Electrical audio signal processing systems and devices – Noise or distortion suppression – Spectral adjustment
Patent
1997-07-28
1999-08-24
Loomis, Paul
Electrical audio signal processing systems and devices
Noise or distortion suppression
Spectral adjustment
704226, H04B 1500
Patent
active
059434295
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to noise suppresion in digital frame based communication systems, and in particular to a spectral subtraction noise suppression method in such systems.
BACKGROUND
A common problem in speech signal processing is the enhancement of a speech signal from its noisy measurement. One approach for speech enhancement based on single channel (microphone) measurements is filtering in the the assumption that the background noise is long-time stationary (in comparison with the speech) a model of the background noise is usually estimated during time intervals with non-speech activity. Then, during data frames with speech activity, this estimated noise model is used together with an estimated model of the noisy speech in order to enhance the speech. For the spectral subtraction techniques these models are traditionally given in terms of the Power Spectral Density (PSD), that is estimated using classical FFT methods.
None of the abovementioned techniques give in their basic form an output signal with satisfactory audible quality in mobile telephony applications, that is
In particular, the spectral subtraction methods are known to violate 1 when 2 is fulfilled or violate 2 when 1 is fulfilled. In addition, in most cases 3 is more or less violated since the methods introduce, so called, musical noise.
The above drawbacks with the spectral subtraction methods have been known and, in the literature, several ad hoc modifications of the basic algorithms have appeared for particular speech-in-noise scenarios. However, the problem how to design a spectral subtraction method that for general scenarios fulfills 1-3 has remained unsolved.
In order to highlight the difficulties with speech enhancement from noisy data, note that the spectral subtraction methods are based on filtering using estimated models of the incoming data. If those estimated models are close to the underlying "true" models, this is a well working approach. However, due to the short time stationarity of the speech (10-40 ms) as well as the physical reality surrounding a mobile telephony application (8000 Hz sampling frequency, 0.5-2.0 s stationarity of the noise, etc.) the estimated models are likely to significantly differ from the underlying reality and, thus, result in a filtered output with low audible quality.
EP, A1, 0 588 526 describes a method in which spectral analysis is performed either with Fast Fourier Transformation (FFT) or Linear Predictive Coding (LPC).
SUMMARY
An object of the present invention is to provide a spectral subtraction noise suppresion method that gives a better noise reduction without sacrificing audible quality.
This object is solved by a spectral subtraction noise suppression method in a frame based digital communication system, each frame including a predetermined number N of audio samples, thereby giving each frame N degrees of freedom, wherein a spectral subtraction function H(w) is based on an estimate .PHI..sub.v (w) of a power spectral density of background noise of non-speech frames and an estimate .PHI..sub.x (w) of a power spectral density of speech frames. The method includes the steps of approximating each speech frame by a parametric model that reduces the number of degrees of freedom to less than N; estimating the estimate .PHI..sub.x (w) of the power spectral density of each speech frame by a parametric power spectrum estimation method based on the approximative parametric model; and estimating the estimate .PHI..sub.v (w) of the power spectral density of each non-speech frame by a non-parametric power spectrum estimation method.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:
FIG. 1 is a block diagram of a spectral subtraction noise suppression system suitable for performing the method of the present invention;
FIG. 2 is a state diagram of a Voice Activity Detector (VAD) that may b
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Loomis Paul
Mei Xu
Telefonaktiebolaget LM Ericsson
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