RECENT DEVELOPMENTS IN LONG DISTANCE CABLES 491 



Importance of High Velocity Circuits in Cable 



Echo suppressors have proven quite effective in reducing the echo 

 effects on long distance circuits. For very long distance cable circuits, 

 however, echo is still a matter for concern, particularly with losses held 

 down to figures such as those given in the paper by Mr. H. S. Osborne 

 entitled "A General Switching Plan for Telephone Toll Service." 



When cable circuit lengths become very great the actual delay suf- 

 fered by the speech waves in traveling from end to end of the circuit 

 becomes important quite apart from echo. We must look forward to 

 the time when a subscriber in San Francisco will talk by cable across 

 the United States to New York, then by cable and open wire to New- 

 foundland, by submarine cable to England and then by a long cable 

 circuit, let us say, to Constantinople; in other words, a 10,000-mile 

 circuit length. The highest velocity long distance cable circuits in use 

 today will give, for conversations over such a circuit, about >^-second 

 delay in going from one end to the other so that when one subscriber 

 speaks the other's reply cannot possibly reach him in less time than 

 one second. This is quite a long time interval. By utilizing speaking 

 tube delay circuits, connections have been set up involving delays as 

 great'as this. Conversations are possible over circuits with such delays 

 but the delay is a serious interference particularly when voice-operated 

 devices are added which tend to lock out portions of the conversations. 



It is thus evidently important to seek higher velocity circuits. 



Telephone Carrier in Cable 



In seeking ways for obtaining high velocity circuits in cable in an 

 economical manner, consideration has been given to the proposition of 

 applying telephone carrier to long distance cables. For large groups of 

 long distance circuits it appears likely that a carrier-frequency range 

 can be advantageously used in cables, as wide or wider than the fre- 

 quency range which has been exploited on open-wire lines. Experi- 

 mental work on systems of this kind is actively under way at the 

 present time. 



The higher frequencies involved together with the accompanying 

 attenuations and increased coupling between circuits introduce some 

 very interesting and unusual noise and crosstalk problems, as well as 

 problems of equalization and maintenance of transmission stability. 

 Also, there are interesting economic problems of conductor size and 

 type, loading versus no loading, repeater spacing, etc. 



It is interesting to note that if non-loaded circuits are utilized, a 

 velocity of transmission of about 100,000 miles per second would be 



