674 



BELL SYSTEM TECHNICAL JOURNAL 



employs a single sideband with suppressed carrier, using a copper-oxide 

 modulator associated with a quartz crystal channel filter. The 

 terminal apparatus required for two-way transmission over a two- 

 path circuit is shown diagrammatically on the left-hand side of Fig. 16. 



OTHER 

 TRANSMITTING 



CHANNEL 



BAND FILTERS 



(64-108 KC) 



OF THIS GROUP 



CHANNEL 

 MODULATOR 



68-72 I--* 

 KC ^-r 



64-68 r-t 

 KC Y-*- 



TELEPHONE 

 SET ■ 



HYBRID 



■^m fW^- NET- 



COIL 



60-64 

 KC 



TRANSMITTING 



CHANNEL 



BAND FILTER 



^1^ 



DEMODULATOR 

 AMPLIFIER 



M 



RECEIVING 



CHANNEL 



BAND FILTER 



60-64 

 KC 



CHANNEL 

 DEMODULATOR 



OTHER 



RECEIVING 



CHANNEL 



BAND FILTERS 



(64-108 KC) 



OF THIS GROUP 



GROUP 

 MODULATOR 



60-108 

 KC 



972- 

 1020 

 KC 



I f-; 876- 

 t-^-l924KC 



I 1 —.' 



I ♦•-I 924- 1 

 |"}-(972KC| 



I I ' "■- 



) I 



[ I TRANS- 



I I MITTING 



I 'AMPLIFIER 



I I 

 -4-1. 



OTHER 



[transmitting 



^ GROUP 

 BAND FILTERS 



1080 

 KC 



TRANSMITTING 



GROUP 

 BAND FILTER 



RECEIVING 



GROUP 

 BAND FILTER 



EAST 



nf 



COAXIAL LINE 

 60-1020 KC 



COAXIAL LINE 

 60-1020 KC 



60-108 

 KC 



972- 

 1020 

 KC 



I GROUP 

 I DEMODULATOR 



64-68 I- -♦ 

 KC h-i- 



68-72 I--* 

 KC 1—1- 



r^ 



I RECEIVING 

 I AMPLIFIER 



f-i 972- : 



f-t-]924KC; 



f-i 924- ; 

 f-T-|876KC! 



WEST 



OTHER 

 RECEIVING 

 GROUP 

 BAND FILTERS 



Fig. 16— Schematic of four-wire circuit employing two steps of modulation. 



A frequency allocation which has been used for experimental pur- 

 poses employs carriers from 64 to 108 kilocycles for the initial step of 

 modulation. The lower sidebands are selected and placed side by 

 side in the range from 60 to 108 kilocycles, as illustrated in Fig. 17, 

 forming a group of 12 channels. 



Double Modulation 

 In order to extend the frequency range of a system to accommodate 

 a very large number of channels, it appears to be more economical to 

 add a second step of modulation rather than carry the individual 

 channel modulation up to higher frequencies. Such a second step of 

 modulation has been used experimentally to translate the initial group 

 of 12 channels en bloc from the range 60 to 108 kilocycles up to higher 

 frequencies. It is possible to place such groups of channels one above 

 another as illustrated in the upper part of the diagram of Fig. 18, up 



