218 BELL SYSTEM TECHNICAL JOURNAL 



receiving device operates. Then the magnitude of the fortuitous effect is 

 different on mark-to-space and space-to-mark transitions. If the effect is 

 greater on the space-to-mark transitions, MB and SB displacements are 

 greater than SE and ME. If the opposite, SE and ME are greater than 

 MB and SB. In all cases, however, the orientation range is reduced equally 

 at both ends. 



Fortuitous distortion also lengthens and shortens the character since it 

 does not affect all transitions alike. 



Internal Distortion 



Telegraph signal distortion may occur within the start-stop receiver' 

 and it should be expected that the components of distortion will have the 

 same effect on the margins of operation as the same components external 

 to the receiver. Consequently, it should be possible to determine the 

 magnitudes of the various components of internal distortion by their effects 

 on the margins of operation. 



As mentioned previously, the upper end of the orientation range is 

 determined by whichever of the displacements MB and SE is the greater; 

 and the lower end by whichever of the disp'acements SB and ME is the 

 greater. To discover the magnitude of the smaller type of displacement it is 

 necessary to reduce the larger displacement by distorting the transmitted 

 signals. For example, if a receiver has a large internal marking bias, the 

 upper limit of orientation is determined by MB displacement, and hence the 

 amount of SE displacement caused by internal distortion is concealed. 

 However, by transmitting signals affected by SB displacement (in other 

 words, signals biased to spacing), the total MB displacement is decreased 

 until it is less than the internal SE displacement, whose effect on margin 

 can then be found. Thus the internal distortion may be determined by 

 observing the effect of external distortion on the margins of operation. 



It is convenient to regard any start-stop receiver as a theoretically perfect 

 receiver affected by certain types of internal distortion. The internal 

 distortion is usually considered to be composed of bias, "skew" (defined 

 later) and fortuitous distortion. (The internal characteristic distortion is 

 generally included in "internal fortuitous distortion," since it is usually very 

 small, and a fairly elaborate testing procedure is required to separate its 

 effects from those of internal fortuitous distortion.) Internal bias and 

 internal fortuitous distortion are of the same nature as the external effects 

 previously described. Skew is said to occur when there exists the type of 

 distortion, previously mentioned, in which the fortuitous effect on space-to- 

 mark transitions differs in magnitude from that on mark-to-space transi- 

 tions. When the former is greater the skew is said to be positive; when the 

 latter, negative. Hence in positive skew, MB and SB displacements tend to 



