230 BELL SYSTEM TECHNICAL JOURNAL 



spacing condition it becomes locked and cannot again operate to marking 

 during that character. The Siemens-Halske five-selector teleprinter^ is an 

 example of this type. 



In a mechanism of type P, the selector may be in either the marking or 

 spacing condition initially, according to the type of the previous signal ele- 

 ment to which it has responded. When subjected to the action of a received 

 pulse the selector may go in either direction, and it remains responsive to the 

 action of the signal during the entire selecting interval. The No. 14 and 

 No. 15 teletypewriters^ (not equipped with holding magnet selector) of the 

 Teletype Corporation are examples of type P. 



^^ k^ 



SE 



(A) TYPE M RECEIVER 



■f^^^f K^ 



SE 



(B) TYPE S RECEIVER 



K^ m^ 



I -n - ^ ^ ^ ""^ .H^^^^-f 



(C) TYPE P RECEIVER 

 Fig. 10 — Effect of selector action on internal distortion. 



Figure 10 (A) illustrates the action of a type M selector. A portion of a 

 teletypewriter character is shown, consisting of the spacing start pulse, a 

 marking first selective pulse and a spacing second selective pulse. The 

 undistorted signal is shown in solid lines. The maximum amounts of mark- 

 ing and spacing bias that the receiver will tolerate are shown by dashed lines 

 and are designated MB and SB. The limiting amounts of marking and 

 spacing end displacement are shown by dotted lines and are designated ME 

 and SE. Above the signal train is shown a schematic representation of the 

 action of the selective system. The periods of time T are those during which 

 the selector is subject to the action of the received signal, and / is the time 

 that the selector must be subjected to the operative force in order that it 



