Noise reduction apparatus using spectral subtraction or scaling

Details Noise reduction apparatus using spectral subtraction or scaling Noise reduction apparatus using spectral subtraction or scaling

1995-10-11

1998-04-21

MacDonald, Allen R.

Data processing: speech signal processing, linguistics, language

Speech signal processing

For storage or transmission

704219, 704209, 704233, G10L 904

Patent

active

057429278

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. The Field of the Invention
The present invention relates to noise reduction, and more particularly, to a noise reduction apparatus using spectral subtraction or scaling and signal attenuation in the regions of the frequency spectrum lying between the formant regions.
2. Description of the Related Art
Broadband noise when added to a speech signal can impair the quality of the signal, reduce intelligibility, and increase listener fatigue. Since in practice much speech is recorded and transmitted in the presence of noise, the problem of noise reduction is vital to the world of telecommunications, and has gained much attention in recent years.
Various classes of noise reduction algorithms have been developed, including noise suppression filtering, comb filtering, and model based approaches. Known noise suppression techniques include spectral and cepstral subtraction, and Wiener filtering.
Spectral subtraction is a very successful technique for reducing noise in speech signals. This operates (see for example, Boll "Suppression of Acoustic Noise in Speech using Spectral Subtraction", IEEE Trans. or Acoustics, Speech and Signal Processing, Vol. ASSP-27, No. 2, April 1979, p.113) by converting a time domain (waveform) representation of the speech signal into the frequency domain, for example by taking the Fourier transform of segments of speech to obtain a sen of signals representing the short term power spectrum of the speech. An estimate is generated (during speech-free periods ) of the noise power spectrum and these values are subtracted from the speech power spectrum signals; the inverse Fourier transform is then used to reconstruct the time-domain signal from the noise-reduced power spectrum and the unmodified phase spectrum.
A related technique is that of spectral scaling, described by Eger "A Nonlinear Processing Technique for Speech Enhancement" Proc. ICASSP 1983 (IEEE) pp 18A.1.1-18.A.1.4; again the signals are transformed into frequency domain signals which are then multiplied by a nonlinear transfer characteristic so as preferentially to attenuate ion-magnitude frequency components, prior to inverse transformation. Developments of this technique, are described in our international patent application No. PCT/GB89/00049 (published as WO89/06877) or U.S. Pat. No. 5,133,013.
Due to non-stationarity in the noise, the estimated noise spectrum used for spectral subtraction will be different from the actual noise spectrum during speech activity. This error in noise estimation tends to affect small spectral regions of the output, and is perceived as short duration random tones, or musical noise. Whilst much lower in overall energy than the original noise, this musical noise tends to be very irritating to listen to. A similar effect occurs in the case of spectral scaling.
Several methods have been employed in an attempt to minimise the musical noise. Magnitude averaging can be used to reduce these artifacts, although this can result in temporal smearing, due to the non-stationarity of the speech. Another method consists of subtracting an overestimate of the noise spectrum, and preventing the output spectrum from going below a pre-set minimum level. This technique can be very effective, but can lead to greater distortion to the speech.


SUMMARY OF THE INVENTION

According to the present invention there is provided a noise reduction apparatus comprising: representing the magnitudes of spectral components of the input signals; low-magnitude ones of the said spectra component signals relative to that of higher magnitude ones of the said spectral component signals; and time-varying signal;
characterized by means to identify formant regions of the speech spectrum; and
means to attenuate those frequency components lying outside the formant regions.


BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a noise reduction apparatus using spectral subtraction;
FIG. 2 is an embodiment of the noise reduction apparatus of the present invention using signal attenuation in the regions of the frequenc

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