Patent GB-867764-A (GB)
Abstract: 867,764. Controlling and actuating by sounds; electric selective signalling. ZENITH RADIO CORPORATION. April 4, 1957 [April 16, 1956; Jan. 2, 1957; March 11, 1957], No. 11001/57. Classes 40 (1) and 118. [Also in Group XL (c)] A control system, e.g. for the remote control of a television receiver, comprises means for generating an ultrasonic signal of predetermined frequency and a circuit responsive to said signal and comprising an input circuit including a microphone wherein an input signal having a frequency corresponding to said predetermined frequency is developed, a limiter to which said input signal is applied, and a circuit coupled to the output of said limiter and producing a control signal for actuating a utilization circuit only in response to signals of appropriate frequency and predetermined minimum duration applied thereto from the limiter. Preferably the input signal has the same frequency as the ultrasonic generator, though the input circuit may, if desired, include a heterodyning frequency-changer. Again the output signal from the limiter is preferably chosen to be the third or fifth harmonic of the input signal, though the fundamental-frequency signal may be used. This signal is selected by appropriate filter circuits and applied to a resistance capacitance averaging circuit so that only when the received signal is of appropriate frequency and duration will a sufficient signal be developed to actuate a relay determining the control function effected by the system. As described, the ultrasonic generator comprises means for generating four ultrasonic frequencies; two of these are used for controlling a tuning motor in a television receiver, one is used for switching the receiver on and off and the fourth is used for optional muting of the loud-speaker of the receiver. The system may alternatively be used for the remote control of garage doors or of a furnace. Ultrasonic generator, Fig. 5.-This comprises a plurality of cylindrical rods 201, of which one only is shown, preferably composed of aluminium, though glass, ceramic or other metals may be used. These rods are supported by spring clamps at their mid-points so that they may be excited into longitudinal vibration at their resonant frequencies. The arrangement shown comprises four rods which resonate at 38, 39, 40 and 41 kc/s. on depressing a pushbutton 206, a rod 207 carrying a cam lever 211 is likewise depressed and a projection from the lever 211 engages in a recess in a rod 218 carrying a hammer 220 so that the rod 218 is simultaneously pulled down. Eventually, however, the lever 211 engages a stop 223 which causes it to rotate against the action of a spring 213, thereby releasing the rod 218. This moves upwards under the action of spring 219 causing hammer 220 to strike the rod 201 which then generates an oscillation at its resonant frequency. Keceiver and control circuits, Fig. 2.-The ultrasonic signal generated at the transmitter is received by a microphone 62 in an input circuit 31 comprising a three-stage tuned amplifier having a pass-band covering 38 to 41 kc/s. The output from circuit 31 is applied to a limiter circuit 34 comprising a suitable valve 102 whose anode circuit includes tuned circuits 115 and 116 forming parts of selector circuits 35, 36, respectively. Circuits 115 and 116 are tuned to 115.5 and 121.5 kc/s. respectively and are coupled to further tuned circuits 117, 119 and 150 to form two conventional frequency discriminators having respective positive and negative resonance peaks at 114 and 117 kc/s. and at 120 and 123 kc/s., i.e. at respective third-harmonics of the four possible ultrasonic input signals. Consequently in selector circuit 35 a positive output is developed at the cathode 123 of double diode 122 when the microphone receives an input signal of 38 kc/s. Similarly, a positive output is obtained at cathode 126 when a 39 kc/s. input is received. In the selector circuit 36 double diode 151 delivers outputs at respective ones of its cathodes for inputs of 40 and 41 kc/s. The diode outputs are applied through resistance capacity filter networks to the control grids of respective triode amplifiers having relays connected in their anode circuits. These filter networks average the signals applied thereto so that a relay is operated only when the corresponding ultrasonic input signal persists for a certain minimum duration; these prevent actuation of the circuit by noise and stray ultrasonic vibrations of short duration. In a modified circuit, Fig. 6 (not shown), the frequency-discriminating circuits in the circuits 35, 36 are replaced by four simple parallel resonant circuits tuned respectively to 114, 117, 120 and 123 kc/s. Specification 867,765 is referred to.