208 BELL SYSTEM TECHNICAL JOURNAL 



operation of start-stop receivers may well begin with a brief review of the 

 nature and causes of the various types of distortion commonly experienced 

 by telegraph signals. Telegraph distortion is generally considered to be 

 divided into three types or components: bias, characteristic distortion, and 

 fortuitous distortion.^ The magnitude of the distortion is expressed in per 

 cent of a unit pulse. 



The Components of Telegraph Distortion 



Bias, which is the simplest and most common component of distortion, 

 may be positive (marking) or negative (spacing). Positive bias appears 

 as a uniform lengthening of all marking pulses and an equal uniform shorten- 

 ing of all spacing pulses. Conversely, negative bias appears as a uniform 

 lengthening of all spacing pulses and an equal uniform shortening of all 

 marking pulses. 



Bias is caused by an improper relation between the levels at which the 

 relay or other receiving device responds and the steady-state marking and 

 spacing levels of the signal. For example. Fig. 1(B) shows the signals of 

 Fig. 1(A) as they might appear as a symmetrical wave on a line. With 

 such a wave zero bias will be received when the currents at which the 

 receiving relay operates from spacing to marking and from marking to 

 spacing are symmetrically located with respect to the average of the steady- 

 state marking and spacing currents. That is, zero bias will be received if the 

 relay operates from spacing to marking and from marking to spacing at 

 B-B, or if the relay operates from spacing to marking at A-A and from 

 marking to spacing at C-C, or if the relay operates from spacing to marking 

 at C-C and from marking to spacing a.t A-A. Negative bias will be received 

 if the relay operates in both directions at A-A, and positive bias will be 

 received if it operates in both directions at C-C. 



In Fig. 1 (C) is shown an unsymmetrical wave, in which the transient from 

 space to mark is more rapid than that from mark to space. In this case, 

 positive bias will result when the relay operates in both directions at B-B or 

 at C-C, but no bias will result if the relay operates in both directions a.t A-A. 



In the remaining diagrams of Fig. 1 it is assumed that the relay operates 

 in both directions at a level midway between the steady marking and spacing 

 levels. Fig. 1(D) shows a wave in which the transients are of such duration 

 that the steady-state value is not attained in the shortest pulse length. It 

 will be seen that the operation of the relay is delayed less after a short pulse 

 than after a long one, and that this is true whether the pulse be marking or 

 spacing. This effect is known as negative characteristic distortion, and it 

 tends to shorten short pulses and lengthen long pulses. When a series of 

 unbiased dots (called telegraph reversals) is transmitted, a steady-state 

 condition is reached, in which the delays become equal on all transitions. 



