ADVANCES IN- CARRIER TELEGRAPH TRANSMISSION 167 



however, all the frequencies are passed through a modulator, where 

 they are transposed as a group to a position above those of system No. 

 2. The lower sideband of the secondary carrier is used, so that the 

 order of the channels is reversed on the line. After modulation, this 



^>^>^>-' 



H44 CIRCUIT 

 WITH MODIFIED REPEATERS 



LOW- 

 •— I PASS 

 FILTER 



WEST TERMINAL 



SECONDARY 

 CARRIER 



(A) 



z o 



- UJ 



■>-in 



4760 SUPPRESSED 

 4335 



EAST TERMINAL 



SECONDARY 

 CARRIER 

 4760 »- 



-r''229b 



_246_5_ 

 2295 



UJ 



UJ 



iL 42511 

 NO.I 



N425 



NO.I 



VOICE-FREQUENCY 

 WEST TERMINAL 



LINE 

 (B) 



VOICE-FREQUENCY 

 EAST TERMINAL 



Fig. 2 — Double-modulation telegraph system. A. Block diagram for one direction 

 of transmission. B. Frequency relations at terminals and on the line. 



group of frequencies passes through a sending band filter, which 

 elminates all the unwanted frequencies, thereby preventing useless 

 overloading of the repeaters and the creation of undesired modulation 

 products therein. The two groups of frequencies pass through com- 

 mon repeaters over the modified H44 circuit and are then separated at 

 the receiving terminal by a combination of filters similar to the one at 

 the transmitting end of the circuit. The signals pertaining to voice- 

 frequency system No. 1 are next demodulated by a secondary carrier 

 having the same frequency as that used at the sending end and thereby 

 reduced to a frequency range adaptable to the standard terminal 

 equipment. The modulators and demodulators were provided with 

 separate oscillators at both ends of the circuit. 



Both modulators and demodulators were of the push-pull type and 

 were arranged as grid-current modulators ^^ instead of as plate-current 



