distortion on Displacement 



Fortuitous distortion causes the start transition to be delayed more or 

 less than normal, and has the same effect on the selective transitions. Since 

 it is usually equally probable that the maximum fortuitous effects will 

 occur on mark-to-space or space-to-mark transitions and will increase or 

 decrease their delay, this type of distortion generally produces the four types 

 of displacement in equal magnitude, and this magnitude is equal to the 

 maximum increase or decrease in the length of pulse. 



An exception to the above statement occurs when the mark-to-space and 

 space-to-mark transients give the wave different slopes at the point where the 



