| 摘要 |
PURPOSE:To detect an external noise component accurately even when a speaker and a microphone are specially close to each other, by providing a rectifying circuit which rectifies the output of a narrow-band amplifier and a subtracting circuit which processes the output of the rectifying circuit by subtraction. CONSTITUTION:The output of a microphone 7 is delayed behind the input signal to a speaker 4 by the time which corresponds to the distance between the speaker 4 and microphone 7, but the distance is extremely short and the delay time is several mS and below, so a time difference and a phase shift due to the delay are absorbed by giving adequate time constant to rectifying circuits 25 and 26. Therefore, variation in rectification output with time is nearly equalized with ease. Consequently, the output of a substracting circuit 27 for processing those rectification outputs by subtraction is nearly zero and an acoustic signal component inputted from the speaker 4 to the microphone 7 is nearly erased, thereby detecting only external noise components passed through narrow- band amplifiers 15-17. |
