Patent GB-1412466-A (GB)
Title: Circuit for discriminating between useful and spurious signals
IPC: H04N5/00
Filed: 1972-10-16
Abstract: 1412466 Controlling by sound SONY CORP 16 Oct 1972 [18 Oct 1971] 47634/72 Headings G4F and G4H [Also in Divisions H3 and H4] In a circuit for discriminating between sound or information signals and noise or spurious signals including a variable gain amplifier 8, Fig. 5, for amplifying the signals, control means 13 varies the gain of the amplifier 8 inversely in proportion to the amplified signal as the amplified signal varies up to a peak value and thereafter restores the gain to its original value at a relatively slower rate and means 9 converts the amplified signals into corresponding unidirectional pulse signals of constant amplitude whose durations are representative of the length of time that the separate portions of the amplified signals are above a first predetermined voltage level. Means 17, 18 integrates each of the unidirectional pulse signals and means 19 responsive to the integrated unidirectional pulse signals produces output control signals representative of each integrated unidirectional pulse signal whose amplitude exceeds a second predetermined voltage level. Ultrasonic control signals (S<SP>1</SP>s, Fig. 4, not shown) and spurious interference noise signals (S<SP>1</SP>n) are received by a transducer (2, Fig. 3, not shown) and are applied via 2a, Fig. 5, to a variable gain amplifier 8 having a BPF 4 tuned to the ultrasonic control frequency. The output from the amplifier 8 is applied via an impedance converter 9 to a detector circuit 14. A feedback peak detector 13 provides negative feedback to the amplifier such that the increased negative feedback provided due to a large input signal is maintained for a delay time after the input signal has fallen in amplitude this causes the output of the amplifier 8 to follow the waveform of the control and interferance signals to produce amplified signals (T<SP>1</SP>s, T<SP>1</SP>n). The detector 14 produces output pulses (U<SP>1</SP>s, U<SP>1</SP>n) corresponding to the control and interference signals exceeding a first amplitude (Vt). These output pulses are integrated by 17, 18 to produce ramp pulses (Q<SP>1</SP>s, Q<SP>1</SP>n) and a transistor 19 conducts to produce a pulse output (R<SP>1</SP>s) representative of the control signal (S<SP>1</SP>s) when the ramp pulse Q<SP>1</SP>s exceeds a second amplitude (Vo). The ramp noise pulses (Q<SP>1</SP>n) do not exceed this second amplitude and so are not present in the final output at 7a. In a modified circuit (Fig. 6, not shown) and Fig. 8, the variable gases amplifier 80 has a pass band of 37 kHz to 41À5 kHz set by a BPF 82 and a BPF 85 and the output of the impedance converter 90 feeds those control channels each comprising a BPF 101, 102, 103 tuned to 40 kHz, 38À5 kHz and 37 kHz, a detector 111, 112, 113 and a gating transistor 121, 122, 123. The feedback circuit 130 includes a transistor 131 and a capacitor. 134 which provides the delay feedback. A feed forward circuit 143 includes a normally saturated on transistor 145 and a transistor 146 which is off. The gating transistors 121, 122, 124 are also off. The control pulses and interference pulses (U<SP>1</SP>s, U<SP>1</SP>n, Fig. 4, not shown) from the output of transistor 144 are integrated by 145, 147 to produce ramp pulses (Q<SP>1</SP>s and Q<SP>1</SP>n) corresponding to the control and interference pulses. When the control ramp pulses (Q<SP>1</SP>s) exceed a second amplitude voltage (Vo) transistor 146 is turned on to produce an output pulse (R<SP>1</SP>s) which turns on that gating transistor 121, 122, 123 to which the control pulse (P<SP>1</SP>s) is also applied. The interference ramp pulses (Q<SP>1</SP>n) do not exceed the second amplitude voltage (Vo) so they do not pass to the output 121a, 122a, 123a. The above circuit of Fig. 8, operates to discriminate between control signals having constant amplitude and duration and having at least one predetermined frequency and spurious interference signals at the predetermined frequency but having an unconstant amplitude and duration. In a modification (Fig. 9, not shown) separate voltage level detectors (141, 142, 143<SP>1</SP>) each corresponding to the circuit of 143 are substituted for the circuits of 121-123 and circuit 143 is removed. A suitable transistor ultrasonic transmitter is disclosed (Fig. 7 not shown) having the different control frequencies selected by a push button switch (220). The circuits may be integrated circuits and may be used for remote control of television receivers such as to control sound volume, the colour balance, to change channels and to turn the television set on and off.