GB1033580A - Radio receiver - Google Patents

Abstract

1,033,580. Stereophonic radio receiving systems. MOTOROLA Inc. Oct. 15, 1963 [Nov. 6, 1962], No. 40682/63. Heading H4L. In a receiver for a stereophonic radio signal the means for reconstituting the subcarrier signal from the received pilot signal includes a normally cut-off valve which is rendered conductive after a predetermined delay on the reception of the pilot signal. By this means the receiver is switched automatically from monophonic to stereophonic reception and noise is reduced when tuning between stations. As shown in Fig. 1, the sum signal L+R, the difference signal L-R amplitude modulated on a suppressed subcarrier wave, and a pilot signal of half the frequency of the subcarrier, are provided at the output of the F.M. receiver and discriminator 12, the pilot signal of 19 kc./s. developed across the tuned circuit 18, 20 being supplied to the control grid of valve 22. The output of the valve 22 is supplied via transformer 26, whose secondary winding is tuned to 38 kc./s., to the control grid of a valve 35 and the resulting subcarrier signal is supplied via transformer 45 to the demodulating circuit comprising diodes 70, 71 for one output channel and 80, 81 for the other. The oppositely poled diodes 70, 71 are fed with opposite phases of the subcarrier so that at the junction of resistors 74, 75 the subcarrier is balanced out but at this point the demodulated difference signal is combined with the audiofrequency sum signal to produce the left audiofrequency signal which is supplied via capacitor 77 to the amplifier 60. The diodes 80, 81 operate in the same way to supply the right audiofrequency signal to the amplifier 62. The valve 35 in the frequency-doubler circuit is biased to cut-off by voltage divider 85, 86 in the absence of a pilot signal. When the pilot signal is received it is rectified by diode 92 and the resulting voltage overcomes the cathode bias of valve 35 and allows it to conduct. RC circuit 94, 97 ensures that switching takes place only when the pilot signal has been present for a predetermined period. Neon tube 49 provides an indication when a stereophonic signal is received. An alternative rectifying circuit for the pilot signal is described, Fig. 2 (not shown).