232 BELL SYSTEM TECHNICAL JOURNAL 



type of mechanism, the armature generally drives a subsidiary selective 

 member, and the time T extends from the instant at which the armature is 

 disengaged by its operating cam until the instant when the subsidiary selec- 

 tor becomes locked. As this period is often long in relation to the magnet 

 releasing time / 1 and the subsidiary selector operating time / 2, holding magnet 

 selectors are often subject to negative internal bias. In those mechanisms 

 in which the subsidiary selector is flexibly coupled to the magnet armature, 

 the former's operation is of type P. It, therefore, may be subject to the 

 characteristic distortion effect noted in the description of type P operation, 

 except that the effect, when it occurs in this type of mechanism, affects only 

 the instant of decision for end displacement and hence resembles negative 

 skew rather than internal fortuitous distortion. 



An interesting, but somewhat unusual, effect occurs in any receiver, of 

 whatever type, in which the lengths of selective period or selector operate 

 time, or both, differ for the various selective pulses, or in which the spacing 

 of the selective periods is improper. In a case of this sort, the receiver ex- 

 hibits an internal bias equal to the difference between the average instant of 

 decision for bias and the average instant of decision for end displacement, 

 an internal fortuitous distortion equal to the variation of the instant of 

 decision having the smaller variation, and a skew equal to the difference 

 between the variations of the instant of decision for bias and the instant of 

 decision for end displacement. 



Conclusions 



A working knowledge of the effect of telegraph distortion on the margins 

 of operation of start-stop receivers is essential in dealing with a plant in 

 which the use of teletypewriters, regenerative repeaters and start-stop dis- 

 tortion measuring sets is as widespread as it is in the Bell System. When a 

 major portion of the communication system operates on a start-stop basis, 

 it is desirable that transmission measurements be made on the same basis. 



The knowledge of this subject that has been gained in recent years has 

 made possible many improvements in technique both in the field and in the 

 laboratory, and these have led to corresponding improvements in the mecha- 

 nisms used in telegraph service. The analysis of new start-stop devices 

 may now be carried out efficiently and accurately, and this often permits 

 the formulation of suggestions leading to improved operation of the devices. 



The general level of service excellence has been raised by the setting up 

 of criteria for the distortion tolerances of station teletypewriters, regenerative 

 repeaters and other start-stop devices used in service, including those pro- 

 vided for switching. The sources of distorted test signals that are now 

 available are useful not only in measuring the tolerances of service receivers. 



