TOLL TRANSMISSION IN THE UNITED STATES 135 



handling only one channel. The present plans call for these inter- 

 mediate repeater stations to be substantially non-attended. The 

 electrical units employed in a cable carrier system are shown sche- 

 matically in Fig. 11. 



Some idea of the possibilities of this carrier cable system may be 

 formed from the results of a trial installation made on a laboratory- 

 scale. In one case, we had circuits as long as 7,500 miles (12,000 km.) 

 set up, over which we carried on satisfactory conversations. The 

 total attenuation over some of these circuits was such as to require 

 power amplifications of 12,000 db, which corresponds to a power 

 ratio of 10^^"° to 1. This amplification was applied at nearly 400 points. 



Broad-Band System for Open- Wire Lines 



In the Bell System, as you know, along many toll routes, there is 

 still much open-wire construction, aggregating tens of thousands of 

 miles. At the present time, many thousands of miles of open-wire 

 lines are equipped with 3-channel two-way carrier systems. These 

 systems employ frequencies up to about 30,000 cycles, and with the 

 regular voice frequency circuit provide facilities for four simultaneous 

 conversations over one pair of wires. This might appear to be an 

 efficient use of the wire plant, but the proposed system employs an 

 additional frequency range from about 30,000 cycles to perhaps 

 150,000 cycles, adding 12 channels in each direction to a pair of wires. 

 This will furnish a total of sixteen simultaneous conversations over a 

 pair of wires. 



Extending the frequency range accentuates the problem of crosstalk 

 and some of the other problems of interference, but it is our present 

 feeling that a substantial number of the pairs on a suitably constructed 

 pole line can be rearranged for operation by this broad-band method. 

 Figure 12 shows schematically the arrangement of apparatus at a 

 terminal to provide these sixteen talking circuits, and Fig. 13 shows 

 diagrammatically the arrangement of apparatus at a repeater station. 



Since increased frequency means higher line attenuations with 

 corresponding higher amplification, the use of higher frequencies 

 means additional repeaters on the line, so that the line currents will 

 not at any point be attenuated below a certain level. The present 

 proposal is to provide repeater spacing of approximately 75 miles, 

 instead of the 150 miles used on the present open-wire carrier systems. 



Coaxial Cable System 



This is the most radical of the broad-band developments that we 

 have attempted to develop practically. Instead of several pairs 



