GB1008935A - Stereophonic radio signal receivers - Google Patents

Abstract

1,008,935. Stereophonic radio receiving systems. RADIO CORPORATION OF AMERICA. May 27, 1963 [June 18, 1962], No. 21128/63. Heading H4L. In a receiver for stereophonic radio signals of the type in which the L+R signals frequencymodulate a main carrier wave, the L - R signals amplitude modulate a subcarrier wave which is suppressed, the resultant sideband signals frequency-modulating the main carrier wave, and a pilot signal being included for regenerating the subcarrier, the pilot signal is selected and supplied via a limiter to control an oscillator at a frequency related to that of the pilot signal. The use of a limiter avoids the phase change in the signal from the oscillator which occurs if the amplitude of the pilot signal varies. As shown, a unit 15, which may be added to an existing receiver, is connected to the output of the frequency discriminator 8. A trap circuit 23 removes a 67 kc/s. subcarrier signal which provides an additional channel independent of the stereophonic transmission, and the 19 kc/s. pilot signal is selected by a circuit 32 in the collector circuit of a transistor 18. The composite signal is derived from a resistor 31 in the emitter circuit of the transistor 18, which is shunted by a circuit 33 tuned to the pilot signal frequency. The pilot signal is applied via a limiter 40 comprising an appropriately biased transistor to a tuned circuit 48, 49 feeding a frequency doubler 50. A grounded base oscillator 55 which is inoperative in the absence of the pilot signal is controlled by the output of the frequency doubler 50, the voltage across resistor 54 allowing oscillations to commence. The 38 kc/s. demodulating or switching voltage from the tank circuit 64 operates a stereophonic reception indicator tube 67 via a transistor amplifier 68, and is also fed to a time division multiplex-type detector circuit 70. The composite signal in which the positive envelope corresponds to one of the stereophonic signals and the negative envelope corresponds to the other, is fed into the detector via the centre tap of the inductor portion 72 of the detector circuit, and the 38 kc/s, switching voltage synchronously switches the diodes 73, 74 to sequentially sample the positive and negative envelope. The right and left output signals from diodes 73, 74 respectively are fed through filters 87, 88, to remove unwanted high frequency components, and de-emphasis circuits to the respective reproducing devices. Negative potential appearing on the emitter of the transistor 18 provides a slight forward bias for the diodes 73, 74 to prevent distortion during monaural operation.