In proportion to practical use of a speech recognition or a cellular phone in actual environment, a signal processing method for excluding a noise from an acoustic signal on which the noise is superimposed in order to emphasize a voice signal becomes important. Especially, Spectral Subtraction (SS) method is often used because it is effectively easy to be realized. The Spectral Subtraction method is disclosed in “S. Boll, “Suppression of Acoustic Noise in Speech Using Spectral Subtraction”, IEEE Trans., ASSP-27, No. 2, pp. 113-120, 1979”.
The Spectral Subtraction method includes a problem that it often causes a perceptually unnatural sound (called “a musical noise”). Musical noise is especially notable in a noise section. Because of statistical variance of the noise signal, removing an average value of noise signal from an input signal causes discontinuity in the remaining signal of the reduction. The musical noise is due to the remaining signal of reduction. In order to solve this problem, an excess suppression method is utilized. In the excess suppression method, by reducing a value larger than an estimation noise from the input signal, all variation elements of the noise are suppressed. In this case, if a reduction result becomes a negative value, the negative value is replaced by a minimum value. However, in the excess suppression method, suppression overflows in a voice section. As a result, a voice is distorted in the voice section. For example, the excess suppression method is disclosed in “Z. Goh, K. Tan and B. T. G. Tan, “Postprocessing Method for Suppressing Musical Noise Generated by Spectral Subtraction”, IEEE Trans., SAP-6, No. 3, May 1998”.
Furthermore, a method for executing some processing on a section generating musical noise in order not to perceive the musical noise is utilized. For example, a small gain is multiplied with each input signal and the multiplication result is superimposed to the output signal. However, in this method, if a sufficient signal is superimposed so as not to perceive the musical noise, a noise level raises by the superimposed signal. As a result, effect of noise suppression is lost.
As mentioned-above, excess suppression using a large suppression coefficient reduces musical noise. However, distortion often occurs in the voice section. Furthermore, in the post processing method for superimposing the input signal on the musical noise, by superimposing the sufficient signal so as not to perceive the musical noise, effect of noise suppression is lost.