122 BELL SYSTEM TECHNICAL JOURNAL 



of a carrier supply system in which the carriers are all derived from a 

 common 4000-cycle source, etc. 



The new system, as in the case of the type C, uses single sideband 

 transmission with carrier elimination. Copper-oxide units are em- 

 ployed as translator elements of various kinds, — modulators, de- 

 modulators, and harmonic producers. Methods of mounting the 

 equipment, and methods and apparatus for testing follow lines already 

 worked out for the type K cable carrier system, which was described a 

 year ago in two A. I. E. E. papers.^ 



Channel Terminals 



A terminal of the type J system changes twelve independent voice 

 channels into a compact block of twelve carrier channels properly 

 allocated in frequency for transmission over the open-wire line. In- 

 versely, such a block received from the open-wire line is separated and 

 transformed into twelve independent voice channels. The first step 

 in transmitting the twelve voice channels is to modulate them on 

 twelve carrier frequencies 4 kilocycles apart from 64 to 108 kilocycles 

 and to select the lower sidebands by means of quartz crystal channel 

 band filters. The last step in the conversion from a received twelve- 

 channel block to the twelve independent voice channels consists in the 

 division of the block by twelve quartz crystal channel filters and the 

 demodulation of these messages to produce voice frequency transmis- 

 sions. These two frequency changes and separations are performed by 

 the same equipment that is used in the type K cable carrier system 

 terminals. 



Figure 3 shows the circuit of a modulator and a demodulator for the 

 opposite directions of a single conversation with indicated connections 

 for the eleven others which make up this fundamental twelve-channel 

 block. The modulator consists of a bridge assembly of copper-oxide 

 varistors and is supplied with about 0.5 milliwatt of carrier power from 

 the carrier supply system which is described later. Of the two result- 

 ing sidebands, the lower is selected by the crystal band filter follow- 

 ing the modulator. The line sides of twelve modulator band filters 

 are joined in parallel and a compensating network is connected to 

 preserve the band characteristics of the upper and lower channels. 



On the receiving side, after separation by one of the twelve parallel 

 filters the sideband is applied to a demodulator supplied with the 



3 "A Carrier Telephone System for Toll Cables," C. W. Green and E. I. Green, 

 Bell System Technical Journal, January 1938 and Electrical Engineering, May 1938. 

 "Cable Carrier Telephone Terminals," R. W. Chesnut, L. M. Ilgenfritz and A. 

 Kenner, Bell System Technical Journal, January 1938 and Electrical Engineering, 

 May 1938. 



