GB1059007A - Improvements relating to stereophonic radio receivers - Google Patents

Abstract

1,059,007. Stereophonic radio receivers; transistor relay circuits. TELEFUNKEN PATENTVERWERTUNGS-G.m.b.H. Nov. 25, 1965, No. 50130/65. Headings H3T and H4L. In a stereophonic radio receiver, a relay responsive to the transmitted subcarrier when present switches the receiver from mono to stereo reception, and an automatic switch device which is responsive to the temporary closing of a key when the receiver is conditioned for stereo reception conditions the receiver for mono reception, the switch device being held by the rectified subcarrier oscillations. This enables the receiver to be switched over to mono reception when a stereophonic signal is being received under adverse noise conditions, but automatically returns the receiver to its normal mode of operation on retuning. As shown in Fig. 2, when stereo signals are received negative half-cycles of the 38 kc./s. subcarrier cause transistor 9 to conduct and operate relay 6 to switch the receiver for stereo operation, a further contact 13 being closed to connect resistor 14 to reduce the time constant of circuit 10, 11. The reduction of the time constant ensures that a positive voltage does not build up and prevent switching in the event of a rapid change to a different stereo transmission. If the key 7 is pressed and released the positive biss from battery 8 causes relay 6 to be released and the receiver is conditioned for mono operation. Resistor 14 is disconnected and rectification of the subcarrier by the baseemitter circuit of the transistor maintains the transistor blocked. In a modification, Fig. 3 (not shown), the relay is connected in parallel instead of in series with the transistor 9 and is energized for mono reception. In an alternative arrangement, Fig. 1 (not shown), a second relay is used which overrides the stereo-mono relay when the mono-key is pressed and is held operated by the rectified subcarrier. In a further arrangement, Fig. 4 (not shown), the relay 6 is replaced by a trigger circuit and the relay contacts are replaced by diodes.