210 BELL SYSTEM TECHNICAL JOURNAL 



Fig. 1(F) shows a wave which performs a damped oscillation before 

 settling to a steady state. This t>^e of wave tends to produce a negative 

 characteristic effect on certain transitions and a positive characteristic 

 effect on others. 



In general, if, on a given transition, the sum of all previous transients is 

 such as to delay the operation of the receiving device, positive characteristic 

 distortion is said to occur. If, on the other hand, the sum of all previous 

 transients is such as to advance the operation of the receiving device, 

 negative characteristic distortion is said to occur. 



Bias and characteristic distortion, considered together, are called "sys- 

 tematic" distortion, because they occur with some regularity, and obey 

 certain constant laws. There is another type of distortion that is not 

 systematic. This is known as forluitoiis distortion. It may be caused 

 by the effect of various interfering currents on the receiving device. Fig. 

 1(G) shows a wave upon which interfering currents have been superposed. 

 It will be noted that, for a given magnitude of interfering current, the more 

 sloping the wave is in the region of the operating level of the receiving 

 device, the greater will be the resulting fortuitous distortion. 



Fortuitous distortion may also occur, in cases of extremely sloping wave- 

 shape, due to the "indecision" of the receiving device, or, in other words, 

 due to small variations of its effective operating level fiom signal to signal. 



Fig. 1(H) shows a wave that is affected by interfering currents and in 

 which the mark-to-space and space-to-mark transients have different slopes 

 in the region of the operating level of the receiving device. The interfering 

 current therefore causes fortuitous distortion of different magnitudes on 

 mark-to-space and space-to-mark transitions. It will be shown later that 

 distortion of this type affects a start-stop receiver in a particular manner 

 which differs from the effect of distortion of the type illustrated in Fig. 1(G). 



These, then are the generally-recognized components of telgraph dis- 

 tortion. More complicated effects ensue when characteristic distortion 

 occurs on waves having dissimilar transients in the mark-to-space and 

 space-to-mark directions, but a consideration of such phenomena is outside 

 the scope of an elementary explanation of telegraph distortion, and is not 

 necessary to an understanding of the effects of distortion on the margins of 

 operation of start-stop receivers. 



Start-Stop Displacements 



The basic principles of operation of start-stop receivers have been described 

 in previous articles- •'. A brief review of these principles will, therefore, 

 suffice here. 



The start-stop signal train consists of a start pulse, which is generally 

 spacing, several selective pulses, each of which may be either marking or 



