1,026,555. Stereophonic radio receivers; semi-conductor circuits. ZENITH RADIO CORPORATION. Sept. 10, 1963 [Sept. 10, 1962; Jan. 14, 1963], No. 35636/63. Headings H3T and H4L. In a receiver for a stereophonic radio system of the type in which a subcarrier is suppressed-carrier amplitude-modulated by a difference signal A-B and a main carrier is frequency modulated by the suppressed carrier difference signal and by the sum signal A+B, transistor circuitry is used to demodulate the two signals and provide automatic chageover from stereophonic to monophonic operation in accordance with the received signal. As shown in Fig. 1, a pilot signal of half the subcarrier frequency which is transmitted with the composite signal is selected from the output of the frequency discriminator 13 by a tuned amplifier 30, 41 and is frequency-doubled in a circuit comprising transistor 40 and fullwave rectifiers 50, 51, a resistor 53 providing regenerative feedback. Amplifier 40 is cut-off in the absence of a pilot-signal by the potential applied to its emitter via lamp 45 which lights to provide an indication of a stereophonic programme when transistor 40 conducts. A tuned amplifier 58, 61 supplies the subcarrier signal via a transformer 28 to the base circuits of a pair of amplitude detectors comprising transistors 20, 20<SP>1</SP>, the composite stereophonic signal developed across resistor 34 being supplied via capacitor 64 to the centretap of the secondary winding of the transformer 28. Transistors 20, 20<SP>1</SP> are biased to cut-off in the absence of a stereophonic signal, the emitters being connected via resistors 65, 66 to the lamp 45. The applied subcarrier signal acts as a switching signal so that the transistors 20, 20<SP>1</SP> conduct alternately and supply the A and B signals to the amplifiers 70, 72 respectively. To reduce cross-talk a portion of the composite signal is fed via an adjustable resistor 67 to the common impedance 23 in the collector circuits of transistors 21, 21<SP>1</SP>. Capacitors 24, 24<SP>1</SP> introduce the usual de-emphasis. During monaural reception the signal from the discriminator 13 is supplied via amplifier 30, resistor 67, and the output circuits of the cutoff transistors 21, 21<SP>1</SP> to the amplifiers 70, 72. In a modification, Fig. 2 (not shown), amplitude detectors 21, 21<SP>1</SP> are replaced by a single transistor 80 of the symmetrical type, the functions of the emitter and collector being interchanged at each half cycle of the subcarrier signal applied across the coil 28. The composite input signal is applied to the base and the A and B outputs are taken from the respective collectors, a portion of the composite signal being applied to the centre tap of the transformer secondary. In this arrangement the A-B signal is derived and combined in a sum and difference manner in the output circuits with the A+B signal. In a further modification, Fig. 3 (not shown), the transistor of Fig. 2 is replaced by an unsymmetrical transistor. In an alternative arrangement, Fig. 3 (not shown), the suppressed carrier amplitude modulated component is supplied to the base circuits of transistors 200, 200<SP>1</SP> and the A + B audio signal is supplied to the centre tap of transformer secondary winding 28, the reconstituted subcarrier being applied to the transformer primary. The subcarrier causes the collector electrodes to be alternately effective to develop difference signals A-B of opposite polarities which are combined with the A+B signals in resistors 202, 202<SP>1</SP> respectively to produce the required A and B signals.