| 摘要 |
PURPOSE: An analog tone detecting method in a private exchanger is provided to widen a detection band of a tone signal and stably detect a tone signal without a variation of a detection band even for a signal input beyond a predetermined input level. CONSTITUTION: A center frequency of a band-pass filter is set by f0 and a detection band width is set ΔHz. In this state, when a signal is inputted, the band-pass filter performs first filtering with a center frequency of f0-Δ/2 (or f0+Δ/2). In a second filtering, the signal outputted from the first filter is again filtered with a center frequency of f0+Δ/2 (or f0-Δ/2). After undergoing the first and the second filtering, a signal outputted from the band-pass filters has a level value between f0-Δ/2-f0+Δ/2 Hz. This signal is level-compared with a non-filtered input by comparators. If it has the same level as the input, frequencies within the detected band width have the same level as the input signal and are recognized as a signal. If the signal has a different level as that of the input signal, frequencies existing at other than the detected band width have a lower level than that of the input signal and is not recognized as a signal.
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