Patent GB-670703-A (GB)
Title: Improvements in or relating to a selective control system
IPC: H04W88/02
Filed: 1948-11-25
Abstract: 670,703. Intercommunication systems. STEVENS, A. H. (Motorola, Inc.). Nov. 25, 1948, No. 30607/48. Class 40 (iv). [Also in Group XXXVIII] In a selective control system, a control signal comprising two or more different frequencies is selected and transmitted together with audio signals over a line or radio link to a plurality of receivers, in each of which a selective calling system responsive to a predetermined frequency combination is operated to charge a condenser from a direct voltage source (which may comprise a tuned rectifier circuit responsive to one of the control frequencies) to electronically energize a relay which connects the reproducer of the selected receiver to the audio signals or operates an alarm or control circuit. The combination of plural frequencies in the control signal confers immunity from erroneous operation due to noise or interference. In Fig. 1 a radio communication system comprises a transmitter 12 which is amplitude, pulse, angularly, or frequency modulated by audio signals from a microphone 10 and by control signals selected by a switch 13 from a plurality of signal generators 14, 15, each comprising two or more frequency generators 16 and a mixer-amplifier 17 modulating the transmitter. Plural receivers 19, 20 each comprise an R.F. amplifier, a detector or discriminator 23, and an audio amplifier 24 supplying a sound reproducer 25, and a selective calling system 26 responsive to the predetermined frequency combination corresponding to the receiver, which selectively connects the reproducer to the audio amplifier. Thus audio signals from the transmitter are reproduced only by the receiver corresponding to the accompanying control signal. In Fig. 2 (not shown), the transmitter and receivers are connected over a line link. Fig. 3 shows a selective calling system for such systems wherein the transformer 30 connected, e.g., to the output of a receiver has plural secondary windings of which 32 is connected to a reproducer 35 through the normally open contacts of a relay 37 in the cathode circuit of a valve 45. Winding 34 supplies a tuned filter-rectifier unit 50 which develops a direct voltage across the terminals 43, and a winding 33 energizes the winding 38 of a tuned electromechanical contactor 39 to intermittently close a contact, bridged by a condenser and in series with the terminals 43 and a resistor 41, to charge a condenser 42 shunted by a discharge resistor 44 to a voltage which increases the anode current of the valve 45 to operate the relay 37 in its cathode circuit and close the reproducer circuit when the frequencies to which the electro-mechanical contactor and the filter-rectifier are respectively tuned (e.g. 200-400 c.p.s. and 3000 c.p.s.) occur in the input signal. If 10 different tuning frequencies are available for the contactor and the filter-rectifier, any one of 100 receivers may be selectively operated. Fig. 7 shows a filter-rectifier circuit in which a transformer secondary 139 supplies the tuned primary and secondary 140, 142 of a transformer feeding a half-wave rectifier 144 to produce a direct voltage which is smoothed by a resistor-condenser network 145, 146. In a modification (Fig. 8, not shown), the rectifier is supplied through a triode amplifier in a negative resistance capacity band-pass circuit. Numerous modifications are described. In Fig. 4 (not shown), the condenser biasing the relay-operating valve is charged from the filter-rectifier output through the contacts in series of two electro-mechanical contactors tuned to different frequencies, so that the reproducer is connected only when three predetermined frequencies are present in the signal. Thus, if 10 different frequencies are available for each contactor and the rectifier filter, 450 different receivers may be selectively called. When the system shown in Fig. 1 operates over a frequency-modulated channel the contactor frequencies may be modulated on the filter-rectifier frequency which in turn modulates the carrier frequency. Fig. 5 shows a selective calling system responsive to such signals wherein the transformer 90 energized from the receiver output has a secondary winding 91 energizing the reproducer through relay contacts 99 and a secondary winding 92 supplying a filter 94 which passes the rectifier frequency and its side-bands through an amplifier to a detector or discriminator 96 feeding a selective responsive device as described and a rectifier 97 supplying a direct voltage from which the relay valve biasing condenser is charged through electro-mechanical contactors, to close the relay contacts 99 and connect the reproducer to the signal. In Fig. 6 (not shown), a detailed system of the kind shown in Fig. 5 for operation by a control signal, frequency-modulated by two other signals, comprises a tuned amplifier feeding a discriminator separating out the modulating frequencies for application to a selective calling system operated by two frequencies together, and a rectifier producing a direct voltage to charge the biasing condenser in the selective calling system. Since the input amplifier and the discriminator are selective to the carrier signal, the rectifier is replaceable by a steady source of direct voltage. A selective calling system may be responsive to plural operating signals to connect the reproducer to the signal and disconnect the selective responsive device, Fig. 9 (not shown), which device may be operated by two or more predetermined frequencies, Figs. 10, 11, 12 (not shown), or by one or two selected frequencies and disabled by the presence of a third frequency, Figs. 13, 14 (not shown). Fig. 15 shows a selective control system, e.g. for the remote control of a radio station, wherein a transformer connected to the signal input has three secondaries 283, 282, 281 which respectively transmit audio signals to a communication channel through a band-pass filter excluding calling signals, and energize selective responsive devices 284, 285 (as hereinbefore described) to operate lock-up relays 288, 289 on reception of the predetermined frequencies so that when the valve 286 becomes conductive it continues so until the valve 287, becomes conducting, and vice versa. Additional relay contacts operate control circuits. Fig. 16 shows a selective responsive device in which two control frequencies on the line terminals 310, 311 initially operate electromechanical contactors 316, 317 to charge a condenser 326, and on arrival of two further control frequencies operating the contactors 318, 319, the condenser 331 is charged from the condenser 326 to operate relay 322 through the triode 335, and connect the reproducer 342 to the line while disconnecting the selective responsive device. Two incoming control frequencies may operate tuned electro-mechanical contactors to charge a condenser and operate a relay through a triode to switch the input to further contactors, which on arrival of a second pair of control frequencies operate a second relay through a triode to close contacts which operate a control or an alarm, or selectively connecting a reproducer to the line, Figs. 17, 18 (not shown). Fig. 19 shows a selective responsive device in which the arrival of successive pairs of control frequencies operate tuned contactors 440, 441 to charge a condenser 442 to a voltage such as to release a normally cut-off triode 443, which develops frequencies across its cathode resistor 444 which operate tuned contactors 445, 446 to similarly release the triode 448. Successive pairs of contactors are thus operated in a chain, and the final triode 458 energizes a relay 459 to operate switching or control contacts.