TELEPHONE TRANSMISSION OVER LONG CABLE CIRCUITS 7.1 



sion. Evidently if the first echoes are small as compared to the 

 direct transmission, the later echoes will be much smaller in mag- 

 nitude. For example, if the power in the first echo, heard by the 

 listener, is 1-10 as great as the directly transmitted power, the second 

 echo will have only 1-100 as much power, the third echo 1-1000 etc. 

 The velocity of an X.L.L. circuit is approximately 20,000 miles 

 (32,000 kilometers) per second, while the velocity with M.H.L. is 

 only 10,000 miles (16,000 kilometers) per second. It is thus seen 

 that the time required for voice energy to travel from one end to the 

 other of an X.L.L. circuit 1,000 miles (1600 kilometers) long is 0.05 



D fa mp fa Of f fa Off [ j 



•Paths of Echoes Affeclino 



yjrrZTD 



Transmitted Power 



jDEjrrm 



■ Paths of Echoes Affecting Listener 



ccmrcct ! j tijixxxf 





\2ZCLZT t — t lJXlrf 



" Tcxrrrr j ^ thtd 



CCrtZ±J3 \ ; ■ , : ^ LXZYJlT 



Fig. 4 — Echo paths in two-wire repeatered circuit. 



second. An echo traveling from orie end of the circuit to the other 

 and back again would, therefore, arrive 0.1 second behind the impulse 

 which started the echo. With M.H.L. circuits these times are of 

 course doubled. 



Figure 4 illustrates the condition existing in a two-wire circuit. 

 For simplicity, the first echoes' only are shown, the later echoes being 

 less important owing to their comparative weakness as explained 

 above. In such a circuit reflections occur not only at the terminals, 

 but at a number of intermediate points in the circuit, the condition 

 of balance between the networks associated with the telephone repeat- 

 ers and the corresponding lines being necessarily imperfect. This 

 imperfection of balance is due in part to lack of perfect balance of 

 the apparatus closely associated with the repeater, and in part to 



