| 摘要 |
PURPOSE:To allow separate demodulation circuits to extract a demodulated audio signal and a control tone respectively by applying AM modulation to a carrier subject to FM modulation by the audio signal through the use of the control tone. CONSTITUTION:An AM modulation circuit 1 for control tone signal modulation is added to the system. The AM modulation circuit 1 implements AM modulation by using the control tone signal to AM-modulate a power supply for a power amplifier section 13 of an FM transmitter to change a level of a carrier signal subject to FM modulation. On the other hand, an amplitude limiter of an FM demodulation circuit in an FM receiver excludes an AM component. Furthermore, a discriminator demodulates the FM-modulated audio signal to obtain an audio output component, and an AM detector applies envelope detection to the output of a 2nd intermediate frequency to obtain a voltage change for the AM component. Then a tone detector detects a control tone components from the voltage change obtained from the AM detector, then the detector rectifies the component and the result is fed to a comparator circuit, in which a desired control output is selected and extracted. |
