LONG DISTANCE TELEPHONE CIRCUITS 



525 



underground transmission regulators will function satisfactorily from 

 the electrical standpoint spaced as far apart as 300 miles (500 kilo- 

 meters) although circuit flexibility considerations usually call for 

 breaking circuits up into shorter regulator sections. In general, 

 regulators are provided for all circuits in underground cable in excess 

 of approximately 180 miles (300 kilometers) in length. 



TABLE I 



Characteristics of 19-Gauge Repeatered Toll Cable Circuits 



Gains of Repeaters 



In four-wire circuits the spacing of the repeaters and their gains 

 depends largely on "one-way circuit" considerations. The maximum 

 levels are fixed by the repeater and loading coil capacities to handle 

 speech waves without distortion, the lower levels being fixed primarily 

 by noise. The lower levels also depend somewhat on crosstalk con- 

 siderations, particularly crosstalk between circuits transmitting in 

 opposite directions but this crosstalk is usually not controlling. The 

 upper level limit used is + 10 db while the lower is — 24 db, these 

 being referred to the level at the transmitting end of the toll circuit 

 as zero. 



In two-wire circuits, the above "one-way circuit" considerations 

 are unimportant in determining the levels, these being largely fixed 



