TELEPHONE TRANSMISSION OVER LONG CABLE CIRCUITS 83 



an X.L.L. four-wire circuit 1080 miles long (1750 kilometers) which 

 was set up for purposes of test. The heavy line in this figure shows 

 the overall result which was actually obtained with repeaters and 

 associated apparatus designed to equalize the transmission, while 

 the dotted line shows what the characteristic would have been had 

 the repeaters introduced exactly the same amount of gain at all 

 frequencies. 



VI. Transients 



In comparatively short telephone circuits, good quality will usually 

 be assured if the transmission, as measured at different single fre- 

 quencies within the voice range, is kept approximately constant. 



s 9 



500 1000 1500 2000 2500 1000 



Frequency in Cycles per Second 



Fig. 9 — Transmission frequency characteristic of long extra light loaded four-wire 



circuit. 



For electrically long circuits, however, this is not sufficient. Not only 

 must the "echo" effects be kept within proper limits, but considera- 

 tion must be given to the fact that when electrical impulses are applied 

 to such circuits, peculiar transient phenomena are experienced. These 

 transient phenomena occur in equal degree in two-way circuits and 

 in circuits arranged to transmit in one direction only, that is, they are 

 not related to "echo" effects. 



In order to give an idea of the nature of some of the transient effects, 

 some oscillograms are shown in Figures 10, 11, 12 and 13. Figure 10 

 shows an 1800-cycle current before and after traversing a cable cir- 

 cuit of an earlier type 1050 miles (1700 kilometers) long. This par- 

 ticular circuit was No. 13 A.W.G. weighing 82 pounds per wire mile 



