676 BELL SYSTEM TECHNICAL JOURNAL 



to about 1000 kilocycles, wasting no frequency space between groups 

 and thus keeping the channels spaced at intervals of 4 kilocycles 

 throughout the entire range. 



The apparatus required for this purpose is shown schematically in 

 Fig. 16, which illustrates the complete terminal arrangements for a 

 single channel employing double modulation. The figure indicates by 

 dottled lines where the other channels and groups of channels are con- 

 nected to the system. 



A modulator for shifting the frequency position of a group of chan- 

 nels inherently yields many different modulation products as a result 

 of the intermodulation of the signal frequencies with the carrier fre- 

 quency and/or with one another. Out of these products only the 

 "group sideband " is desired. The number of the modulation products 

 resulting merely from the lower ordered terms of the modulator re- 

 sponse characteristic is extremely large. All such products must be 

 considered from the standpoint of interference either with the group 

 which is wanted in the output or with other groups to be transmitted 

 over the system. Various expedients may be used to avoid inter- 

 ference as follows: (1) A proper choice of frequency allocation will 

 place the undesired modulation products in the least objectionable 

 location with respect to the wanted signal bands; (2) a high ratio of 

 carrier to signal will minimize all products involving only the signal 

 frequencies; (3) the use of a balanced modulator will materially reduce 

 all products involving the second order of the signal ; (4) selectivity in 

 the group filters will tend to eliminate all products removed some dis- 

 tance from the wanted signal group. Giving due regard to these 

 factors, balanced vacuum tube group modulators have been developed 

 which are satisfactory for the frequency allocations employed. 



Triple Modulation 

 For systems involving frequencies higher than about 1000 kilo- 

 cycles it may be desirable to introduce a third step of modulation. 

 In some experiments along this line a "super-group" of 60 channels, 

 or five 12-channel groups, has been chosen. The lower part of Fig. 

 18 illustrates, for a triple modulation system, the shifting of super- 

 groups of 60 channels each to the line frequency position. This 

 method has been employed experimentally up to about 5,000 kilo- 

 cycles. It is of interest to note that even in extending these systems 

 to such high frequencies, channels are placed side by side at intervals 

 of 4000 cycles to form a practically continuous useful band for trans- 

 mission over the line. 



