| 摘要 |
PURPOSE:To prevent the occurrence of the malfunction due to a noise and a band external signal by a simple constitution by multiplying the output signal and the sound multiplex compound signal of the band filter, detecting a phase difference component of both signals and discriminating a direct current level component of the output signal of the phase detecting circuit. CONSTITUTION:A phase detecting circuit 20 multiplies the carrier signal of a sound multiplex compound signal outputted from a band filter 19 and the sound multiplex compound signal supplied to an input terminal 18, this is detected and made average, thereby detecting the phase difference component of both signals. By supplying the phase difference signal outputted from the phase detecting circuit 20 to a level detecting circuit 21 and detecting the direct current level of the phase difference signal, the presence and absence of the sub-carrier component are detected. Consequently, since a band filter 19 of 2fH is used, attention is paid to the sub-carrier only and detection is executed, the circuit is easy in the constitution, suitable to miniaturization and economical advantageous.
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