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BELL SYSTEM TECHNICAL JOURNAL 



struction was to provide larger conductors. With the development of 

 the vacuum tube, high amplification became available as an alternative 

 method. Since increasing the size of the conductor in order to decrease 

 attentuation involves large expense, we are naturally led to consider 

 the use of as much amplification as possible. 



Before referring further to the utilization of high amplification, 

 I wish to point out that at the present time for distances greater than 

 about 150 miles (240 km.) in cable, we utilize the so-called 4-wire 

 method to obtain two-way telephone transmission ; that is, transmission 

 in one direction is carried by one pair of wires, and transmission in 

 the other direction is carried by a second pair of wires. What is in 

 effect the same method is employed in our present carrier systems, 

 transmission in one direction being superposed on one frequency, and 

 transmission in the other direction being superposed on a different 

 frequency. 



l-WAY 

 REPEATER 



STATION 

 A 



Mm 



X 



2 





NET- 

 WORK 



NET- 

 WORK 





X 



X 



X 



mw 



Fig. 7 — Block schematic of a four-wire circuit showing two two-wire circuits with 



one-way repeaters. 



Figure 7 shows diagrammatically a 4-wire telephone circuit in cable. 

 You will note that one-way amplifiers are introduced in each pair of 

 wires at points which in present practice are about fifty miles (80 km.) 

 apart. The question naturally arises: Why not increase the distance 

 between amplifiers and at the same time increase their amplification, 

 and so reduce the cost? The answer is that two sources of noise 

 disturbance have to be considered: first, induction from neighboring 

 circuits; second, the noise of thermal agitation. 



The line circuit, depending in degree upon the type of construction, 

 receives unwanted interference from the outside, such as induced 

 currents from power lines, lightning, and crosstalk from adjacent 

 circuits, and it is not possible, as a result, to allow the transmitted 

 speech signals to be attenuated below a certain level with respect to 

 such noise interference. As a consequence, the amount which we can 

 allow a speech signal to be attenuated before it reaches an amplifier, 



