| 摘要 |
PURPOSE:To demodulate an audio signal with fidelity to fluctuations of a local oscillation frequency by discriminating an audio intermediate frequency signal outputted from a tuner by a single frequency discriminating circuit, and then by extracting its DC components as the automatic fine tuning voltage of the tuner while outputting the AC component as the audio signal. CONSTITUTION:The output of tuner 1, while supplied to video intermediate frequency amplifying circuit 2, is applied to sound intermediate frequency (SIF) amplifying circuit 9 provided additionaly, an SIF signal from which is inputted to single frequency discriminating circuit 10. The linear detection region of this discriminating circuit 10 is set in a sufficiently wide range to the frequency shift of the audio signal of the SIF signal, and this discriminating circuit 10 discriminates its AC component and DC component. While the AC component is outputted as an audio signal via sound amplifying circuit 8, the Dc component is applied to AFT characteristic converting circuit 11 and, after converted into characteristics having the steep gradient of the linear region, is applied to tuner 1 as its automatic fine tuning voltage to demodulate the audio signal with fidelity. |
