82 BELL SYSTEM TECHNICAL JOURNAL 



General Considerations 



The type K system, the elements of which are illustrated schematic- 

 ally in Fig. 1, operates on a "four- wire" basis, using the same frequency 

 range, but different electrical paths, for opposite directions of trans- 

 mission. Thus it differs from open-wire carrier systems, for which the 

 line is not suitable for four-wire operation, and which therefore require 

 complicated and expensive filters to separate the different frequency 

 bands used for transmission in opposite directions. A high degree of 

 shielding between the two cable paths is necessary to avoid the effects 

 of near-end crosstalk, which would be serious because of the large 

 level differences existing at the repeaters and the terminals. On 

 routes where two or more cables exist, such shielding is obtained by 

 employing two separate cables, with transmission in one direction 

 only, in each section of cable. On single-cable routes, a similar ar- 

 rangement is obtained by adding a small cable. Where there is no 

 cable, two small cables may be provided. Also, satisfactory shielding 

 between the carrier pairs used for opposite directions of transmission 

 has been obtained in short experimental lengths of cable by the use of 

 a layer shield. 



Frequency Allocation 



In contrast to the original Morristown system, which gave nine 

 one-way channels per pair in the range from 4 to 40 kilocycles, the 

 type K system has twelve channels in the range from 12 to 60 kilo- 

 cycles. As shown in Fig. 2, the frequency range of the type K system 

 is roughly double that of preceding open-wire carrier systems.^ The 

 choice of 12 and 60 kilocycles as the lower and upper frequency limits 

 was governed by economic considerations, and there is nothing tech- 

 nically insurmountable either in going to considerably higher fre- 

 quencies or in utilizing the lower frequency range, which is now idle 

 except for the use of the d-c. path for purposes of transmission regu- 

 lation and fault location. Important factors influencing the selection 

 of the upper frequency are the crosstalk, which depends on the number 

 of pairs utilized for carrier in one cable and the extent to which special 

 crosstalk balancing means are used, and the attenuation, which largely 

 controls the spacing between repeaters. Factors affecting the lower 

 limit include the difficulty of maintaining accurate transmission 

 regulation over the whole frequency range, and the design o( the 

 repeater, which becomes harder as the ratio of maximum to minimum 

 transmitted frequency is increased. 



The frequency range between 12 and 60 kilocycles accommodates 

 12 speech channels, each occupying a gross band of 4 kilocycles. The 



