| 摘要 |
PROBLEM TO BE SOLVED: To securely prevent the erroneous detection of a tone signal without using a band pass filter (BPF) for extracting a signal component other than the tone signal. SOLUTION: A first signal component detection means 1 extracts the frequency component of the tone signal by BPF 11 and an average level detection circuit 12 detects an average level L1. A second signal component detection means 2 makes a signal except for the pass band of BPF 11 pass through by LPF 21 and HPF 23. Average level detection circuits 22 and 24 detect the average level of the output signals of LPF and HPF. The threshold setting part 31 of a detection decision means 3 sets a threshold based ion the average level L1 of the tone signal and the average levels L2 and L3 of the signal component except for the tone signal. A comparison decision part 32 compares the average level of the tone signal with the threshold Lth. When the average level L1 exceeds the threshold Lth, it decides that the tone signal is detected and a detection signal D is outputted.
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