AN IMPROVED THREE-CHANNEL CARRIER TELEPHONE SYSTEM 57 



The pilot current is also used to indicate through an audible or 

 visual alarm any trouble which results in large sudden changes in 

 transmission such as would be occasioned by an open or short circuit 

 on the line itself. 



The ability of the regulating mechanism to stabilize the transmission 

 over the system is shown on Fig. 5 which shows the deviations recorded 

 in daily measurements on all three channels of a 250-mile system over 

 a period of one month. The actual changes in line loss at the pilot 

 frequency are also shown for comparison. During this period various 

 conditions of temperature, rain and fog were experienced. 



With the transmitting level that has been provided and for ordinary 

 line conditions it is found practicable to employ repeater spacings of 

 from 125 miles to over 250 miles. The exact distance in any par- 

 ticular case depends upon many factors, such as: wire size, length of 

 toll entrance or intermediate cables, location of existing offices and 

 the susceptibility of the line to sleet or frost. Where this latter 

 condition is prevalent conservative spacings are desirable. 



"- pPj^l — \W-' 



-Wv- 



TO 



MODULATOR 



BAND 



FILTtR 



OSCILLATOR 



7- 



I I 



\ ^ / 



TO 

 HEATER 

 CURRENT 

 SUPPLY 



Fig. 7 — Schematic of modulator. 



Terminals 



The general theory of operation of the terminal may be understood 



from the block diagram shown in Fig. 3. On the voice-frequency side 



each channel terminates as a four-wire circuit. The input to the 



carrier system from the voice circuit is designed to operate at a level 



