START-STOP RECEIVERS 211 



spacing, and a stop pulse which is generally marking. The receiving 

 mechanism is started by the transition at the beginning of the start pulse, 

 and its speed is such that it arrives at the stop position before the end of the 

 stop pulse occurs, and remains stopped until the succeeding start transition 

 takes place. Thus any speed difference between the transmitter and 

 receiver is prevented from cumulating for more than the duration of one 

 signal train. 



Since the receiving device starts anew at each start transition, and the 

 instants of selection of the selective pulses are spaced in time relative to the 

 instant of starting, as shown in Fig. 2(A), the start transition acts as a basic 

 reference point to which all other instants of time during the selective cycle 

 may be referred. 



The advances and delays of the transitions of the start-stop signal train 

 from their normal times of occurrence, relative to the start transition, are 

 known as "start-stop displacements." Fig. 2(B) shows the four types of 

 displacement that may occur: MB or "marking beginning displacement," 

 which is the advance of a space-to-mark transition (beginning of a marking 

 pulse) relative to the start transition; SB or "spacing beginning displace- 

 ment," which is the delay of a space-to-mark transition relative to the start 

 transition; SE or "spacing end displacement," which is the advance of a 

 mark-to-space transition (end of a marking pulse) relative to the start 

 transition; and ME or "marking end displacement," which is the delay of a 

 mark-to-space transition relative to the start transition. 



Effect of Bias on Displacement 



Since bias affects all pulses alike, and since in the usual start-stop receiver 

 the start transition is mark-to-space, the succeeding mark-to-space transi- 

 tions of the signal train are not shifted relative to the start transition. 

 Hence the total effect of the bias appears on the space-to-mark transitions. 

 Positive bias causes MB displacement alone, as shown in Fig. 2(C). Nega- 

 tive bias causes SB displacement alone, as illustrated in Fig. 2(D). 



The total range through which the selective periods may be shifted, 

 relative to the start transition, without producing an incorrect selection is 

 known as the orientation range of the receiver. Its limits are read on a 

 scale calibrated from to 100 in per cent of a unit pulse-length. Figure 3 

 is a graph of teletypewriter orientation range versus input signal bias, for a 

 receiver whose range is from 10 to 90 on unbiased signals. Diagrams of this 

 type are called "bias parallelograms." 



Effect of Characteristic Distortion on Displacement 



Characteristic distortion does not affect all pulses of miscellaneous signals 

 alike, because, as explained above, the effect on each transition depends 



