124 BELL SYSTEM TECHNICAL JOURNAL 



harmonic, that is, 484 kilocycles, are obtained in this way from the 

 carrier supply system. Because of the importance of the carrier 

 supply two sources are provided, with automatic equipment to transfer 

 rapidly from the regular to the emergency source. 



Group Modulation 



As shown in Fig. 1, the type J system uses a band of 36 to 84 kilo- 

 cycles for the west to east direction of transmission and 92 to 140 kilo- 

 cycles for the east to west direction. The output of the fundamental 

 twelve-channel unit consists of twelve lower sidebands from carriers of 

 64-108 kilocycles. This must, therefore, be translated to the two type 

 J directional groups for line transmission. Since the frequencies 

 in the fundamental unit overlap those in both directions of line 

 transmission, this transfer must be made in two steps. Figure 4 shows 

 these frequency translations. The first group modulation is the same 

 for both directions of transmission. By modulating the fundamental 

 unit with a carrier of 340 kilocycles there is obtained a block of lower 

 sidebands extending from 400 to 448 kilocycles. A second modulation 

 with a 484-kc carrier then gives, for transmission from west to east, 

 a twelve-channel block of upper sidebands extending from 36 to 84 

 kilocycles. For the east to west transmission the second modulation 

 uses a 308-kc carrier, producing a twelve-channel block of lower side- 

 bands between 92 and 140 kilocycles. 



Frequencies as high as 308, 340 and 484 kilocycles are chosen for 

 group modulation in order that undesired products shall be well 

 separated from desired products to permit their elimination by simple 

 filter structures. 



The same group modulation processes that have been described 

 above for adapting the twelve-channel group for line transmission are 

 used in the opposite sequence for receiving the block from the line and 

 preparing it for separation by the channel band filters of the receiving 

 terminal; thus, for instance, at an east terminal the block of upper 

 sidebands, extending from 36 to 84 kilocycles as received from the line, 

 is first modulated with 484 kilocycles producing lower sidebands be- 

 tween 400 and 448 kilocycles. These are next modulated with 340 

 kilocycles, which produces a block of twelve lower sidebands extending 

 from 60 to 108 kilocycles, which is the group that the fundamental 

 twelve-channel terminal unit is designed to handle. 



Figure 5 shows the essential features of the group modulating and 

 group demodulating circuits. As in the type K system, group modula- 

 tion is performed at a very low level of the message material and with 

 a high level, about 25 milliwatts, of the group carrier supply, in order 



