| 摘要 |
PURPOSE:To eliminate the need of an AGC system circuit and to require no adjusting point by separating and selecting a picture intermediate frequency signal (PIF) and a sound intermediate frequency signal (SIF) in an input step, passing the respective signals through a limiter amplifier and detecting a specific sound component in a multiplication circuit. CONSTITUTION:After a signal in the vicinity of the carrier frequency of the PIF of 58.75 MHz is selected in a filter 2, and only the SIF of 54.25 MHz is selected in a filter 3, they are respectively fed to the limiter amplifiers 4, 5. In the input of the limiter amplifier 4, the carrier and the AM modulated part thereof are included as a sideband wave, however, limitter amplified, so that the AM part of the output is attenuated to have only the carrier component. In the input of the amplifier 5, only the SIF of an FM signal is present, the FM part is amplified as it is and outputted, when the two inputs are multiplied in the multiplication circuit 6, the sound FM signal of 4.5 MHz is obtained in the output. Namely, the circuits 4, 5 are limiter amplifier, so that a required sensitivity can be easily maintained and it is not required to apply the AGC.
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