Noise reduction apparatus and method

• US 6,757,395 B1
• Filed: 01/12/2000
• Issued: 06/29/2004
• Est. Priority Date: 01/12/2000
• Status: Expired due to Term

• Current Assignee: Sonic Innovations Incorporated (William Demant Fonden)
• Original Assignee: Sonic Innovations Incorporated (William Demant Fonden)
• Inventors: Fang, Xiaoling, Nilsson, Michael J.
• Primary Examiner(s): Harvey, Minsun Oh
• Assistant Examiner(s): PENDLETON, BRIAN T
• Application Number:
  US09/482,192
• Time in Patent Office: 1,630 Days
• Field of Search: 381/94.3, 381/94.1, 381/94.2, 704/226, 704/233
• US Class Current: 381/94.3
• CPC Class Codes: G10L 21/0208 Noise filtering

Abstract

A multi-band spectral subtraction scheme is proposed, comprising a multi-band filter architecture, noise and signal power detection, and gain function for noise reduction. In one embodiment, the gain function for noise reduction consists of a gain scale function and a maximum attenuation function providing a predetermined amount of gain as a function of signal to noise ratio (“SNR”) and noise. In one embodiment, the gain scale function is a three-segment piecewise linear function, and the three piecewise linear sections of the gain scale function include a first section providing maximum expansion up to a first knee point for maximum noise reduction, a second section providing less expansion up to a second knee point for less noise reduction, and a third section providing minimum or no expansion for input signals with high SNR to minimize distortion. According to embodiments of the present invention, the maximum attenuation function can either be a constant or equal to the estimated noise envelope. The disclosed noise reduction techniques can be applied to a variety of speech communication systems, such as hearing aids, public address systems, teleconference systems, voice control systems, or speaker phones. When used in hearing aid applications, the noise reduction gain function according to aspects of the present invention is combined with the hearing loss compensation gain function inherent to hearing aid processing.

Claims

1. A method for reducing noise in audio processing applications, the method comprising:

separating audio signals through an analysis filter into a plurality of processing bands, wherein each said processing band processes said audio signals within a predetermined frequency band;

generating a gain function for noise reduction in each said processing band, wherein said gain function comprises a gain scale function providing a predetermined amount of gain as a function of a ratio of a signal envelope to a noise envelope and a maximum attenuation function providing a predetermined maximum attenuation;

combining the output of each said gain function with the input of each said gain function in a multiplying circuit; and

combining the outputs of said multiplying circuits in a synthesis filter to produce a stream of processed audio samples,

wherein said generating a gain function for noise reduction in each said processing band comprises;

(1) calculating the magnitude of each of a stream of input samples;

(2) converting the output of step (1) into the decibel domain;

(3) estimating the signal envelope of the output of step (2);

(4) estimating the noise envelope based on the output of step (3);

(5) generating a decibel domain gain scale function for noise reduction as a function of the outputs of steps (3) and (4);

(6) generating a decibel domain maximum attenuation function;

(7) combining the outputs of steps (5) and (6); and

(8) converting the output of step (7) from the decibel domain to the magnitude domain.

2. The method according to claim 1, wherein said decibel domain gain scale function comprises at least three linear sections with a first section providing maximum expansion up to a first knee point for maximum noise reduction, a second section providing less expansion up to a second knee point for less noise reduction, and a third section providing minimum or no expansion to minimize distortion and wherein the amount of expansion in any or all of the first, second, and third sections depends on the ratio of the signal envelope to the noise envelope, and

wherein said decibel domain maximum attenuation function is either a constant or equal to said noise envelope.

3. A noise reduction apparatus comprising:

an analysis filter for separating audio signals into a plurality of outputs;

a plurality of processing bands, wherein the number of processing bands equals the number of outputs and one of said plurality of processing bands is connected to each one of said plurality of outputs, wherein each of said plurality of processing bands processes said audio signals within a predetermined frequency band, and wherein each of said plurality of processing bands comprises;

circuitry for generating a gain function for noise reduction, wherein said gain function comprises a gain scale function providing a predetermined amount of gain as a function of a ratio of a signal envelope to a noise envelope and a maximum attenuation function providing a predetermined maximum attenuation; and

a multiplier having a first input coupled to the output of said circuitry and having a second input coupled to the input of said circuitry; and

a synthesis filter for combining the outputs of all of said plurality of processing bands into a stream of processed audio samples,

wherein said circuitry for generating a gain function for noise reduction comprises;

an absolute value circuit having an input coupled to one of said outputs of said analysis filter;

a logarithmic circuit coupled to the output of said absolute value circuit for converting the output of said absolute value circuit into the decibel domain;

a signal envelope estimator coupled to the output of said logarithmic circuit;

a noise envelope estimator coupled to the output of said signal envelope estimator;

a decibel domain amplifier having a first input coupled to the output of said signal envelope estimator and having a second input coupled to the output of said noise envelope estimator; and

an exponential circuit coupled to the output of said decibel domain amplifier for converting the output of said decibel domain amplifier from the decibel domain to the magnitude domain.

4. The apparatus according to claim 3, wherein said decibel domain amplifier generates a decibel domain gain scale function and a decibel domain maximum attenuation function, wherein said decibel domain gain scale function comprises at least three linear sections with a first section providing maximum expansion up to a first knee point for maximum noise reduction, a second section providing less expansion up to a second knee point for less noise reduction, and a third section providing minimum or no expansion to minimize distortion and wherein the amount of expansion in any or all of the first, second, and third sections depends on the ratio of the signal envelope to the noise envelope, and

wherein said decibel domain maximum attenuation function is either a constant or equal to said noise envelope.

5. A noise reduction apparatus comprising:

an analysis filter for separating audio signals into a plurality of outputs;

a plurality of processing bands, wherein the number of processing bands equals the number of outputs and one of said plurality of processing bands is connected to each one of said plurality of outputs, wherein each of said plurality of processing bands processes said audio signals within a predetermined frequency band, and wherein each of said plurality of processing bands comprises;

circuitry for generating a gain function for noise reduction, wherein said gain function comprises a gain scale function providing a predetermined amount of gain as a function of a ratio of a signal envelope to a noise envelope and a maximum attenuation function providing a predetermined maximum attenuation; and

a multiplier having a first input coupled to the output of said circuitry and having a second input coupled to the input of said circuitry; and

a synthesis filter for combining the outputs of all of said plurality of processing bands into a stream of processed audio samples,

wherein said circuitry for generating a gain function for noise reduction further comprises a gain function for hearing loss compensation and wherein the circuitry for generating a gain function for noise reduction and hearing loss compensation comprises;

an absolute value circuit having an input coupled to one of said outputs of said analysis filter;

a logarithmic circuit coupled to the output of said absolute value circuit for converting the output of said absolute value circuit into the decibel domain;

a signal envelope estimator coupled to the output of said logarithmic circuit;

a noise envelope estimator coupled to the output of said signal envelope estimator;

a decibel domain amplifier for noise reduction having a first input coupled to the output of said signal envelope estimator and having a second input coupled to the output of said noise envelope estimator;

a first summing circuit having a first input coupled to the output of said decibel domain amplifier for noise reduction and having a second input coupled to the output of said signal envelope estimator;

a decibel domain amplifier for hearing loss having an input coupled to the output of said first summing circuit;

a second summing circuit having a first input coupled to the output of said decibel domain amplifier for hearing loss and having a second input coupled to the output of said decibel domain amplifier for noise reduction; and

an exponential circuit coupled to the output of said second summing circuit for converting the output of said second summing circuit from the decibel domain to the magnitude domain.

6. The apparatus according to claim 5, wherein said decibel domain amplifier for noise reduction applies a decibel domain gain scale function and a decibel domain maximum attenuation function, wherein said decibel domain gain scale function comprises at least three linear sections with a first section providing maximum expansion up to a first knee point for maximum noise reduction, a second section providing less expansion up to a second knee point for less noise reduction, and a third section providing minimum or no expansion to minimize distortion and wherein the amount of expansion in any or all of the first, second, and third sections depends on the ratio of the signal envelope to the noise envelope, and

wherein said decibel domain maximum attenuation function is either a constant or equal to said noise envelope.

7. A method for reducing noise in audio processing applications, the method comprising:

separating audio signals through an analysis filter into a plurality of processing bands, wherein each said processing band processes said audio signals within a predetermined frequency band;

generating a gain function for noise reduction in each said processing band, wherein said gain function comprises a gain scale function providing a predetermined amount of gain as a function of a ratio of a signal envelope to a noise envelope and a maximum attenuation function providing a predetermined maximum attenuation;

combining the output of each said gain function with the input of each said gain function in a multiplying circuit; and

combining the outputs of said multiplying circuits in a synthesis filter to produce a stream of processed audio samples,

wherein said generating a gain function for noise reduction in each said processing band further comprises a gain function for hearing loss compensation in each said processing band and wherein said generating a gain function for noise reduction and hearing loss compensation comprises;

(1) calculating the magnitude of each of a stream of input samples;

(2) converting the output of step (1) into the decibel domain;

(3) estimating the signal envelope of the output of step (2);

(4) estimating the noise envelope based on the output of step (3);

(5) generating a decibel domain gain scale function for noise reduction as a function of the outputs of steps (3) and (4);

(6) generating a decibel domain maximum attenuation function;

(7) combining the outputs of steps (5) and (6);

(8) generating a decibel domain gain function for hearing loss as a function of the output of step (3);

(9) summing the outputs of steps (7) and (8); and

(10) converting the output of step (9) from the decibel domain to the magnitude domain.

8. A noise reduction apparatus comprising:

an analysis filter for separating audio signals into a plurality of outputs;

a plurality of processing bands, wherein the number of processing bands equals the number of outputs and one of said plurality of processing bands is connected to each one of said plurality of outputs, wherein each of said plurality of processing bands processes said audio signals within a predetermined frequency band, and wherein each of said plurality of processing bands comprises;

circuitry for generating a gain function for noise reduction, wherein said gain function comprises a gain scale function providing a predetermined amount of gain as a function of a ratio of a signal envelope to a noise envelope and a maximum attenuation function providing a predetermined maximum attenuation; and

a multiplier having a first input coupled to the output of said circuitry and having a second input coupled to the input of said circuitry; and

a synthesis filter for combining the outputs of all of said plurality of processing bands into a stream of processed audio samples,

wherein said circuitry for generating a gain function for noise reduction further comprises a gain function for hearing loss compensation and wherein the circuitry for generating a gain function for noise reduction and hearing loss compensation comprises;

an absolute value circuit having an input coupled to one of said outputs of said analysis filter;

a logarithmic circuit coupled to the output of said absolute value circuit for converting the output of said absolute value circuit into the decibel domain;

a signal envelope estimator coupled to the output of said logarithmic circuit;

a noise envelope estimator coupled to the output of said signal envelope estimator;

a decibel domain amplifier for noise reduction having a first input coupled to the output of said signal envelope estimator and having a second input coupled to the output of said noise envelope estimator;

a decibel domain amplifier for hearing loss compensation having an input coupled to the output of said signal envelope estimator;

a summing circuit having a first input coupled to the output of said decibel domain amplifier for hearing loss compensation and having a second input coupled to the output of said decibel domain amplifier for noise reduction; and

an exponential circuit coupled to the output of said summing circuit for converting the output of said summing circuit from the decibel domain to the magnitude domain.

9. A method of reducing noise in audio applications, the method comprising:

generating a gain function for noise reduction to include (1) a gain scale function and (2) a maximum attenuation function, wherein said gain scale function provides a predetermined amount of gain as a function of a combination of (A) the ratio of a signal envelope to a noise envelope and (B) the noise envelope, wherein said gain scale function is a piecewise linear function in the logarithmic domain, and wherein said maximum attenuation function provides a predetermined maximum attenuation.

10. The method according to claim 9, wherein said piecewise linear function comprises a plurality of linear sections with at least a first section providing expansion up to a first knee point for noise reduction and at least a second section providing minimum or no expansion to minimize distortion and wherein the amount of expansion in any or all of said plurality of sections depends on the ratio of the signal envelope to the noise envelope.

11. The method according to claim 9, wherein said maximum attenuation function is either a constant or proportional to said noise envelope.

12. The method according to claim 9, further comprising:

(1) calculating the magnitude of each of a stream of input samples;

(2) converting the output of step (1) into the logarithmic domain;

(3) estimating the signal envelope of the output of step (2);

(4) estimating the noise envelope based on the output of step (3);

(5) combining the outputs of said gain scale function and said maximum attenuation function; and

(6) converting the output of step (5) from the logarithmic domain to the magnitude domain.

13. The method according to claim 9, wherein said generating a gain function for noise reduction further comprises a gain function for hearing loss compensation and wherein said generating a gain function for noise reduction and hearing loss compensation comprises:

(1) calculating the magnitude of each of a stream of input samples;

(2) converting the output of step (1) into the logarithmic domain;

(3) estimating the signal envelope of the output of step (2);

(4) estimating the noise envelope based on the output of step (3);

(5) combining the outputs of said gain scale function and said maximum attenuation function;

(6) summing the outputs of steps (3) and (5);

(7) generating, a logarithmic domain gain function for hearing loss as a function of the output of step (6);

(8) summing the outputs of steps (5) and (7); and

(9) converting the output of step (8) from the logarithmic domain to the magnitude domain.

14. The method according to claim 9, wherein said generating a gain function for noise reduction further comprises a gain function for hearing loss compensation and wherein said generating a gain function for noise reduction and hearing loss compensation comprises:

(1) calculating the magnitude of each of a stream of input samples;

(2) converting the output of step (1) into the logarithmic domain;

(3) estimating the signal envelope of the output of step (2);

(4) estimating the noise envelope based on the output of step (3);

(5) combining the outputs of said gain scale function and said maximum attenuation function;

(6) generating a logarithmic domain gain function for hearing loss as a function of the output of step (3);

(7) summing the outputs of steps (5) and (6); and

(8) converting the output of step (7) from the logarithmic domain to the magnitude domain.

15. An audio processor for reducing noise in audio applications, the audio processor comprising:

circuitry for generating a gain function for noise reduction to include (1) a gain scale function and (2) a maximum attenuation function, wherein said gain scale function provides a predetermined amount of gain as a function of a combination of (A) the ratio of a signal envelope to a noise envelope and (B) the noise envelope, wherein said gain scale function is a piecewise linear function in the logarithmic domain, and wherein said maximum attenuation function provides a predetermined maximum attenuation.

16. The audio processor according to claim 15, wherein said piecewise linear function comprises a plurality of linear sections with at least a first section providing expansion up to a first knee point for noise reduction and at least a second section providing minimum or no expansion to minimize distortion and wherein the amount of expansion in any or all of said plurality of sections depends on the ratio of the signal envelope to the noise envelope.

17. The audio processor according to claim 15, wherein said maximum attenuation function is either a constant or proportional to said noise envelope.

18. The audio processor according to claim 15, wherein said circuitry for generating a gain function for noise reduction comprises:

an absolute value circuit having an input and an output;

a logarithmic circuit coupled to the output of said absolute value circuit for converting the output of said absolute value circuit into the logarithmic domain;

a signal envelope estimator coupled to the output of said logarithmic circuit;

a noise envelope estimator coupled to the output of said signal envelope estimator;

a logarithmic domain amplifier having a first input coupled to the output of said signal envelope estimator and having a second input coupled to the output of said noise envelope estimator; and

an exponential circuit coupled to the output of said logarithmic domain amplifier for converting the output of said logarithmic domain amplifier from the logarithmic domain to the magnitude domain.

19. The audio processor according to claim 15, wherein said circuitry for generating a gain function for noise reduction further comprises a gain function for hearing loss compensation and wherein the circuitry for generating a gain function for noise reduction and hearing loss compensation comprises:

an absolute value circuit having an input and an output;

a logarithmic circuit coupled to the output of said absolute value circuit for converting the output of said absolute value circuit into the logarithmic domain;

a signal envelope estimator coupled to the output of said logarithmic circuit;

a noise envelope estimator coupled to the output of said signal envelope estimator;

a logarithmic domain amplifier for noise reduction having a first input coupled to the output of said signal envelope estimator and having a second input coupled to the output of said noise envelope estimator;

a first summing circuit having a first input coupled to the output of said logarithmic domain amplifier for noise reduction and having a second input coupled to the output of said signal envelope estimator;

a logarithmic domain amplifier for hearing loss having an input coupled to the output of said first summing circuit;

a second summing circuit having a first input coupled to the output of said logarithmic domain amplifier for hearing loss and having a second input coupled to the output of said logarithmic domain amplifier for noise reduction; and

an exponential circuit coupled to the output of said second summing circuit for converting the output of said second summing circuit from the logarithmic domain to the magnitude domain.

20. The audio processor according to claim 15, wherein said circuitry for generating a gain function for noise reduction further comprises a gain function for hearing loss compensation and wherein the circuitry for generating a gain function for noise reduction and hearing loss compensation comprises:

an absolute value circuit having an input and an output;

a logarithmic circuit coupled to the output of said absolute value circuit for converting the output of said absolute value circuit into the logarithmic domain;

a signal envelope estimator coupled to the output of said logarithmic circuit;

a noise envelope estimator coupled to the output of said signal envelope estimator;

a logarithmic domain amplifier for noise reduction having a first input coupled to the output of said signal envelope estimator and having a second input coupled to the output of said noise envelope estimator;

a logarithmic domain amplifier for hearing loss compensation having an input coupled to the output of said signal envelope estimator;

a summing circuit having a first input coupled to the output of said logarithmic domain amplifier for hearing loss compensation and having a second input coupled to the output of said logarithmic domain amplifier for noise reduction; and

an exponential circuit coupled to the output of said summing circuit for converting the output of said summing circuit from the logarithmic domain to the magnitude domain.