630 



BELL SYSTEM TECHNICAL JOURNAL 



A certain combination which consists of certain sequences of the 600 

 and 1500-cycIe tones is assigned to each boat. The 600 and 1500- 

 cycle tones selected by band-pass filters are rectified and actuate a 

 polar relay which then delivers to the part marked "selector" signals 

 corresponding to those made by the telephone dial. The selector is a 

 standard train dispatcher selector which can be set for various com- 

 binations of signals and when the correct combination arrives it will 

 close a switch and ring the bell. 



For calling the shore operator from the ship, the Codan mentioned 

 previously connects the receiver to the line and also operates a relay 

 to light the shore operator's switchboard light whenever a boat operator 

 starts his transmitter and puts on his carrier. 



OSCILLATORS 



LU—m 



600 

 CYCLES 



1500 

 CYCLES 



RADIO 

 TRANS- 

 MITTER 



BAND- PASS 

 FILTERS 



RADIO 

 RECEIVER 



:n: 



RECTIFIERS 



TELEPHONE 

 DIAL 



Fig. 12 — Calling system for boats. 



These two methods of calling in the two directions are not the only 

 ones which are used on radio links. For most transoceanic and service 

 to the large liners, prearranged schedules or continuous watch allow 

 of calling by voice. In the Green Harbor- Provincetown circuit of 

 Fig. 3 and some transoceanic circuits calling is accomplished by 

 transmitting 1000 cycles interrupted 20 times a second which is a 

 standard means of ringing over telephone lines. 



Referring again to Fig. 5, note the third diagram and the element 

 marked "SSBTR" ^ which means single sideband transmitter. Single 

 sideband transmission has been used on transoceanic radio telephone 

 circuits since the first circuit was opened. It is not used in broad- 

 casting, at least in this country, although it has been proposed a 

 number of times. Single sideband communication was first used on 

 wire line carrier circuits with their inception in 1918. It was utilized 

 when the long-wave radio circuit was tested experimentally in 1923 



