542 BELL SYSTEM TECHNICAL JOURNAL 



the amount lost apparently is not a serious handicap. The C. C. I. 

 has tentatively recommended that 0.1 second be taken as a limit 

 towards which it is desirable to work if practicable. 



Better Cable Circuits Desirable for Extreme Distances 



These difficulties with the present type of cable construction all 

 become more pronounced if, instead of a 3600-mile (6000-kilometer) 

 circuit, consideration is given, for example, to possible future inter- 

 continental circuits in all cable construction. Such a circuit between 

 San Francisco and Istanbul, for example, would be about 10,000 miles 

 (16,000 kilometers). The time of propagation for such a connection 

 would be about .6 of a second in each direction so that two-way 

 conversations would be seriously impeded. Serious difficulties would 

 also be experienced with the voice-operated devices and because of 

 the accumulated distortions, including non-linear effects. While, 

 therefore, it is possible that a circuit of this sort could be used for 

 two-way telephony between Istanbul and San Francisco, the imper- 

 fections of such a circuit would be so outstanding as to warrant a 

 serious effort to develop something better if this could be done at a 

 reasonable cost. 



Telephone Carrier in Cable 



In order to obtain better transmission results over very long cable 

 circuits in an economical manner the development of a carrier system 

 for cables has been actively undertaken. The development work of 

 the Bell System has now been carried to the point where it seems 

 assured that it will be successful and that telephone carrier will have 

 an important field of use in long distance cables on heavy routes. 



This carrier system uses non-loaded cable conductors whose velocity 

 is very high as compared to voice-frequency loaded circuits, the 

 effective circuit velocity including delays introduced by apparatus 

 being about 100,000 miles (160,000 kilometers) per second. An 

 experimental trial system has been set up by looping circuits back 

 and forth in a cable so as to produce the equivalent of the system 

 shown in Fig. 7. It will, of course, be understood that this figure 

 represents merely the experimental setup and should therefore not be 

 considered the ultimate in such matters as carrier channels per pair, 

 repeater spacing, etc 



Talking tests which were made using this experimental setup 

 showed very satisfactory quality of transmission and no appreciable 

 interference between circuits In addition to testing each of the nine 

 telephone circuits shown in the sketch, these nine circuits were con- 



