START-STOP RECEIVERS 215 



preceding paragraph, repeated "BLANK" signals satisfy this description. 

 Fig. 4(F) illustrates the effect of negative characteristic distortion on this 

 signal combination. It will be noted that the resulting SB displacement 

 extends into the stop pulse, where usually no selection takes place, and 

 hence it would not affect the margin of operation of the start-stop receiver 

 except, of course, in the case of a type of receiver, such as a regenerative 

 repeater, in which selection of the stop pulse does occur. For the case of a 

 receiver which does not select the stop pulse, the maximum SB displacement 

 affecting the margin of operation occurs at the second transition of the 

 letter "T" following repeated "BLANK" signals. Except in the case of 

 very large distortions, this displacement will be of nearly the same mag- 

 nitude as that illustrated in Fig. 4(F). 



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

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

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

 combination of pulses as predominantly marking as possible. As noted 

 previously, this description is satisfied by a sufficiently long marking signal 

 to permit the attainment of steady state, followed by any signal train having 

 the first selective pulse marking. Figure 4(C) illustrates the effect of 

 positive characteristic distortion on this type of signal. 



The maximum SE displacement will occur when the start transition is 

 delayed as much as possible and some mark-to-space selective transition is 

 delayed as little as possible. This will take place, in the case of negative 

 characteristic distortion, when a long marking signal precedes the start 

 transition and a combination of pulses as predominantly spacing as possible 

 precedes the mark-to-space transition in question. This description is 

 answered by a long marking signal followed by a "CARRIAGE RETURN" 

 signal train, as shown in Fig. 4(H). The SE displacement occurs at the 

 end of the fourth selective pulse. 



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

 displacement will occur when the start transition is preceded by a combina- 

 tion of pulses which is as predominantly spacing as possible, and some 

 mark-to-space selective transition is preceded by the longest possible marking 

 signal. This description is satisfied by repeated "BLANK" signal trains 

 followed by the letter "A'," and, as shown in Fig. 4(L), the SE displacement 

 occurs at the end of the fourth selective pulse. 



The maximum ME displacement will occur when the start transition is 

 delayed as little as possible and some mark-to-space selective transition is 

 delayed as much as possible. This will take place, in the case of negative 

 characteristic distortion, when the start transition is preceded by a combina- 

 tion of pulses which is as predominantly spacing as possible, and some mark- 

 to-space selective transition is preceded by the longest possible markins^ 



