Selective calling arrangement

In a selective calling system of the type wherein a fixed transmitter transmits a binary-encoded calling signal to a plurality of portable receivers, said calling signal comprising radio-frequency pulses of uniform width separated by first and second intervals respectively representative of the two binary states in said calling signal, and wherein each said receiver comprises means for receiving the encoded pulses transmitted by said transmitter, comprising an RF demodulator for generating DC pulses corresponding to said radio-frequency pulses, a first timing circuit triggered in dependence of said DC pulses, the holding time of said first timing circuit being intermediate said first and said second pulse intervals and being restored with every triggering pulse, means to detect the binary value of each interval by detecting the state of said first timing circuit at the end of said interval, a memory for storing the binary signal uniquely assigned to call said receiver, means for comparing said detected binary signal with said stored binary signal and for generating an alarm signal if there is a correspondence between said two binary signals and means for generating a reset signal for restoring said comparing means if there is no generation of DC pulses during a certain time interval, the improved receiver which comprises:

• a first differentiating circuit connected to the output of said demodulator for generating a train of short pulses corresponding to the leading edges of said DC pulses;
• a second timing circuit having a triggering input connected to the output of said first differentiating circuit, said second timing circuit, when triggered, generating a pulse which has a duration substantially equal to the duration of each of said pulses of uniform width, said pulse duration is unaffected by repeated triggering of said second timing during the generation of said pulse, said pulse exclusively determining the triggering and the moment of detecting the state of said first timing circuit.