Patent GB-611203-A (GB)
Title: Improvements in calling and noise suppression circuits for radio receivers
IPC: H03G3/26
Filed: 1946-04-25
Abstract: 611,203. Valve circuits for radio reception. BRITISH THOMSON-HOUSTON CO., Ltd. April 25, 1946, No. 12499. Convention date, April 25, 1945. [Class 40 (v)] A receiver in a communication system has the audio amplifier stages rendered inoperative in the absence of a carrier wave, the amplifier being rendered operative again when a carrier wave having a predetermined modulation characteristic is received. Fig. 1 shows the stages following the demodulator of a superheterodyne receiver. Triode 8 is normally biassed beyond cut-off by the voltage across resistor 25, thus rendering the audio amplifier 9 inoperative, to signals from the receiver. When a carrier is received, modulated by the frequency to which the resonant circuit 11, 12 is tuned, diode 15 conducts and the potential developed across resistor 17 biasses the triode section of valve 14 beyond cut-off, thus reducing the bias voltage across resistor 25 and rendering valve 8 conducting. The circuit 11, 12 is preferably tuned to a sub-audio frequency, thus enabling this frequency to be filtered from the output circuit by a simple high-pass filter 27. In a modification, Fig. 2, the noise suppression circuit responds not only to a particular modulation frequency but also to undesired noise components. Limiter 28 and discriminator 31 enable the receiver to handle frequency or phase modulated signals, the audio frequency modulation appearing across potentiometer 32 and being applied to triode 8 which is normally biassed beyond cut-off by the potential developed across resistor 25. Signals modulated at the frequency of tuned circuit 11, 12 render valve 8 conducting as described in connection with Fig. 1 above. In the absence of a received carrier, positive bias is applied to the control grid of valve 38 from the rectifier 34 and the audio amplifier valve 8 is biassed to cut-off, thus preventing the reproduction of undesirable noises in the receiver output. If the carrier is frequency or phase modulated with the desired keying tone and undesired noise is also present, the noise appears as amplitude modulation of the carrier and is rectified by the circuit 34. The resistorcapacitor circuit 43, 36 has a short time-constant and negative peaks of the unwanted noise modulation reduce the negative bias across resistor 36 due to the rectified carrier the current in valve 38 increases and thus biasses valve 8 to cut-off, the time-constant of the resistor-capacitor combination 25, 44 being of sufficient length that positive peaks of noise do not appear across resistor 25 and thus are not produced in the receiver output. Controls 40 and 20 enable the levels to be adjusted at which the incoming carrier wave and the audio control frequency cause the suppression circuits, to. operate. In Fig. 3 (not shown), a number of receivers are shown used in a communication system, each receiver being rendered operable by a different modulation frequency. The transmitter is modulated with any or all of the frequencies at which the receivers operate and thus it is possible for communication to be made between the transmitter and any particular receiver or all of the receivers simultaneously.