Patent ID: 7941315

Claim:
A noise reducer comprising: a speech accepting device that accepts a speech on which a noise is superimposed and converts the speech into a time-domain signal on a time axis of the speech; a signal transforming part transforming the signal on the time axis of the speech into a frequency-domain signal on a frequency axis of the speech; an amplitude calculating part calculating an amplitude component for each predetermined frequency band of the frequency-domain signal; a noise target value estimating part estimating a noise target value |N (xn, f)| through the expression | N ( xn, f )|=α( f )| N ( x ( n− 1), f )|+(1−α( f ))| IN ( xn, f )|, where |IN (xn, f)| is an amplitude of the accepted speech, |N (x(n−1), f)| is an amplitude of a noise target value in a last analysis window (x(n−1)), and α(f) is an average coefficient for each frequency; a coefficient calculating part calculating a noise reduction coefficient to reduce the noise for each frequency band on the basis of the amplitude component calculated by the amplitude calculating part; a noise reducing part multiplying the frequency-domain signal by the calculated noise reduction coefficient to obtain a reduced-noise converted signal on the frequency axis; a comparator comparing an amplitude of the noise target value to an amplitude of the frequency-domain signal, wherein if the converted signal is equal to or larger in amplitude than an amplitude of the estimated noise target value, then the converted signal is not reduced in the reducing part, and wherein if the converted signal is smaller in amplitude than an amplitude of the estimated noise target value, then the converted signal is replaced by the noise target value in the reducing part; a signal restoring part transforming the frequency-domain signal from the noise reducing part into another time-domain signal on the time axis; and a speech output device that outputs the another time-domain signal as sound.