Patent GB-1010327-A (GB)
Title: Frequency-modulation receivers for stereophonic or monophonic signals
IPC: H03D1/22
Filed: 1962-08-03
Abstract: 1,010,327. Stereophonic radio receiving systems. ZENITH RADIO CORPORATION. Aug. 3, 1962 [Oct. 27, 1961], No. 29888/62. Heading H4L. A frequency-modulation radio-receiver for the reception of monaural or stereophonic signals comprises means for demodulating the carrier signal to derive a detected signal representative of the modulation function of the carrier, means for applying the detected signal to a second demodulator which passes audio signals without detection during monaural reception but which derives separated stereophonic signals during stereophonic reception, and means for adjusting the intensity of the signals applied to the second demodulator in response to a change between monaural and stereophonic reception to maintain the mean level of reproduction approximately constant during monaural and stereophonic reception. The signal consists of an A+B audio signal which frequency-modulates a main carrier, a suppressed subcarrier amplitude-modulated with the A-B signal which frequency-modulates the main carrier, and a pilot carrier for reconstituting the subcarrier at the receiver, which also frequencymodulates the main carrier. As shown, in Fig. 1, the receiver is conventional as far as the frequency discriminator 13. The output of the discriminator 13 is supplied via a filter 26, 27 selecting the stereophonic signal, and adjustable attenuator 31, 32, to an amplifier 25 supplying oppositely phased outputs via capacitors 60, 61 respectively. These oppositely phased signals are supplied to a synchronous detector including diodes 40, 41 which also receives via valve 67 and the primary of transformer 42 a subcarrier signal regenerated at 65 from the pilot signal supplied via capacitor 66. The diodes 40, 41 are biased to be non-conductive during monaural reception by a potentiometer 68, a counter-bias being derived from the subcarrier regenerator 65 during stereophonic reception to render them conductive. The signal from anode impedance 33 is made larger than that from cathode impedance 34 and causes the diodes 40, 41 to derive the A audio signal at load resistor 44 and the B audio signal at load resistor 43. The slight amount of each signal appearing in the opposite channel is eliminated by the application of the antiphased signal from resistor 34 to the common connection of resistors 43, 44. The A and B audio signals are supplied via de-emphasis networks 45, 46 and 47, 48 to conventional output stages. During monaural reception the diodes 40, 41 are blocked and the detected signal from resistor 34 is supplied to the output stages in parallel. In order to maintain the mean signal level constant on both monaural and stereophonic reception the frequency selective attenuator 31, 32 attenuates signals above the audio-frequency range. In addition correct weighting of the respective signals is obtained by adjustment of the resistors 33, 34 and the synchronous detector is arranged to have a 50% duty cycle by adjustment of the potentiometer 68. It is stated that the attenuator 31, 32 may be omitted and similar results achieved by modifying the duty cycle of the synchronous detector. Alternatively, the phase of the subcarrier may be varied. Instead of attenuating the subcarrier component by circuit 31, 32, the subcarrier may be pre-emphasized. The signal from the filter 26, 27, 29, 30, Fig. 1, is fed via a coupling capacitor 80, Fig. 5, through a preemphasis network 81, 82. The regenerated subcarrier (demodulation signal) is supplied to the primary of transformer 42 and a D.C. bias derived from the demodulation signal generator is supplied via a resistor 59 to the junction of resistors 43, 44 in the synchronous detector, and to the cathode of an auxiliary diode 88. During stereophonic reception the potential applied to the anodes of diodes 40, 41 is the same as that supplied to their cathodes from network 83, 84 and they function normally to derive the A and B audio signal components, the diode 88 being biased to cut off. During monophonic reception the diodes 40, 41 are biased to cut-off and the audio signal is supplied via diode 88 to the A and B amplifiers. Appropriate adjustment of resistors 59, 86, 87 enables the mean output level to be maintained constant for both stereophonic and monophonic reception. Specification 1,009,265 is referred to.