5.W BELL SYSTEM TECHNICAL JOURNAL 



dividually, simultaneously or in groups.^ This signaling system has 

 also been adapted to radio transmission.^ Its use permits broadcast- 

 ing from a central radio transmitting station to police organization 

 districts, patrol boats and automobiles without reciuiring the constant 

 attention of operators at the receiving stations. 



Requirements of the System 



Before describing the system which was finally worked out to meet 

 the requirements of the New York Police Department, it seems best 

 to state the nature of these requirements. For a system employed 

 to handle communications ranging all the way from routine messages 

 between police headquarters and its difTerent outlying police stations 

 and patrols to general alarms for insuring the capture of escaping 

 criminals, absolute reliability and flexibility are of the utmost im- 

 portance. The central station must be able to call the receiving 

 stations individually, collectively, or in a number of designated group 

 combinations corresponding to the police organization districts. To 

 accomplish this result effectively, means must be provided whereby 

 the desired signal may be sent automatically by a simple manual 

 operation. The apparatus for this purpose must be in the form of an 

 attachment which may be used with a standard radio telephone 

 transmitter w'ithout extensive modifications. 



As the receiver will be in operation continuously, the difficult and 

 expensive maintenance of batteries must be avoided by energizing 

 the vacuum tubes from the commercial power supply system. The 

 tuning arrangements of the receiver must be of the simplest possible 

 character and must be capable of being locked to insure that the 

 receiver remains tuned to the transmitting frequency. The selectivity 

 and sensitivity must be sufficient to insure reliable operation under all 

 conditions. The receiver must provide means for listening to all 

 material broadcast by the central broadcasting station but the signaling 

 system should respond only to signals from the transmitter signaling 

 attachment, irrespective of broadcast speech, music and telegraph 

 signals which may involve the same frequencies. Visible indications 

 should be provided to show when the receiver is in operating condi- 

 tion. The receiver should respond to a call from the central station 

 l)y operating another visible indicator, in addition to a bell or other 

 audible signal, if desired. 



'"Modern Methods in Train Dispatching," by J. C. Latham, Electrical Com- 

 munication, Vol. Ill, No. 1, July, 1924. 



-"Radio Telephone Signaling Low-Frequency System," by C. S. Demarest, 

 M. L. Almquist and L. M. Clement, Journal of the A. I. E. E., Vol. XLIII, No. 3 

 March, 1924. 



