Method and system for updating noise estimates during pauses in an information signal

• US 6,108,610 A
• Filed: 10/13/1998
• Issued: 08/22/2000
• Est. Priority Date: 10/13/1998
• Status: Expired due to Fees

• Current Assignee: NCT Group, Inc.
• Original Assignee: Noise Cancellation Technologies, Inc. (NCT Group, Inc.)
• Inventors: Winn, Steve
• Primary Examiner(s): WACHSMAN, HAL D
• Application Number:
  US09/170,594
• Time in Patent Office: 679 Days
• Field of Search: 702/57, 702/60, 702/66-77, 702/79, 702/106, 702/111, 702/193, 702/124, 702/126, 702/185, 702/189-191, 702/195, 702/197, 702/198, 702/FOR 103, 702/FOR 104 , 702/FOR 107, 702/FOR 108, 702/FOR 110, 702/FOR 166, 702/FOR 167-FOR 169, 704/226, 704/208, 704/205, 704/210, 704/228, 704/227, 704/229, 704/225, 704/203, 704/233, 704/234, 381/317, 381/318, 381/320, 381/93, 381/98, 381/94.1-94.3, 708/322, 708/323, 708/309, 708/311, 324/76, 324/19, 324/21, 324/22, 324/76.24, 324/613, 324/614, 375/232-234
• US Class Current: 702/77
• CPC Class Codes: G10L 2021/02168 the estimation exclusively taking place during speech pauses
• CPC Class Codes: G10L 21/0208 Noise filtering
• CPC Class Codes: G10L 25/78 Detection of presence or absence of voice signals switching of direction of transmission by voice frequency in two-way loud-speaking telephone systems H04M9/10

Abstract

The invention relates to an improved adaptive spectral estimator for estimating the spectral components in a signal containing both an information signal, such as speech, and noise. A method and system provide for generating noise estimates and then only updating the noise estimates during pauses in an information signal, when speech or other information is not detected, rather than continuously updating the noise estimates. A noise estimate is calculated for each frequency band and provides for the inclusion of a variable mathematical factor that can be set by the user to produce the best sound quality.

Claims

1. A method for estimating the power of frequency components of an information signal from an input signal containing both the information signal and noise and updating an estimation of noise power of the frequency components, said method comprising:

producing a set of frequency components of the information signal;

calculating the total power in each frequency component of the set of frequency components;

estimating the power of a previous noise reduced output;

calculating a gain for each frequency component as a function of the total power in each frequency component of the information signal, the estimated power of the previous noise reduced output and an estimate of a noise power of the noise;

multiplying each frequency component of the set of frequency components by a corresponding said gain to produce an estimate of the power of each frequency component of said information signal;

detecting a pause in the information signal, further comprising for each frequency component;

determining whether the total power in each frequency component of said information signal exceeds a first predetermined threshold, and

if the total power in each frequency component of said information signal exceeds the first predetermined threshold, then determining whether a threshold value exceeds a second predetermined thresholds wherein the pause is detected if the threshold value exceeds the second predetermined threshold, and

updating the estimate of the noise power during the pause detected in the information signal.

2. The method as in claim 1 wherein if the threshold value does not exceed the second predetermined threshold, then incrementing the threshold value.

3. The method as in claim 1, further comprising:

estimating an overall signal to noise ratio of the input signal from a weighted sum of an estimated signal to noise ratio of each frequency component of the set of frequency components.

4. The method as in claim 3 further comprising

using said estimated overall signal to noise ratio to determine the presence of the information signal in the input signal.

5. The A method as in claim 1, further comprising

combining the calculations of the total power of each frequency component of said set of frequency components of said information signal to produce a noise reduced output signal.

6. The method as in claim 1 in which the estimated power of the previous noise reduced output is estimated from a combination of a previous power estimate of a frequency component of the set of frequency components of said information signal and the positive difference between the total power in the frequency component and the estimate of the noise power.

7. The method as in claim 6, in which the gain in each frequency component of the set of frequency components is determined by:

estimating a Wiener gain from said estimate of the noise power and the estimated power of the previous noise reduced output;

multiplying said Wiener gain by the ratio of the total power of each frequency component of the information signal to the estimated noise power to produce an estimate of a signal to noise ratio of the frequency component;

calculating a function of the estimated signal to noise ratio; and

dividing said function of the estimated signal to noise ratio by the ratio of the total power of each frequency component of the information signal to the estimated noise power to produce a modified gain.

8. The method as in claim 1 which is used for preprocessing the input signal prior to being provided to a speech or voice recognition system.

9. The method as in claim 1 which is used for reducing noise in the input signal provided to a communications system.

10. The method of claim 1 wherein producing the set of frequency components comprises filtering the input signal through a set of band pass filters.

11. The method of claim 1 wherein producing the set of frequency components comprises calculating the Fourier Transform of the input signal.

12. The system for estimating the noise power of frequency components of an information signal from an input signal containing both the information signal and noise, said system comprising:

means to produce a set of frequency components of the information signal;

a first calculating means for calculating the total power in each frequency component of the set of frequency components;

an estimating means for estimating the power of each frequency component of the information signal and for updating a previously made estimate of a noise power of the noise only during a pause detected in the information signal by the estimating means, wherein the estimating means comprises;

an adder that is provided with an input spectral power signal and a first predetermined threshold value;

a first comparison element that receives the estimate of the power of the information signal and the first predetermined threshold value from the adder, wherein the first comparison element determines whether the estimate of the power of the information signal exceeds the first predetermined threshold value; and

a second comparison element coupled to the first comparison element that determines whether a threshold value exceeds a second predetermined threshold value if the estimate of the power of the information signal exceeds the first predetermined threshold value, wherein if the threshold value exceeds the second predetermined threshold value then the pause is detected and the estimating means updates the previously made estimate of the noise power;

a second calculating means for calculating a modified gain for each frequency component as a function of the total power of each frequency component of the information signal, the estimate of the power of a previous noise reduced output and the updated estimate of the noise power; and

gain multiplying means for multiplying each frequency component by a corresponding gain to produce an updated estimate of the power of each frequency component of said information signal.

13. The system as in claim 12 further comprising:

an increment element that increments the threshold value if the threshold value does not exceed the second predetermined threshold value.

14. The system as in claim 12 in which the second calculating means comprises:

means for estimating a Wiener gain from said updated estimate of the noise power and the estimated power of the previous noise reduced output;

Wiener multiplying means for multiplying said estimated Wiener gain by the ratio of the total power of each frequency component to the updated estimate of the noise power to produce an estimate of a signal to noise ratio for each frequency component of the set of frequency components;

function calculating means for calculating a function of the estimated signal to noise ratio; and

division means for dividing said function of the estimated signal to noise ratio by the ratio of the total power of each frequency component to the updated estimate of the noise power to produce a modified gain.

15. The system of claim 12 wherein said means to produce a set of frequency components filters the input signal through a set of band pass filters.

16. The system of claim 12 wherein said means to produce a set of frequency components is capable of calculating the Fourier Transform of the input signal.