228 BELL SYSTEM TECHNICAL JOURNAL 



tolerance. This is why the orientation of a service receiver is generally- 

 adjusted to the center of bias tolerance, and small amounts of internal bias 

 or negative skew are not considered objectionable, since they do not affect 

 the tolerance to bias at the center of bias tolerance. By the same token, the 

 presence of positive skew, which indicates a lowered bias tolerance, usually 

 calls for a readjustment of the receiver to reduce the fortuitous effect on the 

 space-to-mark transitions. As explained above, removing the skew by 

 introducing a fortuitous effect on the mark-to-space transitions will not, of 

 course, improve the bias tolerance. 



It is the present practice in the field to specify a minimum bias tolerance 

 about 5 per cent greater than the minimum permissible "end distortion" 

 tolerance, the orientation being adjusted to the center of bias tolerance for 

 both measurements. 



Some Causes of Internal Distortion 



Up to this point internal distortion has been considered without regard 

 to its probable causes. The more obvious causes will be found to be 

 analogous to those which produce equivalent distortions in telegraph trans- 

 mission circuits. 



Bias will result when an element (whether electrical, mechanical, or elec- 

 tronic) of a receiver possesses dissymmetry toward marking or spacing. 

 For example, a mechanical element may travel more slowly from spacing to 

 marking than from marking to spacing and thus cause spacing bias, or its 

 range of travel may be divided unequally into marking and spacing portions, 

 thus producing an equivalent efi'ect. 



Characteristic distortion will result when an element (whether electrical or 

 mechanical) of a receiver fails to attain a steady state before being acted 

 upon by a succeeding transition, or otherwise depends, in its action, upon the 

 previous history of the signal train. An example of characteristic distortion 

 is found in the 20-milliampere holding magnet selector when it is equipped 

 with a resistive shunt. In this type of selector the armature is actuated by a 

 cam, which presents it to the pole-face at about the middle of each pulse, 

 and then disengages it. The armature is then free to release or remain 

 operated, according as the received pulse is spacing or marking. The shunt 

 that is normally used presents so low an impedance to the magnet winding 

 that the motional impedance effect which is produced by the sudden mechani- 

 cal presentation of the armature to the pole-faces causes a sizeable reduction 

 in the magnet current. In the case of a short marking pulse, the current 

 fails to attain steady state before the next mark-to-space transition occurs. 

 The magnet therefore releases sooner than it does at the end of a long 

 marking pulse, during which the current has had time to attain steady state. 

 It will be seen that this is really a characteristic distortion effect, since it is 

 due to a failure to reach steady state and depends upon the previous history 



