220 BELL SYSTEM TECHNICAL JOURNAL 



space-to-mark transitions. It may also result from an equivalent electro- 

 mechanical effect in a start-stop receiver, as will be described later. 



Measurements of Receiver Distortion Tolerance 



In measurements of the distortion tolerance of start-stop receivers there is 

 used a distributor which is arranged to transmit signals having any of the 

 four types of displacement MB, SB, SE and ME. Positively biased 

 signals are transmitted for MB displacement and negatively biased signals 

 for SB displacement. The test signals having SE or ME displacement 

 are said to be aflfected by "end distortion." These differ from any ex- 

 perienced on transmission circuits in that only the mark-to-space transitions 

 of the selective pulses are shifted relative to the start transition, being de- 

 layed for ME displacement and advanced for SE displacement, as shown 

 in Fig. 2(E) and (F). "End distortion" simulates the mark-to-space displace- 

 ments produced by characteristic and fortuitous distortion, and it has been 

 found in practice that it yields results which enable a receiver's tolerance to 

 these components of distortion to be predicted with a high degree of accuracy. 



When fixed values of displacement are transmitted, the limits of orienta- 

 tion are measured by means of the range scale of the receiver. Alternately, a 

 distributor may be used in which the magnitude of displacement may be 

 continuously varied, and this enables measurements of internal distortion 

 to be conducted with the orientation fixed, or, indeed, on receivers having 

 no means or a Hmited means of varying the orientation. 



Orientation Settings for Best Tolerance to Test Distortions 



Obviously, the best orientation setting is that which permits the receiver 

 to tolerate the greatest amount of any distortion which is expected. If all 

 four types of displacement are considered equally likely, the orientation 

 should be set at that point at which the minimum tolerance to any type of 

 displacement is as large as possible. For example, consider a receiver 

 which, with an orientation setting of 49, has the following tolerances to test 

 displacements: 



MB 44 



SB 2>^ 



SE 42 



ME 44 



Let the orientation setting be raised 2 per cent, to 51. Then the tol- 

 erances are as follows: 



MB 42 



SB 40 



SE 40 



ME 46 



