| 摘要 |
PURPOSE:To shorten considerably the detection time of amplitude reduction of information signals by adding the second signal, which is obtained by shifting the phase of the first signal at about 90 deg. near the carrier frequency and multiplying this phase-shifted signal, to the first signal obtained by multiplying an information signal. CONSTITUTION:An FM luminance signal (a) from an amplifier 14 is supplied to a phase shifter 20. The phase shifter 20 is so set that its shift quantity is about +90 deg. or -90 deg. near the carrier frequency of the supplied FM luminance signal (a). An FM luminance signal (c) obtained by shifting the phase of the signal (a) in the phase shifter 20 is supplied to a multiplier 21, which performs the same operation as a multiplier 15, and is multiplied by 2 to obtain a signal (d) which has a maximum amplitude for a minimum amplitude (namely, zero amplitude) of an output signal (b) of the multiplier 15. The output signal (b) of the multiplier 15 and the output signal (d) of the multiplier 21 are supplied to an adding circuit 22. If a minimum value of shorter amplitude periods of an output signal (e) of the adding circuit 22 is lower than a reference voltage E0, amplitude reduction of the FM luminance signal (a) is discriminated. |
