×

LOW FREQUENCY SIGNAL TRANSMISSION AND INDICATING SYSTEM

  • US 3,818,345 A
  • Filed: 12/29/1972
  • Issued: 06/18/1974
  • Est. Priority Date: 12/29/1972
  • Status: Expired due to Term
First Claim
Patent Images

1. A low frequency signal transmission and indication system comprising one or more portable radio transmitters each transmitting a modulated wave signal having the same carrier and obtained by modulating this carrier by a respectively different low frequency signal which is assigned to each transmitter, one or more radio receivers each removing said low frequency signal from said modulated wave when it receives said modulated wave, one or more control center transmitters each transmitting a modulated wave signal having the same carrier and obtained by modulating this carrier by one of a plurality of signals which respectively differ from said low frequency signals assigned to each transmitter by a constant frequency fO, one or more receivers assigned to respective geographical areas removing the low frequency signal from said modulated wave signal transmitted from said center transmitter, means for detecting the difference between the low frequency signal from said radio receiver and the low frequency signal from said geographically assigned receiver, one or more transmitters assigned to respective geographical areas each transmitting a modulated wave signal having the same carrier and obtained by modulating said carrier by another respectively different low frequency signal only when said detected difference in frequency is identical with the constant frequency fO, and a control center receiver receiving the signal from said geographically assigned transmitter, removing the low frequency signal contained in this signal and activating an indication circuit corresponding to this low frequency signal, whereby, when a carrier modulated by a selected specific low frequency signal is transmitted from said center transmitter, only a geographically assigned transmitter connected to a radio receiver which has received from one of said portable radio transmitters a wave modulated by the low frequency signal which is different from this selected specific low frequency signal by the frequency fO is activated to inform that this particular portable radio transmitter is in the vicinity of said particular geographically assigned receiver.

View all claims
  • 0 Assignments
Timeline View
Assignment View
    ×
    ×