START-STOP RECEIVERS 229 



of the signal train. However, when miscellaneous signals are being received 

 the effect appears similar to a fortuitous distortion occurring on mark-to- 

 space selective transitions, and hence it is usually thought of as negative 

 skew. 



Fortuitous distortion will result when an element is irregular in its action, 

 and if such action is more irregular on one type of transition than on the 

 other, the result will appear as skew. For example, irregular action of the 

 receiving clutch affects the selector alike in regard to all selective transi- 

 tions, and appears as internal fortuitous distortion. Another source of 

 internal fortuitous distortion is the period of indecision that occurs during 

 the passage of a selective element past a locking member, at which time the 

 choice between marking and spacing is largely fortuitous. 



A common cause of skew in teletypewriters may occur in the following man- 

 ner: If the armature stops are so adjusted that, for example, the armature 

 travel is greater on the marking side than on the spacing side of the armature 

 lock, positive internal bias results. If, now, this bias is compensated for by 

 so adjusting the armature air-gap and retractive spring tension as to cause the 

 receiving magnet to operate in a negatively biased manner (rather than by 

 correcting the improper armature travel), the armature will be forced to op- 

 erate in a region of the operating wave that is more sloping than the region 

 in which it releases. Hence, it will operate more irregularly than it releases, 

 and thus will be affected by positive skew. 



Selector Action 



Over and above the sources of internal distortion which are analogous in 

 effect to sources of distortion encountered in telegraph transmission cir- 

 cuits, there is another whose action in causing internal distortion is not so 

 obvious as those just described. This source of internal distortion may be 

 termed "selector action,''^ and it depends upon the relation between the 

 operating time of a selector element and the period of time allowed for said 

 element to act. For the purpose of explaining the effect of time relations 

 within the selector on internal distortion, selector mechanisms may be 

 classified as of three basic types: M, S, and P. 



In a mechanism of type M each selector is initially in the spacing condition 

 and either remains spacing or operates to marking when subjected to the 

 action of the corresponding received signal element. When it attains the 

 marking condition it becomes locked for the duration of the character. 

 Early types of start-stop prin'.ers having an individual selector magnet 

 for each pulse of the code and employing a separate receiving distributer,^ 

 are illustrative of type M. 



In a device of type 5 each selector is initially in the marking condition 

 and either remains marking or operates to spacing when subjected to the 

 action of the corresponding received signal element. When it attains the 



