PHASE DISTORTION AND PHASE DISTORTION CORRECTION 197 



The distortion on the loaded cable from the transient point of 

 view is represented in Fig. 2, which shows the envelope of the building- 

 up of current of frequency wjliv in the TVth section of a periodically 

 loaded line.* 



The oscillograms of Fig. 3 show the received current in response to 

 sinusoidal waves on a 600-mile medium heavy loaded (H-174) line.^ 



1.2 



I.I 

 1.0 

 0.9 

 0.8 

 0.7 

 0.6 

 0.5 

 0.4 



0.3 

 0.2 

 0.1 



-5-4-3-2-1 I 2 3 4 5 



VALUES OF X 



Fig. 2 — Building-up of alternating currents in long periodically loaded line 



Figs. 3a and 3& are for frequencies of 1,000 cycles and 1,500 cycles 

 respectively and Fig. 3c is for a compound wave made up of 800 and 

 1,600 cycles. The last oscillogram shows clearly the greater delay of 

 the higher frequency. Due to the relative weakness of this component 

 the building-up transients while quite apparent are not pronounced. 

 It will be observed that an appreciable time has elapsed in each case 

 before the received wave has built up to the amplitude or frequency 

 of the steady state. 



The detrimental effects of transient distortion were first studied in 

 the long submarine cable. Early in 1918 a theory of distortion 

 correction was developed by John R. Carson from the transient point 

 of view. The principle was arrived at that the modified arrival curve 

 of prescribed form (a square-topped wave in the case of long line 

 telegraphy) may be obtained by combining derivatives with respect 

 to time of the datum arrival curve (the current obtained at the end of 



* See reference 4. 



* In this symbolism "H" refers to coil spacing of 6,000 ft., and the following 

 number gives the inductance in millihenrys. 



