SHORT-HAUL CARRIER TELEGRAPH 



683 



100 

 80 



^20 



a. 6 



-24 -22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 



SIGNAL-TO-INTFRFERENCE* RATIO IN DECIBELS 



Fig. 11 — Comparison of amplitude and frequency shift modulation with static 

 interference. 



35 db or more. The telegraph channels are next combined with the voice 

 frequency circuit by means of a 150A filter, and are connected to the 

 composite set and line through the low-pass section of the 121 A (type 

 -C) carrier line filter. As a result of the cut-offs of the 150A high-pass 

 and 121A low-pass filter components, the pass band of the telegraph is 

 about 3.7 to 5.4kc. At the outlying terminal of the open-wire line, the 

 telegraph is separated from the voice and type-C carrier circuits by 

 similar filters and connected to the individual subscriber stations by a 

 branching network and branch lines. 



The typical arrangement on a two-wire circuit in the voice frequency 

 range is sho^\'n in Fig. 13. Six channels are available, using six of the 

 twelve frequency bands for transmission east to west and the other six 

 bands west to east. As in the high frequency case, a branching network 

 and branch lines at the outlying end connect the circuits to the sub- 

 scribers. Fig. 14 shows a layout in which branch lines are connected at 

 intermediate points in the telephone circuit. At these intermediate points 

 the impedance of the branching network is made high, in order to keep 

 the balance at the telephone repeater from being harmed excessively. 

 Though the network attenuates greatly the signals through it, the tele- 

 graph level is usually sufficiently high so that this loss can be tolerated. 



