Noise suppression based on Bark band weiner filtering and modified doblinger noise estimate

• US 20050240401A1
• Filed: 04/23/2004
• Published: 10/27/2005
• Est. Priority Date: 04/23/2004
• Status: Active Grant

• Current Assignee: Cirrus Logic Incorporated
• Original Assignee: Acoustic Technologies Incorporated (Cirrus Logic Incorporated)
• Inventors: Ebenezer, Samuel Ponvarma
• Granted Patent: US 7,492,889 B2
• Time in Patent Office: Days
• US Class Current: 704/226
• CPC Class Codes: G10L 21/0208 Noise filtering
• CPC Class Codes: G10L 21/0216 characterised by the method used for estimating noise

Abstract

In a noise suppresser, an input signal is converted to frequency domain by discrete Fourier analysis and divided into Bark bands. Noise is estimated for each band. The circuit for estimating noise includes a smoothing filter having a slower time constant for updating the noise estimate during noise than during speech. The noise suppresser further includes a circuit to adjust a noise suppression factor inversely proportional to the signal to noise ratio of each frame of the input signal. A noise estimate is subtracted from the signal in each band. A discrete inverse Fourier transform converts the signals back to the time domain and overlapping and combined windows eliminate artifacts that may have been produced during processing.

Claims

1. In a noise suppression circuit including an analysis circuit for dividing an input signal into a plurality of frames, each frame containing a plurality of samples, a circuit for calculating a noise estimate, a circuit for subtracting the noise estimate from the input signal, and a synthesis circuit for reconstructing the frames into an output signal, the improvement comprising:

a plurality of band pass filters for dividing an input signal into a plurality of bands;

means for calculating a noise suppression factor inversely proportional to the signal to noise ratio of each frame in each band.

2. The noise suppression circuit as set forth in claim 1 wherein said band pass filters define Bark bands.

3. The noise suppression circuit as set forth in claim 2 and further including a circuit for limiting spectral gain in said circuit for calculating a noise estimate.

4. The noise suppression circuit as set forth in claim 3 and further including a speech detector, wherein the spectral gain limit is higher when speech is detected than when speech is not detected.

5. The noise suppression circuit as set forth in claim 3 and further including a first smoothing circuit coupled to said circuit for calculating a noise estimate, wherein said first smoothing circuit smoothes gain across the frequency spectrum of the input signal.

6. The noise suppression circuit as set forth in claim 5 wherein said first smoothing circuit smoothes gain across bands below approximately 2 kHz.

7. The noise suppression circuit as set forth in claim 1 wherein said circuit for calculating a noise estimate includes:

a smoothing filter with a slower time constant for updating the noise estimate of a frame when a noisy speech spectrum deviates from a noise estimate by more than a predetermined amount than when the noisy speech spectrum deviates from the noise estimate by less than the predetermined amount, thereby reducing the noise estimate and slowing the change in estimate from frame to frame.

8. The noise suppression circuit as set forth in claim 7 wherein said filter is a first-order exponential averaging smoothing filter.

14. A telephone having an audio processing circuit including a receive channel and a transmit channel, wherein the improvement comprises a noise suppression circuit as set forth in claim 1 in at least one of said channels.

9. In a noise suppression circuit including an analysis circuit for dividing an input signal into a plurality of frames, each frame containing a plurality of samples, a circuit for calculating a noise estimate, a circuit for subtracting the noise estimate from the input signal, and a synthesis circuit for reconstructing the frames into an output signal, the improvement comprising:

a smoothing filter in said circuit for calculating a noise estimate, said smoothing filter having a slower time constant for updating the noise estimate of a frame when a noisy speech spectrum deviates from a noise estimate by more than a predetermined amount than when the noisy speech spectrum deviates from the noise estimate by less than the predetermined amount, thereby reducing the noise estimate and slowing the change in estimate from frame to frame.

10. The noise suppression circuit as set forth in claim 9 and further including a circuit to adjust a noise suppression factor inversely proportional to the signal to noise ratio of each frame.

11. The noise suppression circuit as set forth in claim 10 and further including a circuit for calculating a discrete Fourier transform of each frame of the input signal to convert each frame to frequency domain.

12. The noise suppression circuit as set forth in claim 11 wherein said circuit for calculating a discrete Fourier transform divides the frame into a plurality of bands of progressively higher center frequency.

13. The noise suppression circuit as set forth in claim 12 wherein said bands are Bark bands.

15. A telephone having an audio processing circuit including a receive channel and a transmit channel, wherein the improvement comprises a noise suppression circuit as set forth in claim 9 in at least one of said channels.