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BELL SYSTEM TECHNICAL JOURNAL 



distance of about 95 miles (153 km.), we are employing two coaxial 

 structures within a single lead sheath so as to provide transmission in 

 each direction. The complete structure is shown in Fig. 16. Some 

 of the space within the circular lead sheath is filled with ordinary 

 cable pairs which are tapped out at repeater points for testing and 

 trial purposes. Repeaters are being provided along this coaxial 

 circilit at intervals of about 10 miles (16 km.), which allows frequencies 

 up to about 1,000,000 cycles to be transmitted. Figures 17 and 18 

 show schematically this coaxial broad-band system. 



At each repeater point there is a single amplifier for each coaxial 

 unit, that is, one for each direction of transmission. This amplifier 

 handles the entire number of simultaneous conversations obtainable, 

 which is 240 for the million-cycle band. The amplifiers are equipped 

 with automatic regulating arrangements to adjust the amplification 

 to correspond to the attenuation of the cable as the temperature 

 changes. As you would expect, the attenuation variations with 

 temperature are not the same at different frequencies, but the regu- 

 lating system meets this condition. 



Development of means for combining and separating the channels 

 at the terminals is an interesting feature. Some of our engineers 

 have termed the means employed "unit group." The twelve carrier 

 channels employed in both the cable and open-wire broad-band systems 

 are provided in a unit called a "12-channel terminal." The coaxial 

 system employs essentially the same 12-channel terminal, but it 

 employs twenty of them to provide the total of 240 channels. The 

 output of one 12-channel terminal is put directly on the line, but the 

 outputs of the other nineteen are modulated a second time, and raised 

 to successive positions in the frequency spectrum. 



The use of double modulation has two principal advantages. In 

 the first place it simplifies the apparatus by requiring fewer different 

 carrier frequencies, and since it employs a channel terminal that is 

 used by all broad-band systems, considerable economies in production 

 are secured. The chief advantage of double modulation in the coaxial 

 system, however, is that it simplifies the separation of the side-bands 

 resulting from modulation, which is necessary because only one of the 

 side-bands is transmitted. If only a single modulation were employed, 

 the two sidebands of the upper carrier frequency would be separated 

 by only about .05% of the carrier frequency, while with double 

 modulation the narrowest separation is about ten times this amount. 



With such a coordinated program of broad-band telephony, toll 

 transmission takes on a new appearance. Not only will the provision 



