Speech coding apparatus, linear prediction coefficient analyzing apparatus and noise reducing apparatus

A noise reducing apparatus, comprising:

• speech signal receiving means for receiving a plurality of frames of analog speech signals in which a noise exists and converting the frames of analog speech signals into a plurality of frames of digital speech signals one after another;
• Fourier transforming means for performing a discrete Fourier transformation for each of the frames of digital speech signals obtained by the speech signal receiving means and producing an input spectrum and a phase spectrum corresponding to each of the frames of digital speech signals for each of frequency values;
• noise assuming means for selecting a particular input spectrum having a minimum value from among a current input spectrum, produced by the Fourier transforming means, corresponding to a current frame of digital speech signals and a predetermined number of past input spectra, produced by the Fourier transforming means, corresponding to past frames of digital speech signals preceding to the current frame and assuming the particular input spectrum as a noise spectrum corresponding to the current frame of digital speech signals for each of the frequency values;
• noise reducing degree determining means for determining a degree of a noise reduction according to each of the frames of digital speech signals obtained by the speech signal receiving means;
• noise reducing means for adjusting a value of each of the noise spectra assumed by the noise assuming means according to the degree of the noise reduction determined by the noise reducing degree determining means to produce an adjusted noise spectrum having an adjusted value corresponding to the current frame of digital speech signals for each of the frequency values, subtracting the adjusted noise spectrum from the current input spectrum produced by the Fourier transforming means for each of the frequency values to reduce the noise existing in the current frame of digital speech signals, and producing a noise-reduced input spectrum corresponding to the current input spectrum for each of the frequency values;
• and
• inverse Fourier transforming means for performing an inverse Fourier transformation for the noise-reduced input spectra produced by the noise reducing means according to the phase spectra, produced by the Fourier transforming means, corresponding to the current frame of digital speech signals, producing a current frame of first-order output signals corresponding to the current frame of digital speech signals, and outputting a plurality of frames of first-order output signals corresponding to the frames of analog speech signals received by the speech signal receiving means, one after another as a plurality of frames of output signals.