268 BELL SYSTEM TECHNICAL JOURNAL 



II. Description of Tests 



1. Preliminary Considerations 



Telegraph transmission quality may be considered perfect when the 

 received signals at one end of the circuit correspond exactly to the 

 signals as sent at the other end of the circuit. Any departure in the 

 length of a signal or part of a signal at the receiving end is, therefore, 

 a quantitative measure of the degradation of the quality of trans- 

 mission and has been termed telegraph distortion. Since telegraph 

 signals are composed of dots, dashes, and spaces, the measurement of 

 degradation in transmission quality consists of measuring the lengths of 

 these signal elements at the receiving end of the circuit and comparing 

 them with their lengths at the sending end of the circuit. Means for 

 doing this, together with a discussion of the various types of distortion 

 affecting the lengths of telegraph signals, have been given in a recent 

 paper. ^ 



As brought out in the paper referred to, distortion of telegraph 

 signals may be divided into three components, namely, bias, character- 

 istic distortion and fortuitous distortion. Each of these may be either 

 positive or negative, depending upon whether the distortion causes a 

 lengthening or a shortening of the signal part under consideration. 

 The three components may be described briefly as follows: 



Bias consists of a substantially uniform lengthening of the marks 

 and a corresponding shortening of the spaces of telegraph signals, or 

 vice versa. The first condition is called positive (marking) bias and 

 the reverse condition negative (spacing) bias. It is usually due to 

 lack of symmetry in the marking and spacing battery voltages or in 

 the adjustment of repeating relays. 



Characteristic distortion is distinguished from other types of distortion 

 by the fact that it is a function of the signal combination as well as of 

 the electrical and mechanical characteristics of the circuit. For 

 example large inductance in a circuit may prevent the signaling current 

 from building up to its full value during a short impulse following a 

 long impulse of opposite polarity, thereby causing a decrease in the 

 length of the short impulse which would be called negative character- 

 istic distortion. 



Fortuitous distortion is an erratic lengthening and shortening of 

 marks and spaces such as that due to the superposition of extraneous 

 interfering currents upon the signaling currents in the line, or due to 

 chattering and sparking at the contacts of repeating relays. 



In the case of machine telegraphy such as printing telegraph, the 



1 Nyquist, Shanck and Cory, Trans. A. I. E. E., Vol. Xl.VI, 1927, p. 231-240. 



