A TWELVE-CHANNEL CARRIER TELEPHONE SYSTEM 121 



available to add to the type C system, thus giving sixteen telephone 

 circuits on an open-wire pair in addition to the two telegraph circuits. 

 Since there are already about 60,000 miles of pole line equipped with 

 type C systems, the new type J system was developed to go in the 

 frequency range above the type C system rather than to supersede it 

 with more channels (Fig. 1). 



The new system has been designed to meet high standards of trans- 

 mission and reliability for distances up to several thousand miles. 

 The frequency band transmitted by the individual derived circuits is 

 exceptionally wide, from about 100 to 3600 cycles for a single system 

 and has been previously discussed ^ in relation to the channel spacing 

 in this and other new broad-band devefopments. 



An important feature of the work on the type J system has naturally 

 been that of making the line circuits suitable for carrying the higher 

 frequencies. The tendency of circuits to crosstalk into one another 

 increases rapidly with frequency. Advances in transposition design 

 and structural improvements have now made it possible to extend the 

 frequency range from 30,000 cycles to 140,000 cycles, which is about 

 the upper frequency of the type J system. The problem of incidental 

 cables in open-wire lines has also been serious, since the losses increase 

 with frequency, and what is usually more important, there may be sub- 

 stantial reflection effects at junctions of the open-wire line and cable. 

 These are serious, not only from the standpoint of the transmission 

 loss which they entail, but from their effect on crosstalk. The in- 

 crease in attenuation at the higher frequencies has also brought other 

 problems into the picture. For example, repeaters are needed at 

 more frequent intervals than with the lower frequency systems. 

 Attenuation variation with frequency due to weather changes is 

 greater than at the lower frequencies. 



Figure 2 shows schematically the complete type J system, with its 

 different major circuit elements, resulting at the terminals in the 

 division of the single line circuit effectively into sixteen talking circuits. 

 In no recent development is the function of the wave filter in providing 

 essential units in a frequency dividing plan more forcefully illustrated 

 than in the application of this new system, in combination with the 

 type C and other facilities which exist. There are about sixty different 

 designs of filters and networks in the terminals and repeaters. Their 

 functions are varied, — as, for example, separating the individual 

 channel bands, separating the opposite directional groups of channels, 

 separating the type J frequency range as shown in Fig. 1 from the 

 type C and other ranges, separating the different carrier frequencies 



^"Transmitted Frequency Range for Circuits in Broad Band Systems," H. A. 

 Affel, Bell System Technical Journal, October 1937. 



