216 BELL SYSTEM TECHNICAL JOURNAL 



signal. As noted in the previous paragraph, the letter "i^" preceded by- 

 repeated "BLANK" signal trains satisfies this description, and the ME 

 displacement is as illustrated in Fig. 4(M). 



In the usual case of positive characteristic distortion, the maximum ME 

 displacement will occur when the start transition is preceded by a long 

 marking signal, and some mark-to-space selective transition is preceded by a 

 combination of pulses as predominantly spacing as possible. As seen 

 previously, this description is answered by a long marking signal followed 

 by a "CARRIAGE RETURN" signal train. Fig. 4(J) illustrates the ME 

 displacement. 



Effect of Characteristic Distortion on Orientation Limits 



In the usual start-stop system which employs a stop pulse longer than the 

 unit selecting pulse, characteristic distortion affects the upper and lower 

 Hmits of orientation differently. This effect is due mainly to the longer stop 

 pulse, although the fact that the start transition is always mark-to-space 

 contributes to the effect. 



In the case of negative characteristic distortion, the displacements (MB 

 and SE) which affect the upper end of the orientation range are those in 

 which the start transition suffers a long delay and a selective transition 

 suffers a short delay. The delay of the start transition can be quite large, 

 since it may be preceded by a long marking pulse. Moreover, the delay 

 of the selective transition may be very short, 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 of as much as four units duration. 

 Hence these displacements, being the difference between a large and a small 

 delay, are large. 



On the other hand, the displacements, SB and ME, which affect the lower 

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

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

 start transition can not be very short for two reasons: first, the start pulse 

 cannot be preceded by a steady spacing pulse; and second, what is of more 

 importance, the stop pulse is of greater than unit length. The delay of a 

 selective transition can be long, as when the transition is preceded by a pulse 

 of four or five units in length. (This delay may not be so long as that 

 suffered by a start transition which follows a steady-state marking condi- 

 tion, but it is not much shorter.) Hence the SB and ME displacements, 

 being the difference between a long selective transition delay and only a 

 fairly short start transition delay, are smaller than the MB and SE dis- 

 placements. 



For this reason negative characteristic distortion affects the upper end 

 of the range more than it does the lower. 



