LOADING FOR TELEPHONE CIRCUITS 249 



commercially available a dev'ice known as an "echo suppressor" 

 which interrupts the path of the echoes without disturbing the main 

 transmission. A description of the device and its field of application 

 was given in a recent Institute paper. ^^ 



Velocity Distortion. In a coil loaded line the steady state velocity 

 of wave propagation varies with frequency. At the upper frequencies 

 the velocity change is principally due to lumpiness effects of the 

 loading and is, therefore, a function of the ratio of the frequency 

 under consideration to the cut-off frequency. As illustrated in Fig. 2, 

 the departure of the actual velocity from the nominal velocity of the 

 corresponding smooth line ( v 1/CL) increases as the frequency is 

 raised, the rate of change increasing rapidly as the cut-off frequency 

 is approached. At frequencies below approximately 0.3 of the cut-off 

 frequency the coil loaded line has substantially the same velocity 

 characteristics as the corresponding smooth line; when the frequency 

 is further reduced, the departure of the actual velocity from the 

 nominal velocity increases as a function of the ratio of the line resist- 

 ance to the inductive reactance per unit length. 



As a result of these velocity-frequency relations, a long loaded 

 repeatered circuit may have seriously objectionable quality, even 

 when the attenuation-frequency distortion is made negligible by the 

 use of special devices at the repeater stations for correcting the at- 

 tenuation-frequency distortion effects. 



The velocity distortion is particularly noticeable during the building- 

 up and dying-down periods, when it manifests itself as transient 

 distortion. The duration of transient distortion depends, among 

 other factors, upon the length of the line, the nominal velocity, and 

 the cut-off frequency of the loading. In the old standard loading 

 systems the high frequency velocity distortion caused by the lumpi- 

 ness effects of the loading was more serious than the low frequency 

 velocity distortion. Accordingly, a substantial reduction in the 

 transient distortion has been obtained in the new standard loading 

 systems by raising the cut-off frequency of the loading. 



For further discussion of velocity distortion reference should be 

 made to Mr. A. B. Clark's paper, '^ previously mentioned, which 

 gives experimental results and to an earlier Institute paper by Mr. 

 J. R. Carson ^^ which gives the results of theoretical studies. 



1* "Echo Suppressors for Long Telephone Circuits," A. B. Clark and R. C. Mathes, 

 Jour. A. I. E. E., p. 618, June, 1925. 



i» Clark, Loc. Cit. 



-" "Theory of the Transient Oscillations of Electrical Networks and Transmission 

 Systems," J. R. Carson, Trans. A. I. E. E., Vol. 38, 1919, p. 345. 



