Electrical audio signal processing systems and devices – One-way audio signal program distribution – Public address system
Patent
1989-09-15
1992-07-21
Kemeny, Emanuel S.
Electrical audio signal processing systems and devices
One-way audio signal program distribution
Public address system
G10L 708
Patent
active
051330130
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of reducing the level of noise in a signal, and to apparatus for reducing noise using this method; particularly but not exclusively this invention relates to a method of reducing noise in a speech signal, and to apparatus for thus producing a speech signal with enhanced intelligibility.
A signal will often acquire broadband noise so that the time-average noise power is spread across a portion of the noise spectrum. In a speech system, noise may cause a listener severe fatigue or discomfort.
It is obviously desirable to reduce noise, and many methods of doing so are known; in speech systems, some types of noise are more perceptually acceptable than others. Especially desirable are methods which may be used with existing transmission equipment, and preferably are easily added at the receiver end.
It is known to reduce noise in high noise environments (-6 to +6 dB signal-to-noise ratio) by so-called spectral subtraction techniques, in which the signal is processed by transforming it into the frequency domain, then subtracting an estimate of the noise power in each spectral band, then re-transforming into the time domain. This technique suffers from several drawbacks, however. Firstly, it is necessary to measure the noise power in each spectral line; this involves identifying `non-speech` periods, which can be complicated and unreliable. Secondly, it requires the assumption that the noise spectrum is stationarry between the instants at which the noise power is measured; this is not necessarily the case. Thirdly, if an estimate of noise power made in one non-speech period is applied to the next non-speech period correctly, there will be a total absence of background noise during non-speech periods, and this modulation of the background noise sounds unpleasant to a listener.
According to the invention, there is provided a noise reduction apparatus comprising; first conversion means for receiving a time-varying signal and producing therefrom output signals representing the magnitude of spectral components thereof, processing means for receiving the output of the first conversion means, the processing means having a nonlinear transfer characteristic such that in use low magnitude inputs thereto are attenuated relative to high magnitude inputs, the transfer characteristic being linear for high magnitude spectral components, and second conversion means for receiving the output of the processing means and reconstituting therefrom a time-varying signal.
Preferably, the transition between the linear and non-linear regions of the characteristic is gradual and substantially without discontinuities in slope, so as to progressively roll off lower magnitude (noise) spectral components.
Preferably, a level adjusting operation is performed so that the signal is maintained in a predetermined relation to the transfer characteristic, which may be an automatic gain control operation on the signal.
Preferably the first conversion operates on frames of the signal and uses a one dimensional or complex transform to produce a series of transform coefficients, and the second conversion applies the inverse transform to reconstitute the signal. In a preferred embodiment a Fast Fourier transform is utilized. Where such a transform is employed, it will be advantageous to provide shaping of each frame using a window function, so as to reduce frequency `leakage` when the frame is transformed. Where such a window function is employed, the sampled data frames are preferably overlapped.
In a second embodiment, several different transfer characteristics are employed within the processing so that a more severe attenuation is effected in certain spectral regions. Where the signal is a speech signal, these regions may be assigned on a fixed basis, employing knowledge of the spectral position of speech formant bands for an average speaker, or may be derived by the apparatus for each speaker by initially measuring formant band time-averaged positions.
In a third embodiment, several different transfer characteristics
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British Telecommunications public limited company
Doerrler Michelle
Kemeny Emanuel S.
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