Radio-telephone audio coded selective calling system - has each receiver with tuned circuits and level discriminators on input to microprocessor for individual code recognition

摘要

The selective calling system for a radio-telephone system uses calling frequencies within the audio band to represents numbers. The audio frequencies modulate HF carriers for transmission and activate resonant circuits in the receive equipment. The calling frequencies must be sustained for a 70 ms period to enable the receive resonant circuit to respond and transmit sufficient level for the microprocessor controlled signalling receiver to accept the digit. The total time for signalling a fire digit call number is 350 ms. The signal incoming to the receiver is fed to all resonant circuits (2) in that receiver. Each parallel resonant circuit arm (A, B, C etc.) has a level discriminator (3) on the output and each individual output is separately fed into the microprocessor (4) for code recognition or rejection.