TELEPHONE SYSTEM OF THE UNITED STATES 55 



electric power received at the end of the circuit. To prevent such vari- 

 tions affecting the net equivalent the long circuits are all provided 

 with automatic regulators which adjust the gains of the telephone 

 repeaters to compensate for the effect of temperature variations on 

 the equivalent of the circuit. 



The effects of transmission delay are also very interesting and 

 important. Voice waves travel considerably more slowly over cable 

 circuits than they do over open wire circuits. For example, the 

 velocity is about 30,000 kilometers per second for " longdistance" 

 type cable circuits as compared to nearly 300,000 kilometers per 

 second for non-loaded open wire circuits. 



One important result of delaying the speech waves is the "echo" 

 effect. The transmitted currents are in part reflected at the distant 

 terminal due to variations in the impedance of the receiving circuit. 

 If the reflected currents transmitted back to the other end are delayed 

 enough they may be heard by the talker as echoes of his voice. They 

 may be again reflected at the sending end of the circuit and returned 

 to the listener as an echo following the directly transmitted speech. 

 The effects of these echoes are largely eliminated by devices known 

 as "echo suppressors" by means of which the transmission of voice 

 waves in one direction over the circuit causes interruption of the 

 path over which the echo currents are transmitted in the opposite 

 direction. However, the effectiveness of echo suppressors is limited 

 by the necessity that they shall not be operated by noise currents of 

 extraneous origin as this would interrupt conversations. The echoes, 

 therefore, are an important factor to be taken into account in deter- 

 mining the type of toll cable circuit to be provided to meet the trans- 

 mission limitations imposed on the long distance circuits. 



In cable circuits introducing considerable transmission delay, the 

 fact that the delay is not exactly the same for waves of different fre- 

 quencies is also important, tending to give rise to what have been 

 sometimes referred to as "transient" effects. In loaded cable circuits 

 the waves of higher frequency are delayed more than those of lower 

 frequency because of the fact that the loading is applied in lumps. 

 The coils and condensers in the repeaters and auxiliary apparatus 

 on the other hand, tend to delay the waves of lower frequency. The 

 result is that the waves of intermediate frequency arrive first, fol- 

 lowed by the waves of higher and lower frequency. Devices known 

 as " phase compensators " can be used to reduce the effects, particularly 

 those caused by the line. To improve the situation at the low end 

 of the frequency scale special attention has been given to the design 

 of the repeaters and auxiliary apparatus. 



