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BELL SYSTEM TECHNICAL JOURNAL 



From here the currents leave for Key West by way of the underground 

 cable, the cable hut, and the submarine cable itself. 



The highest frequency channel is attenuated somewhat over 80 db 

 by the time it reaches the receiving apparatus at Key West. Here it 

 is stepped up about 70 db by the amplifiers, from the output of which 

 it suffers a few db loss in the demodulator circuit and 9 db loss in the 

 receiving attenuator circuit. If the connection is continued to another 

 toll office, such as Miami or New York, this attenuator is adjusted to 

 give a 9 db loss between the hybrid coil at Key West and the receiving 

 toll switchboard in question. 



KEY WEST 



o-^^WP— I I I I — Q^Q"^"*^^^"^"^ 



ID u. < c: < Z ax < nj, 



Fig. 11 — Relative transmission levels in circuit from Havana to Key West. 



The problem of satisfactory transmission of the speech currents in 

 this path involves several specially critical points. The transmitting 

 amplifier must amplify to the high levels required without modulating 

 sufficiently to produce troublesome new frequencies falling within its 

 own group of frequency bands. New frequencies produced by the 

 amplifier and falling outside this group are suppressed by the direc- 

 tional filters and cause no trouble. Additional possible sources of 

 modulation are the directional filters and impedance matching trans- 

 formers. They must transmit the high level currents coming from the 

 amplifier but must not modulate them sufficiently to produce trouble- 

 some new frequencies falling within the oppositely directed group of 

 bands. The latter are at an exceedingly low level and so the modula- 



