Patent GB-919172-A (GB)
IPC: H04W88/18
Abstract: 919,172. Radio signalling. PHILIPS ELECTRICAL INDUSTRIES Ltd. April 20, 1960 [April 23, 1959], No. 13833/60. Class 40 (5). In a system for transmitting a coded information signal by radio between a transmitter and one of a number of co-operating receivers, calling is effected by a combination of at least two calling-frequencies particular to the selected receiver which are transmitted as modulations of a carrier wave, and after reception of the calling signals the receiver is conditioned to receive information signals utilizing the calling frequencies for that receiver. As shown in Fig. 1, at the transmitter the calling signals, coded information signals, and pilot signals are recorded on a punched tape 10 which operates switches 11<SP>1</SP> to 22<SP>1</SP> controlling corresponding low-frequency generators 11 to 22. The outputs of the generators 11 to 22 are supplied via an attenuator 7, adjusted so that each frequency produces a maximum frequency deviation of the carrier, and amplifier 6 to a modulator 3 which frequency modulates a carrier wave generated at 1 which is transmitted in the normal manner. The received signals are supplied to a mixing stage 32, Fig. 2, and via an I.F. amplifier 34, and frequency discriminator 35 to a series of selective tone detectors 36 to 40. All except the first frequency detector 36 are normally inoperative but if the first detector 36 receives the correct frequency a direct voltage is produced in it which releases the next detector and so on. Frequency detectors 36 to 39 respond to the correct succession of calling frequencies and are followed by frequency detector 40 which responds to a pilot frequency and operates a monostable trigger 60 controlling a bi-stable trigger 61. During the time determined by its time constant the trigger 60 opens the gates 56 to 59 and supplies a voltage to hold the frequency detectors simultaneously operative and ready for the reception of coded information signals. As shown, the coded information signal uses up to three different frequencies of the four frequencies used for the calling signal to avoid the possibility of calling up another receiver using as a calling signal the same four frequencies in a different succession. The information signals produce pulses in succession in, say, detectors 39, 36, 37, and via the corresponding gates, bi-stable triggers 49, 46, 47, are operated. The operation of the trigger 61 actuates an acoustical warning device 63 until the user operates a push-button 64 and the trigger 61 is returned to normal. Operation of contact 64 also completes a circuit for indicating lamps 66, 67, 69 corresponding to the operated triggers 46, 47, 49. When only one pilot frequency is used, or if instead of a pilot frequency a time interval is used between each call and the coded information signal and between the information signal and the subsequent call, the information signal frequencies may be transmitted in the same order as the calling frequencies to avoid calling another receiver. The received information signal is stored by the triggers 46 to 49 until a contact 65 is closed by the user, or alternatively it may be arranged that the triggers 46 to 49 are returned when the trigger 60 is operated by the next call. When a second pilot frequency is used at the end of an information signal, a bistable trigger is substituted for the trigger 60 and its return is controlled by a further frequency detector. A field strength indicator is controlled by a cut-off device 71 connected to the discriminator 35 and producing a direct voltage which is amplified at 72 and operates a lamp 73 when a switch 74 is operated. The device 71 is adjusted so that the lamp 73 is operated when the signal to noise ratio exceeds a minimum value below which the receiver is cut off by the device 71. The tone detectors, Fig. 3 (not shown) comprise a tuned circuit whose output is supplied to the base of a normally blocked transistor which is rendered conductive by a forward bias voltage fed as a keying voltage to the base. The collector is coupled to the base of a further transistor whose output is rectified to produce a keying voltage for the following tone detector. Specifications 625,623 and 873,254 are referred to.