| 摘要 |
A PWM signal generator is operable to generate a first pulse signal and a second pulse signal at least one of before and after generating the first pulse signal. A duty ratio of the second pulse signal is gradually changed. A low-pass filter is operable to convert the first pulse signal output from the PWM signal generator into an analog acoustic signal. The low-pass filter is operable to convert the second pulse signal output from the PWM signal generator into the analog acoustic signal having an inaudible frequency.
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