LONG DISTANCE TELEPHONE CIRCUITS 529 



Control of Delay Distortion 



In the case of two-wire circuits, since only relatively short distances 

 are involved, delay distortion does not enter as a design factor. In 

 the case of long four-wire circuits delay distortion is very important 

 and to avoid this, very light weight and high cutoff loadings are used. 

 Delay distortion may be reduced by employing correcting networks 

 which introduce distortion counter to that introduced by the line. 

 Such networks, in addition to their cost, have the disadvantage that 

 they increase the total delay of the circuit. Their use is not necessary 

 for the lengths of circuits and types of construction now used for 

 message telephone circuits in the United States. 



Performance Characteristics 

 Minimum Working Net Loss 



In determining the numbers of circuits of different types of con- 

 struction to be provided in a proposed cable for long distance work, 

 it is necessary to take into account the limiting lengths for which the 

 different types of circuit will meet the transmission requirements 

 established for different conditions. This limitation for repeatered 

 circuits is not set by attenuation but rather by singing margin, crosstalk 

 or echoes. The lowest net loss at which a circuit equipped with re- 

 peaters may be operated without passing the limiting requirements 

 for any of these characteristics is called the minimum net loss of the 

 circuit. Since it is desirable to keep the net loss of a circuit at any 

 time at least as great as the minimum net loss, an allowance for the 

 probable circuit variations is added to the minimum net loss to obtain 

 the minimum working net loss. This minimum working net loss is in 

 general not directly proportional to the length of the circuit although 

 in some cases within the important range of distances it can be con- 

 sidered to be proportional to a sufficient degree of accuracy. 



It is evident that the minimum working net loss is a characteristic 

 of fundamental importance in the design of new toll cables as well as 

 in determining the operating limitations of existing cable circuits. 

 Circuits having the same type of loading and the same spacing of 

 repeaters may have widely different minimum net losses. This can 

 come about because of differences in the accuracy with which crosstalk 

 coupling is reduced in the manufacture and installation of the cable 

 and its associated loading coils and apparatus, differences in the 

 degree of uniformity of these characteristics and differences in the 

 perfection of balance and matching of impedance between the cable 

 conductors and the associated equipment. In the United States the 

 study of the crosstalk, impedance and echo results obtained in various 

 toll cables has led to the development of standard practices as to 



