Patent GB-686027-A (GB)
Title: Selective calling system for radio receiving apparatus
IPC: H04W88/02
Filed: 1950-05-10
Abstract: 686,027. Electric selective signalling systems. TAYLOR, P. H., and PAMPHONIC REPRODUCERS. Ltd. June 19, 1951 [May 10, 1950], No. 11651/50. Class 40 (i) [Also in Group XL(c)] In a selective calling system comprising a transmitting station and a plurality of receiving stations, calling is effected by transmitting from the transmitter a low frequency wave upon which is superimposed a pulse signal of which the duration is appreciably shorter than the duration of one half-cycle of the low frequency wave, the pulse signal being superimposed at different positions on the low frequency wave depending upon the receiving station to be called, and each receiver comprises a gating device which is opened periodically at a predetermined time with reference to the phase of the low frequency wave, the instants in time at which the gating devices of the different receivers are opened being displaced from one another so that only a pulse signal which is superimposed on the low frequency wave at any instant in time corresponding to that at which the gating device is opened can pass therethrough to actuate a calling device. In a radio system in which the transmitter and receivers operate on the same wave-length the transmitter is provided with a 10 c/s RC oscillator 2, 3 Fig. 1, whose frequency is controlled by the variable resistor 4. The 10 c/s oscillations are applied to the control grids of cathodefollower valves 6,7, through a phase shift network 8 such that the oscillations applied to the grids are 90 degrees out of phase with respect to each other; and the cathode circuits comprise coils 9 and 10 of a goniometer type phase shifting device, whose pickup coil 11 is connected to a.triode amplifier circuit 13. The output from the valve 13 is fed to a squaring device comprising biased rectifiers 15 and is differentiated by the short time constant resistance-capacity network 17, whose output is amplified and the negative pulses suppressed by the valve 19, which is cut off on the negative pulses being applied. The output of a 10 kc/s oscillator 24 is supplied via a tuned pr.imary. transformer 27 to the control grids of the push-pull. connected valves 21, 22, which are normally non- conducting to form a gate circuit which is opened by the short duration pulses at a repetition frequency of 10 c/s supplied from the amplifier 19. The output of the 10 c/s oscillator 2, 3, is combined via conductor 29 with the output of the gate circuit 21, 22, to produce a composite wave having one burst of 10 kc/s oscillation positioned at each cycle of 10 c/s oscillation, which is fed to the modulator of the transmitter to produce the modulated carrier. At the receiver, Fig. 2 the output from the demodulator is fed through an amplifier 32a, 32b, to a filter circuit, the 10 kc/s signal being supplied to the tuned primary of the transformer 33, and the 10 c/s signal being fed to a valve 31 connected as a phase splitter, outputs being taken from the anode and cathode and 'through a changeover switch 40 to a resistancecapacity circuit 38 providing a continuous phase variation through 360 degrees which is adjusted to correspond with the phase shift at the transmitter. The 10 c/s phase shifted signal is amplified at 42, squared in the biased rectifier circuit 44, differentiated at 46, and applied to the valve 47 which is cut-off by the negative pulses, to provide a train of short positive pulses at a repetition frequency of 10 c/s. These pulses are applied to the gating circuit 49, 50, so that the valves only conduct when a 10 c/s pulse is applied to allow the coincident 10 kc/s burst to pass through the gating circuit of the one receiver on the system which is correspondingly adjusted. These 10 kc/s  bursts are applied through a tuned transformer to an integrating circuit comprising a diode 55 and capacitor 56. In order to prevent noise actuating the calling bell, the capacitor 56 only attains a sufficient voltage to trigger the thyratron 57 after a predetermined number of pulses. When the valve fires if energizes a relay coil 58, and contacts A1 close and actuate an alarm device. Contacts A2 short circuit the A.V.C. network when the receiver is awaiting a call, to prevent distortion of the incoming 10 c/s signal, the hold-on contacts A3 simultaneously disconnect the valve 57 from the H.T. supply, and contacts A4 are connected across the capacitor 56 to discharge it ready for the next call. A switch 59 is provided for deenergizing the relay when the warning signal has been noted. To ensure that the gate-opening pulse does not drive the grids of valves 49 'and 50 positive a limiter 60 is provided. A number of phaseshifting devices may be provided at the transmitter so that a number of high frequency bursts can be simultaneously superimposed on the low frequency reference wave enabling a group of receivers to be called simultaneously, or the 10 kc/s signal may be continously transmitted to call all the receivers at the same time. The system may also be used for selectively calling receiver stations over a line, in which case the carrier wave could be omitted.