8 BELL SYSTEM TECHNICAL JOURNAL 



is designed to compensate. Three stages of pentodes are used with 

 negative feedback ^ around the last two stages. Attenuation changes 

 due to temperature of the line are compensated automatically by a 

 pilot channel device which has been installed at every second or third 

 repeater. The regulating mechanism uses four small tubes and is 

 added to the normal repeater when desired. The amplifiers shown in 

 Figs. 5 and 6 are regulating. As the cable is underground, the temper- 

 ature changes are very slow and but meagre data on the accuracy of 

 compensation are yet available. 



Terminals 



A schematic diagram of the terminal arrangements for a 240-channel 

 million-cycle system is shown on Fig. 8. In this installation the 

 New York and Philadelphia terminals have each been equipped to 

 handle only 36 two-way telephone conversations. As has been pointed 

 out, the scheme employed involves two steps of modulation, the first 

 of which is used to set up a 12-channel group in the frequency range 

 from 60 to 108 kc. Three such groups have been provided in this 

 installation. In order to transmit at the higher frequencies, a second 

 step of modulation is used in which an entire 12-channel group is 

 moved to the desired frequency location by a "group" modulator. 

 Six such group modulators have been provided at various frequencies 

 throughout the range, including both the top and bottom. Patching 

 facilities have been provided so that any 12-channel group may be 

 transmitted over any one of the high-frequency paths. A typical 

 frequency characteristic of one of the channels is shown in Fig. 9. It 

 may be observed that relatively high quality has been obtained, due 

 largely to the use of quartz crystal electric wave filters, even though 

 the channels are spaced throughout the frequency range at 4000-cycle 

 intervals. "^ 



Preliminary Tests 



As already noted, various long circuits have been built up by 



looping back and forth through the coaxial system. One setup over 



which conversations were successfully carried out consisted of five 



voice-frequency links in tandem, each link being 760 miles long, giving 



a total circuit length of 3800 miles. This setup included, in each 



direction, seventy stages of modulation and the equivalent of 400 line 



amplifiers, the transmission passing twenty times through each one of 



the twenty one-way line amplifiers constituting the ten two-way 



repeaters. 



2 "Stabilized Feedback Amplifiers" by H. S. Black, Bell Sys. Tech. Jour., January, 

 1934. 



