350 



BELL SYSTEM TECHXICAL JOURXAL 



provides a means whereby aiiN- one of about seventy-five radio stations, 

 operating on the same \va\'e length, may be called without signaling 

 the remaining. Obviously this is an important improvement in the 

 radio art for in many cases it permits a radio station operator to 

 pursue other duties which would be impossible if he were retjuired to 

 listen in at all times. 



The engineering problem presented, being remarkably similar to 

 many telephone problems, was solved in a very similar manner. 

 When it is desired to signal a station, an alternating current of a 

 ver>' definite frequency is impressed on the transmitter. This mod- 

 ulates the power radiated similar to the way the undulations of the 

 voice modulate the power when speech is transmitted. The station 

 to be signaled is determined by the code transmitted. This code 

 consists of a definite grouping of dots and spaces and dashes. 



At the receiving station this modulated power is detected in the 

 usual manner and results in an alternating current identical in nature 

 to that used in transmitting the code. A special alternating current 

 relay of high selectivity and sensitivity, in conjuction with a more 

 common direct current relay system, converts the code into a series 

 of direct current impulses. These impulses pass into a selector like 

 that used in common train dispatching circuits. The mechanism 

 of this selector will be unlocked and a local ringing circuit closed 

 if the code is that for which it has been set. Thus it is seen that 

 the code is received by all stations but only one selector of the sys- 

 tem will operate to ring its local annunciator bell. The number of 

 stations which can operate in the same system is determined by the 

 number of possible combinations on the selector. At present this 

 is set at seventy-eight but this may be readily extended to include 

 more than two hundred. 



Because of the high selecti\it>' of the allernating current relay 

 and its associated direct current rela\' system, the apparatus is 

 particularly free from interference such as the operation of nearby 

 spark or I.C.W. Stations. In fact, tests show that the signaling sys- 

 tem will continue to function satisfactorily long after interference 

 is so bad as to make conversation impossible. As designed, the 

 signaling system may be made an integral part of a standard radio 

 system without altering the apparatus already in use. 



