248 BETJ. SYSTEM TECHNICAL JOURNAL 



New Requirements for Cable Loading Systems. In the I'lrst com- 

 mercial applications of telephone repeaters, the new features in the 

 loading were the improved types of coils already described and the 

 improved precision of spacing the coils. No fundamental changes were 

 made in the loading systems then standard. 



The completion of the development of a satisfactory commercial 

 type of telephone repeater marked the beginning of a long period of 

 experimental work for the purpose of determining the commercial 

 possibilities of the use of repeaters over long cable circuits. When 

 loaded cables of improved impedance regularity became available, 

 long circuits were built up for experimental purposes by looping 

 back and forth. As the length of these circuits was increased, phe- 

 nomena not previously observed in cable circuits became increasingly 

 troublesome, and it became apparent that it would be necessary to 

 develop new loading systems having improved velocity and higher 

 cut-ofT frequency characteristics in order to realize the full possi- 

 bilities of repeaters in extending the range and reducing the cost of 

 long distance telephone service over cables. 



The disturbances above mentioned were found to be due to : 



(a) Echo elTects. 



ih) X^elocity distortion. 



These phenomena originate in the lines themselves and are made more 

 apparent by the amplifying action of the repeaters. They are present 

 in non-repeatered circuits but not to a noticeable degree. It is the 

 combination of the extreme length of the circuit and the use of re- 

 peaters to keep the over-all lo.ss low that makes the disturbances 

 troublesome. 



Echoes. Echoes are due to unbalance currents; i.e., to the reflec- 

 tion of electrical energy at points of impedance irregularity in the 

 circuits. When the circuit is so long that the time of transmission 

 from the point of reflection to the disturbed subscriber is appreciable, 

 there will be echo elTects unless the losses in the circuit are so large 

 as to cause the reflected energy to become inappreciably small. On 

 such circuits it may be necessary to work the repeaters at gains well 

 below those at which "singing" occurs or distortion due to "near 

 singing" is experienced. 



vSince the time of transmission is such an important factor in echo 

 phenomena, reductions in the harmful effects of these disturbances 

 have been obtained in the improved loading systems which have been 

 developed for use on long repeatered circuits, by substantially in- 

 creasing the velocity of transmission. Recently there has become 



