Automatic gain selector for a noise suppression system

Claims

What is claimed is:

1. An improved noise suppression system for attenuating the background noise from a noisy input signal to produce a noise-suppressed output signal, said noise suppression system comprising:

means for separating the input signal into a plurality of pre-processed signals representative of selected frequency channels;

means for modifying an operating parameter of each of said plurality of pre-processed signals provided by said signal separating means to provide a plurality of post-processed signals; and

means responsive to said plurality of pre-processed signals for generating a modification signal having a selected modification value for each channel for application to said modifying means to enable the operating parameter to be modified, said modification signal generated by automatically selecting a modification value for each channel from one of a plurality of sets of modification values for that channel.

2. An improved noise suppression system for attenuating the background noise from a noisy input signal to produce a noise-suppressed output signal, said noise suppression system comprising:

means for separating the input signal into a plurality of pre-processed signals representative of selected frequency channels, each of said plurality of pre-processed signals comprised of a plurality of frames, each frame comprised of a plurality of samples of said input signal;

means for modifying an operating parameter of each of said plurality of pre-processed signals provided by said signal separating means to provide a plurality of post-processed signals; and

means responsive to said plurality of pre-processed signals for generating a modification signal for application to said modifying means to enable the operating parameter to be modified, said modification signal generating means including means for smoothing said modification signal multiple times per frame.

3. The improved noise suppression system according to claim 2, wherein said smoothing means operates on a per-sample basis.

4. The improved noise suppression system according to claim 1 or 2, wherein said separating means includes a plurality of bandpass filters.

5. The improved noise suppression system according to claim 1 or 2, wherein said operating parameter of each of said plurality of pre-processed signals is the gain of said signal.

6. The improved noise suppression system according to claim 1 or 2, wherein said modification signal for application to said modifying means is comprised of a plurality of predetermined gain values.

7. The improved noise suppression system according to claim 1 or 2, further comprising:

means for combining said plurality of post-processed signals to produce said noise-suppressed output signal.

8. An improved noise suppression system for attenuating the background noise from a noisy input signal to produce a noise-suppressed output signal, said noise suppression system comprising:

means for separating the input signal into a plurality of pre-processed signals representative of selected frequency channels;

means for generating an estimate of the signal-to-noise ratio (SNR) in each individual channel;

means for producing a gain value for each channel by automatically selecting one of a plurality of gain tables in response to a multi-channel noise parameter, and selecting one of a plurality of gain values from the selected gain table in response to said channel SNR estimates and the channel number; and

means for modifying the gain of each of said plurality of pre-processed signals provided by said signal separating means in response to said gain values to provide a plurality of post-processed signals.

9. An improved noise suppression system for attenuating the background noise from a noisy input signal to produce a noise-suppressed output signal, said noise suppression system comprising:

means for separating the input signal into a plurality of pre-processed signals representative of selected frequency channels, each of said plurality of pre-processed signals comprised of a plurality of frames, each frame comprised of a plurality of samples of said input signal;

means for generating an estimate of the signal-to-noise ratio (SNR) in each individual channel once each frame;

means for producing a raw gain value for each channel in response to said SNR estimates once each frame;

means for smoothing said raw gain values multiple times per frame; and

means for modifying the gain of each of said plurality of pre-processed signals provided by said signal separating means in response to said smoothed gain values to provide a plurality of post-processed signals.

10. The improved noise suppression system according to claim 8 or 9, further comprising:

means for combining said plurality of post-processed signals to produce said noise-suppressed output signal.

11. The improved noise suppression system according to claim 8 or 9, wherein said separating means includes a plurality of bandpass filters covering the voice frequency range.

12. The improved noise suppression system according to claim 8 or 9, wherein said SNR generating means includes means for dividing current input signal energy estimates by previous background noise energy estimates for each individual channel.

13. The improved noise suppression system according to claim 8 or 9, wherein said gain modifying means includes means for multiplying the amplitude of each of said plurality of pre-processed signals by the appropriate predetermined channel gain value, thereby providing said plurality of post-processed signals.

14. The improved noise suppression system according to claim 10, wherein said combining means includes means for summing said plurality of post-processed signals to form a single output signal.

15. The improved noise suppression system according to claim 8, wherein said multi-channel noise parameter is the overall average background noise level of all channels comprising said input signal.

16. The improved noise suppression system according to claim 9, wherein said gain smoothing means operates on a per-sample basis.

17. An improved noise suppression system for attenuating the background noise from a noisy pre-processed input signal to produce a noise-suppressed post-processed output signal by spectral gain modification, said noise suppression system comprising:

signal dividing means for separating the pre-processed input signal into a plurality of selected frequency bands, thereby producing a plurality of pre-processed channels;

channel energy estimation means for generating an estimate of the energy in each of said plurality of pre-processed channels;

channel noise estimation means for generating an estimate of the signal-to-noise ratio (SNR) of each individual channel based upon said channel energy estimates and an estimate of the current background noise energy for that individual channel;

channel gain controlling means for providing channel gain values, said channel gain controlling means having a plurality of gain tables, each gain table having predetermined individual channel gain values corresponding to various individual channel SNR estimates, said channel gain controlling means further having gain table selection means for automatically selecting one of said plurality of gain tables according to the overall average background noise level of said input signal;

channel gain modifying means for adjusting the gain of each of said plurality of pre-processed channels provided by said signal dividing means according to said channel gain values, thereby producing a plurality of post-processed channels; and

channel combination means for recombining said plurality of post-processed channels to produce said post-processed output signal.

18. The improved noise suppression system according to claim 17, wherein each individual channel gain value provided by said channel gain controlling means is selected as a function of (a) the channel number, (b) the current channel SNR estimate, and (c) the overall average background noise level.

19. The improved noise suppression system according to claim 17, further comprising:

gain smoothing means for smoothing the gain values provided by said channel gain controlling means to said channel gain modifying means.

20. The improved noise suppression system according to claim 17, wherein said gain table selection means includes noise level quantization means for providing a digital gain table selection signal in response to the analog level of the average background noise of said input signal.

21. The improved noise suppression system according to claim 20, wherein said noise level quantization means includes hysteresis such that said gain table selection signal is not responsive to minimal changes in the average background noise level of said input signal.

22. The improved noise suppression system according to claim 17, wherein said channel noise estimation means further includes;

background noise estimation means for generating and storing an estimate of the background noise power spectral density of said pre-processed input signal; and

channel SNR estimation means for generating an estimate of the SNR of each individual channel based upon the current background noise energy estimate and the current input signal energy estimate.

23. The improved noise suppression system according to claim 22, wherein said background noise estimation means includes valley detector means for periodically detecting the minima of the input signal energy such that said background noise estimates are updated only during said minima.

24. The improved noise suppression system according to claim 19, wherein said gain smoothing means operates on a per-sample basis.

25. An improved channel gain controller for use with a spectral gain modification noise suppression system having separating means to divide a noisy input signal into a plurality of channels, and a modifying means to adjust the gain of said channels according to gain values provided by the channel gain controller to produce a plurality of noise-suppressed output channels, said channel gain controller comprising:

a plurality of gain tables, each having predetermined individual channel gain values corresponding to various individual channel signal-to-noise ratio (SNR) estimates; and

gain table selection means for automatically selecting one of said plurality of gain tables according to the overall average background noise level of said noisy input signal.

26. The improved channel gain controller according to claim 25, wherein each individual channel gain value provided by said channel gain controller is selected as a function of (a) the channel number, (b) the current channel SNR estimate, and (c) the overall average background noise level.

27. The improved channel gain controller according to claim 25, wherein said gain table selection means further includes noise level quantization means for providing a digital gain table selection signal in response to the analog level of the average background noise of said input signal.

28. The improved channel gain controller according to claim 27, wherein said noise level quantization means includes hysteresis such that said gain table selection signal is not responsive to minimal changes in the average background noise level of said input signal.

29. The improved channel gain controller according to claim 25, further comprising:

gain smoothing means for smoothing the gain values provided by said channel gain controller to said noise suppression system modifying means.

30. The improved channel gain controller according to claim 29, wherein said gain smoothing means operates on a per-sample basis.

31. The method of attenuating the background noise from a noisy input signal to produce a noise-suppressed output signal comprising the steps of:

separating the input signal into a plurality of pre-processed signals representative of selected frequency channels;

modifying an operating parameter of each of said plurality of pre-processed signals to provide a plurality of post-processed signals; and

generating a modification signal responsive to said plurality of pre-processed signals, said modification signal having a selected modification value for each channel to enable the operating parameter to be modified, said modification signal generated by automatically selecting a modification value for each channel from one of a plurality of sets of modification values for that channel.

32. The method of attenuating the background noise from a noisy input signal to produce a noise-suppressed output signal in a noise suppression system comprising the steps of:

separating the input signal into a plurality of pre-processed signals representative of selected frequency channels, each of said plurality of pre-processed signals comprised of a plurality of frames, each frame comprised of a plurality of samples of said input signal;

modifying an operating parameter of each of said plurality of pre-processed signals to provide a plurality of post-processed signals; and

generating a modification signal responsive to said plurality of pre-processed signals, said modification signal having a selected modification value for each channel to enable the operating parameter to be modified, said modification values being smoothed multiple times per frame to reduce discontinuities in said modification signal.

33. The method according to claim 32, wherein said modification values are smoothed on a per-sample basis.

34. The method according to claim 31 or 32, wherein said operating parameter of each of said plurality of pre-processed signals is the gain of said signal.

35. The method according to claim 31 or 32, further comprising the step of:

combining said plurality of post-processed signals to produce said noise-suppressed output signal.

36. The method of attenuating the background noise from a noisy input signal to produce a noise-suppressed output signal by spectral gain modification, comprising the steps of:

separating the input signal into a plurality of pre-processed signals representative of selected frequency channels;

generating an estimate of the signal-to-noise ratio (SNR) in each individual channel;

producing a gain value for each channel by automatically selecting one of a plurality of gain tables in response to a multi-channel noise parameter, and selecting one of a plurality of gain values from the selected gain table in response to said channel SNR estimates and the channel number; and

modifying the gain of each of said plurality of pre-processed signals in response to said gain values to provide a plurality of post-processed signals.

37. The method of attenuating the background noise from a noisy input signal to produce a noise-suppressed output signal by spectral gain modification, comprising the steps of:

separating the input signal into a plurality of pre-processed signals representative of selected frequency channels, each of said plurality of pre-processed signals comprised of a plurality of frames, each frame comprised of a plurality of samples of said input signal;

generating an estimate of the signal-to-noise ratio (SNR) in each individual channel once each frame;

producing a raw gain value for each channel in response to said SNR estimates once each frame;

smoothing said raw gain values multiple times per frame; and

modifying the gain of each of said plurality of pre-processed signals in response to said smoothed gain values to provide a plurality of post-processed signals.

38. The improved noise suppression system according to claim 36, wherein said multi-channel noise parameter is the overall average background noise level of all channels comprising said input signal.

39. The method according to claim 37, wherein said gain values are smoothed on a per-sample basis.

40. The improved noise suppression system according to claim 36 or 37, wherein said SNR estimates are generated by dividing current input signal energy estimates by previous background noise energy estimates for each individual channel.

41. The improved noise suppression system according to claim 36 or 37, wherein the channel gains are modified by multiplying the amplitude of each of said plurality of pre-processed signals by the appropriate channel gain value, thereby providing said plurality of post-processed signals.

42. The method according to claim 36 or 37, further comprising the step of:

combining said plurality of post-processed signals to produce said noise-suppressed output signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to acoustic noise suppression systems, and, more particularly, to a novel technique for automatically selecting gain parameters for a noise suppression system employing spectral subtraction.

2. Description of the Prior Art

The primary objective of acoustic noise suppression systems is to improve the overall quality of speech. The addition of noise suppression to a speech communication system enhances speech intelligibility by filtering environmental background noise from the desired speech signal. This speech enhancement process is particularly necessary in environments having abnormally high levels of ambient background noise, such as a noisy factory, an aircraft, or a moving vehicle.

Numerous approaches have been proposed for enhancement of speech that has been degraded by ambient background noise. An overview of these techniques may be found in J. S. Lim and A. V. Oppenheim, "Enhancement and Bandwidth Compression of Noisy Speech," Proc. IEEE, vol. 67, no. 12 (December 1979), pp. 1586-1604. One very sophisticated technique, described therein, is the process of spectral subtraction. In this approach, the entire input signal spectrum is divided by a bank of bandpass filters, and particular spectral bands (corresponding to the filtered output signals) exhibiting relatively low signal-to-noise ratios (SNRs) are attenuated. All of the spectral bands, including both the attenuated bands and those bands which were not affected due to the their high SNRs, are then recombined to produce the noise-suppressed output signal

Several modifications to the basic spectral subtraction noise suppression technique have been described in the prior art. For example, R. J. McAulay and M. L. Malpass, in the article "Speech Enhancement Using a Soft-Decision Noise Suppression Filter," IEEE Trans. Acoust., Speech, Signal Processing, vol. ASSP-28, no. 2, (April 1980), pp. 137-145, propose a two-state soft-decision maximum-liklihood algorithm which results in a class of various noise suppression curves. In terms of a noise suppression prefilter, these curves determine the amount of suppression applied to a particular frequency channel by utilizing the measured SNR as a pointer for a look-up table to determine the attenuation for that particular spectral band. In other words, the noise suppression gain parameter is determined as a function of the individual channel number and the estimated signal-to-noise ratio.

Alternative methods for determining the noise suppression gain factors are described by Kates, in U.S. Pat. No. 4,454,609 and by Graupe et. al., in U.S. Pat. No. 4,185,168. Kates describes a combinational logic matrix providing weighting factors based upon certain combinations of the envelope-detected input signal energies and empirically-determined constant coefficients. These weights are then compared to a preselected threshold, and a gain factor is selected. Graupe describes an adaptive filter wherein the gain-to-noise parameter relationship approximates that of a Weiner or Kalman filter. Again, the gain parameters are selected as a function of the amount of detected energy in a particular band of input signal.

However, in specialized applications involving abnormally high background noise levels, even the more sophisticated noise suppression techniques become ineffective. One example of such application is the vehicle speakerphone option to a cellular mobile radio telephone system which provides hands-free operation for the automobile driver. The mobile hands-free microphone is typically located at a greater distance from the user, such as being mounted overhead on the visor. The more distant microphone delivers a much poorer signal-to-noise level to the land-end party due to road and wind noise conditions. Although the received speech signal at the land-end is usually intelligible, continuous exposure to such background noise levels often increases listener fatigue.

Although most prior art techniques perform sufficiently well under nominal background noise conditions, the performance of these approaches becomes severely limited when used in such specialized applications of unusually high background noise. Typical spectral subtraction noise suppression systems may reduce the background noise level over the voice frequency spectrum by as much as 10 dB without seriously affecting the speech quality. However, when these prior art techniques are used in relatively high background noise environments requiring noise suppression levels approaching 20 dB, there is a substantial degradation in the quality characteristics of the voice. Furthermore, in rapidly-changing high noise environments, a severe low frequency noise flutter develops in the output speech signal. This noise flutter is inherent to a spectral subtraction noise suppression system, since the individual channel gain parameters are continuously being updated in response to the changing background noise environment.

Hence, acoustic noise suppression systems usually represent a substantial compromise between noise suppression depth and distortion of the desired speech signal. A need, therefore, exists for an improved method and means for selecting noise suppression gain parameters adapted for use in high ambient noise environments without compromising voice quality

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an improved method and apparatus for suppressing background noise in speech communications systems.

Another object of the present invention is to provide an improved noise suppression system which attains sufficient noise attenuation in high background noise environments without significantly degrading the voice quality.

Still another object of the present invention is to provide a means and method for improving noise flutter performance of a noise suppression system used in high background noise environments.

A more particular object of the present invention is to provide a means to automatically select noise suppression gain factors for a spectral gain modification noise suppression system as a function of the average background noise level.

In accordance with the present invention, an improved noise suppression system employing spectral gain modification is provided which performs speech quality enhancement by attenuating the background noise from a noisy pre-processed input signal--the speech-plus-noise signal available at the input of the noise suppression system--to produce a noise-suppressed post-processed output signal--the speech-minus-noise signal provided at the output of the noise suppression system--by spectral gain modification. The noise suppression system of the present invention includes a means for separating the input signal into a plurality of pre-processed signals representative of selected frequency channels, and a means for modifying an operating parameter, such as the gain, of each of these pre-processed signals according to a modification signal to provide post-processed noise-suppressed output signals. The means for generating the modification signal is responsive not only to the noise content of each individual channel, but also to a multi-channel noise parameter such as an average overall background noise level.

Accordingly, the automatic gain selection means of the present invention produces gain factors for each channel by automatically selecting one of a plurality of gain table sets in response to the overall average background noise level of the input signal, and by selecting one of a plurality of gain values from each gain table in response to the individual channel signal-to-noise ratio estimate. Thus, each individual channel gain value is selected as a function of (a) the channel number, (b) the current channel SNR estimate, and (c) the overall average background noise level. This gain table selection technique allows a wider choice of channel gain values adaptable to particular background noise environments, thereby permitting significantly more noise suppression depth without increasing distortion in the noise-suppressed speech.

The problem of severe noise flutter caused by step discontinuities in frame-to-frame noise suppression gain changes is also addressed by the present invention. The automatic gain selector of the present invention includes a means for smoothing these noise suppression gain factors for each individual channel on a per-sample basis. This smoothing of the raw gain factors during every sample of speech, as opposed to every frame of speech, effectively eliminates the discontinuities in the output waveform, such that the noise flutter performance is significantly improved without degradation of the voice quality. Furthermore, the present invention utilizes different smoothing coefficients for each channel to compensate for the different gain table sets employed. This correlation of the per-channel gain smoothing filter time constant to the overall average background noise level results in a further improvement in the audible quality of the speech.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of a basic noise suppression system known in the art which illustrates the spectral gain modification technique;

FIG. 2 is a block diagram of an alternate implementation of a prior art noise suppression system illustrating the channel filter-bank technique;

FIG. 3 is a detailed block diagram illustrating the implementation of the channel filter-bank technique;

FIG. 4 is a detailed block diagram illustrating the preferred embodiment of the present invention channel gain controller block of FIG. 3;

FIGS. 5a and b flowcharts illustrating the general sequence of operations performed in accordance with the practice of the present invention; and

FIGS. 6a and b detailed flowcharts illustrating specific sequences of operations as shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the general principle of spectral subtraction noise suppression as known in the art. A continuous time signal containing speech plus noise is applied to input 102 of noise suppression system 100. This signal is then converted to digital form by analog-to-digital converter 105. The digital data is then segmented into blocks of data by the windowing operation (e.g., Hamming, Hanning, or Kaiser windowing techniques) performed by window 110. The choice of the window is similar to the choice of the filter response in an analog spectrum analysis. The noisy speech signal is then converted into the frequency domain by Fast Fourier Transform (FFT) 115. The power spectrum of the noisy speech signal is calculated by magnitude squaring operation 120, and applied to background noise estimator 125 and to power spectrum modifier 130.

The background noise estimator performs two functions: (1) it determines when the incoming speech-plus-noise signal contains only background noise; and (2) it updates the old background noise power spectral density estimate when only background noise is present. The current estimate of the background noise power spectrum is subtracted from the speech-plus-noise power spectrum by power spectrum modifier 130, which ideally leaves only the power spectrum of clean speech. The square root of the clean speech power spectrum is then calculated by magnitude square root operation 135. This magnitude of the clean speech signal is combined with the phase information 145 of the original signal, and converted from the frequency domain