CARRIER SYSTEMS ON TELEPHONE LINES 567 



are spaced at intervals of 150 to 300 miles depending upon particular 

 transmission considerations. By means of a pilot channel, stability of 

 transmission over the several carrier channels is assured, despite the 

 relatively large inherent variations in high-frequency line transmission 

 due to weather changes. 



The service requirements which present themselves in the application 

 of carrier methods are, of course, basically no different from those 

 for commercial talking circuits obtained by other means. The problem 

 is to establish a toll circuit between long distance offices which meets 

 certain standards of transmission, including speech volume, stability 

 and quality. The latter requires that there must be transmitted a 

 certain band width of frequencies in the voice range. Furthermore, 

 there must exist no appreciable load distortion effects. The circuit 

 must also be relatively free from noise or crosstalk. A signaling 

 system must be provided so that the operators at opposite terminals 

 may call each other. In other respects the system must appear as a 

 normal telephone circuit not distinguishable from an operating stand- 

 point from the other circuits afforded by metallic wire connections. 

 The apparatus installed in the telephone office must conform to certain 

 physical standards of equipment, ruggedness, flexibility, etc. It must 

 be capable of being maintained by trained office forces. Testing 

 facilities must be provided, etc. It is believed that these objectives 

 have been largely realized in the arrangements which are described 

 in this paper. 



The Type "C" System 



The type "C" system embodies those major technical features 

 which our experience with the older systems has indicated as most 

 desirable. It is a carrier-suppressed, single sideband system, in which 

 respect it is similar to the older type "A" system. However, it has 

 been found possible to dispense with the equal frequency spacing of 

 the channels which was characteristic of the type "A" system, and 

 which involved the transmission of a synchronizing current between 

 two terminals and the harmonic generation of higher frequencies from 

 this synchronizing current. A simplification in apparatus has resulted. 

 This non-harmonic arrangement of channels has further made possible 

 a more efficient use of the frequency spectrum by the fact that the 

 channel bands at lower frequencies can be squeezed together more 

 closely than those of the higher frequencies where the band filters are 

 less efficient due to decreasing ratio of band width to frequency. 



The type "C" system requires for each modulator an oscillator as 



a source of carrier supply. Moreover, since a synchronizing current 



is not employed at the receiving terminal of the channel, an oscillator 

 2,7 



