| 摘要 |
PURPOSE:To make it possible to pick up noise from a sounding object, by such a simple structure that noise from an A/D converter is digitaized to be added to a pair of multipliers, a pair of low band pass filters, an effective value calculator and an FET operation apparatus. CONSTITUTION:An input signal is digitized by an A/D converter 11 and inputted to first and second multipliers 12, 13 while a cosine wave signal cos omegact and a sine save signal sinomegact (wherein omegac is angular frequency) being outputs of an oscillator 18. In this case, the output of a first multiplier 12 is inputted to a first low band pass filter 14 and a signal with the frequency band of noise generated from a sounding object is outputted to the output of the first low band pass filter 14. The output of a second low band pass filter 15 is similar. Herein, outputs of first and second low band pass filters 14, 15 are applied to an effective value calculator 16 and an effective value is calculated by the effective value calculator 16 while the output of the effective value calculator 16 is inputted to an FET arithmetic apparatus 17 to undergo frequency analysis.
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