CARRIER SYSTEMS ON TELEPHONE LINES 575 



that for which terminal transmitting apparatus is effective in main- 

 taining the transmission level well above the line noise. The function 

 of the repeater is, therefore, to amplify the carrier currents so that 

 they pass on to the succeeding line section at a magnitude comparable 

 to that sent out from the terminals. Obviously, the design of the 

 repeater with respect to its gain and level carrying capacity, etc., 

 presents a wide range of possibilities depending on the distance of 

 transmission, frequency, etc. 



It has been found most practical to install the repeaters along the 

 route at approximately the spacing of the voice-frequency repeaters 

 on the same wires. This means a spacing of from 150 to 300 miles, 

 and occasionally slightly over 300. To have in the same office both 

 voice-frequency and carrier repeaters reduces the equipment, simplifies 

 the maintenance problem, and makes it possible to use the same 

 sources of power supply. The gain and the load carrying capacity 

 are, therefore, determined by this spacing, the gain being controlled 

 by the attenuation loss between the repeaters, and the load carrying 

 capacity by the output level desired because of noise considerations. 



The higher attenuation of the line in the carrier range of frequencies 

 means that the carrier repeaters must have a maximum gain of 

 approximately four times that of the voice repeaters operated on the 

 same wires. Whereas gains of the order of 8 to 15 TU may be readily 

 supplied by voice repeaters using balance and so-called "two-wire" 

 operation, the 30 to 45 TU gain required by the carrier repeaters 

 necessitates non-balanced or "four-wire" operation or its equivalent, 

 by using different frequencies in opposite directions and directional 

 filters for the prevention of "singing." 



Figure 6 is a schematic diagram of the circuits comprising a typical 

 repeater station including loading, compositing apparatus and line 

 filters. After passing through the high-pass line filter the carrier 

 currents arrive at the high and low group directional filters which 

 distinguish between the oppositely directed currents. These filters 

 are substantially the same as those used for similar purposes at the 

 terminal stations. 



It is, of course, required in the design of the directional filters that 

 in each direction the filters must pass a frequency band sufficient to 

 transmit properly the three carrier channel bands. In addition to 

 this the filters must present a loss outside of the transmission band 

 which is sufficient to prevent the two-way amplifier circuit from 

 "singing." This means that considering the closed loop circuit of 

 the two amplifiers and the four directional filters the attenuation in 

 this loop must be considerably greater than the sum of the gains or 



