Subband acoustic noise suppression

• US 5,933,495 A
• Filed: 02/07/1997
• Issued: 08/03/1999
• Est. Priority Date: 02/07/1997
• Status: Expired due to Term

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Oh, Stephen S.
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): APPIAH, CHARLES NANA
• Application Number:
  US08/797,019
• Time in Patent Office: 907 Days
• Field of Search: 379/406, 379/410, 379/411, 379/388, 379/420, 381/94.3, 370/286, 370/289, 370/290, 370/291
• US Class Current: 379/406.08
• CPC Class Codes: H04M 1/6033 for providing handsfree use or a loudspeaker mode in telephone sets arrangements for preventing acoustic echo H04M9/08

Abstract

A conditioning circuit for use with a microphone 224 placed near a loudspeaker 206 has microphone-in 225 and speaker-line 208 input terminals, an echo canceller circuit (212, 216, 220, 222), a noise-suppresser circuit 230, and a synthesis filter 234 coupling noise-suppresser circuit 230 to a microphone-out output terminal 236. Microphone-in 225 and speaker-line 108 input terminals respectively receive microphone and speaker signals. The echo canceller circuit is coupled between microphone-in 225 and speaker-line 108 input terminals and produces a subband reduced-echo microphone signal by (i) transforming the microphone signal into a subband microphone signal and the speaker signal into a filtered subband speaker signal, and (ii) subband subtracting the filtered subband speaker signal from the subband microphone signal. Noise-suppresser circuit 230 is coupled to the echo canceller circuit to produce a subband reduced-noise, reduced-echo microphone signal by subband noise suppression of the subband reduced-echo microphone signal. Synthesis filter 234 transforms the subband reduced-noise, reduced-echo microphone signal into a fullband reduced-noise, reduced-echo microphone signal. Because the subband signal is not restored to a fullband signal until after undergoing both echo cancellation and noise reduction, my system requires less processing power than systems that apply a synthesis filter between subband echo cancellation and noise reduction. This system is useful in hands-free telephones, especially hands-free cellular telephones used in automobiles. A circuit 538 for subbahd detection of near-end speech in the microphone signal can be provided so echo cancellation filter coefficients can be automatically frozen when near-end speech is detected.

Claims

1. A conditioning circuit, comprising:

microphone-in and speaker-line input terminals for respectively receiving microphone and speaker signals, and a microphone-out output terminal;

an echo canceller circuit, coupled between the microphone-in and speaker-line input terminals, for producing a subband reduced-echo microphone signal by (i) transforming the microphone signal into a subband microphone signal and the speaker signal into a filtered subband speaker signal, and (ii) subband subtracting the filtered subband speaker signal from the subband microphone signal;

a subband noise-suppresser circuit, coupled to the echo canceller circuit and receiving said subband reduced-echo microphone signal, for producing a subband reduced-noise, reduced-echo microphone signal by subband noise suppression of the subband reduced-echo microphone signal; and

a synthesis filter, coupled between the noise-suppresser circuit and the microphone-out terminal, for transforming the subband reduced-noise, reduced-echo microphone signal into a fullband reduced-noise, reduced-echo microphone signal;

wherein the echo canceller circuit comprises an adaptive filter having changeable filter coefficients responsive to feedback of the subband reduced-echo microphone signal, for transforming the speaker signal into a filtered subband speaker signal;

wherein the detector circuit comprises;

a circuit for determining an echo path energy ratio (EPR) as an energy ratio between the output signal and the input signal and an echo canceller energy ratio (ECER) as an energy ratio between an output of the echo cancellation circuit and the output signal;

a circuit for comparing EPR to a first predetermined threshold level and ECER to a second predetermined threshold level; and

a sensing circuit for determining that near-end speech is present when EPR exceeds the first predetermined threshold level and at the same time ECER exceeds the second predetermined threshold level, further comprising a detector circuit responsive to the subband microphone signal, subband reduced-echo microphone signal, and reduced-noise, reduced-echo microphone signal for generating a FREEZE control signal only when the microphone signal contains near-end speech (speech actually voiced near the microphone);

the adaptive filter being responsive to the FREEZE control signal for disabling the filter coefficients from changing.

2. The conditioning circuit of claim 1, further comprising a subband residue echo cancellation circuit coupling the synthesis filter to the noise-suppresser circuit, which uses the subband microphone signal to reduce any residual echo in the reduced-noise, reduced-echo microphone signal.

3. The conditioning circuit of claim 1 wherein in each subband i the noise-suppresser circuit estimates a current power ρ

_i (t), updates a noise spectrum estimate NS_i computes a noise suppression gain G_i, determines an adjusted gain by adjusting for a minimum gain, and scales the subband reduced-echo microphone signal with the adjusted gain.

4. A method of operating a hands-free telephone comprising a loudspeaker coupled to an output signal source for generating a corresponding acoustic signal and a microphone for generating an input signal, the method comprising the steps of:

reducing the presence of the output signal in the input signal by subband domain acoustic echo cancellation for generating a subband reduced-echo signal; and

suppressing noise in the subband reduced echo signal without generation an intermediary fullband signal, by subband domain noise suppression for generating a subband reduced-noise reduced echo signal;

further comprising a step of reducing any residual echo signal in the subband reduced-echo reduced-noise signal.

5. The method of claim 4 wherein the step of suppressing noise for each subband i comprises estimating a current power ρ

_i (t), updating a noise spectrum estimate NS_i, computing a noise suppression gain G_i, determining an adjusted gain by adjusting for a minimum gain, and scaling the subband reduced-echo microphone signal with the adjusted gain.

11. The method of claim 5 wherein

12. The method of claim 11 wherein ##EQU5## where k is a predetermined value to reduce the fluctuation of gain due to the power spectrum estimate and NS_i is the noise spectrum estimate for the i^th sub-band.

6. An echo cancellation and noise suppression apparatus comprising:

a loudspeaker responsive to an output signal source for generating a corresponding first acoustic signal;

a microphone responsive to a second acoustic signal which includes a component of the first acoustic signal for generating a corresponding input signal;

a subband echo cancellation circuit coupled between the output signal source and the microphone for generating a subband reduced-echo signal by reducing the component of the first acoustic signal in the input signal; and

a subband noise suppression circuit responsive to the subband reduced-echo signal for generating a subband reduced-noise reduced-echo signal without generating an intermediary full-band signal by reducing noise in the subband reduced-echo signal;

further comprising a subband residue-echo suppression circuit coupled to receive the subband reduced-echo reduced noise signal and the subband signal from the microphone'"'"'s input signal for reducing any residual echo signal.

7. The apparatus of claim 6 wherein the subband noise suppression circuit is for estimating a current power ρ

_i (t), updating a noise spectrum estimate NS_i, computing noise suppression gain G_i, adjusting for a minimum gain, and scaling the reduced-echo signal with the computed gain.

8. The apparatus of claim 7 wherein

9. The apparatus of claim 8 wherein ##EQU4## where k is a predetermined value to reduce the fluctuation of gain due to the power spectrum estimate and NS_i is the noise spectrum estimate for the i^th sub-band.

10. The method of claim 6 wherein EPR and ECER are determined based on frequency components beyond 500 Hz.

13. An echo cancellation and noise suppression apparatus comprising:

a loudspeaker responsive to an output signal source for generating a corresponding first acoustic signal;

a microphone responsive to a second acoustic signal which includes a component of the first acoustic signal for generating a corresponding input signal;

a subband echo cancellation circuit coupled between the output signal source and the microphone for generating a subband reduced-echo signal by reducing the component of the first acoustic signal in the input signal; and

a subband noise suppression circuit responsive to the subband reduced-echo signal for generating a subband reduced-noise reduced-echo signal without generating an intermediary full-band signal by reducing noise in the subband reduced-echo signal further comprising a near-end speech detector comprising;

echo detector for determining an echo path energy ratio (EPR) as an energy ratio between the output signal and the input signal;

an echo canceller output detector for determining an echo canceller energy ratio (ECER) as an energy ratio between an output of the echo cancellation circuit and the output signal;

a comparator circuit for simultaneously comparing (i) EPR to a first predetermined threshold level and (ii) ECER to a second predetermined threshold level; and

a detector circuit responsive to the comparator circuit for indicating that near-end speech is present when EPR and ECER simultaneously respectively exceed the first and second predetermined threshold levels.

14. A conditioning circuit, comprising:

microphone-in and speaker-line input terminals for respectively receiving microphone and speaker signals, and a microphone-out output terminal;

an echo canceller circuit, coupled between the microphone-in and speaker-line input terminals, and comprising an adaptive filter having changeable filter coefficients responsive to feedback of the subband reduced-echo microphone signal, for producing a subband reduced-echo microphone signal by (i) transforming the microphone signal into a subband microphone signal and the speaker signal into a filtered subband speaker signal, and (ii) subband subtracting the filtered subband speaker signal from the subband microphone signal;

a noise-suppresser circuit, coupled to the echo canceller circuit, for producing a subband reduced-noise, reduced-echo microphone signal by subband noise suppression of the subband reduced-echo microphone signal;

a detector circuit comprising a circuit for determining an echo path energy ratio (EPR) as an energy ratio between the output signal and the input signal and an echo canceller energy ratio (ECER) as an energy ratio between an output of the echo cancellation circuit and the output signal;

a circuit for comparing EPR to a first predetermined threshold level and ECER to a second predetermined threshold level; and

a sensing circuit for determining that near-end speech is present when EPR exceeds the first predetermined threshold level and at the same time ECER exceeds the second predetermined threshold level, and responsive to the subband microphone signal, subband reduced- echo microphone signal, and reduced-noise, reduced-echo microphone signal for generating a FREEZE control signal only when the microphone signal contains near-end speech (speech actually voiced near the microphone);

the adaptive filter being responsive to the FREEZE control signal for disabling the filter coefficients from changing; and

a synthesis filter, coupled between the noise-suppresser circuit and the microphone-out terminal, for transforming the subband reduced-noise, reduced-echo microphone signal into a fullband reduced-noise, reduced-echo microphone signal.

15. The conditioning circuit of claim 14 further comprising a subband residue echo cancellation circuit coupling the synthesis filter to the noise-suppresser circuit, which uses the subband microphone signal to reduce any residual echo in the reduced-noise, reduced-echo microphone signal.

16. The conditioning circuit of claim 14 wherein in each subband in the noise-suppresser circuit estimates a current power ρ

_i (t), updates a noise spectrum estimate NS_i, computes a noise suppression gain G_i, determines an adjusted gain by adjusting for a minimum gain, and scales the subband reduced-echo microphone signal with the adjusted gain.

17. The conditioning circuit of claim 14 further comprising a loudspeaker and a microphone respectively coupled to the speaker-line terminal and the microphone-in terminal.

18. A method of operating a hands-free telephone comprising a loudspeaker coupled to an output signal source for generating a corresponding acoustic signal and a microphone for generating an input signal, the method comprising the steps of:

reducing the presence of the output signal in the input signal by subband domain acoustic echo cancellation for generating a subband reduced-echo signal; and

suppressing noise in the subband reduced echo signal by subband domain noise suppression for generating a subband reduced-noise reduced echo signal, and reducing any residual echo signal in the subband reduced-echo reduced-noise signal;

wherein the step of suppressing noise for each subband i comprises estimating a current power ρ

_i (t), updating a noise spectrum estimate NS_i, computing a noise suppression gain G_i, determining an adjusted gain by adjusting for a minimum gain, and scaling the subband reduced-echo microphone signal with the adjusted gain.

19. An echo cancellation and noise suppression apparatus comprising:

a loudspeaker responsive to an output signal source for generating a corresponding first acoustic signal;

a microphone responsive to a second acoustic signal which includes a component of the first acoustic signal for generating a corresponding input signal;

a subband echo cancellation circuit coupled between the output signal source and the microphone for generating a subband reduced-echo signal by reducing the component of the first acoustic signal in the input signal;

a subband noise suppression circuit responsive to the subband reduced-echo signal for generating a subband reduced-noise reduced-echo signal by reducing noise in the subband reduced-echo signal; and

a subband residue-echo suppression circuit coupled to receive the subband reduced-echo reduced noise signal and the subband signal from the microphone input signal for reducing any residual echo signal;

wherein the subband noise suppression circuit is for estimating a current power ρ

_i (t), updating a noise spectrum estimate NS_i , computing noise suppression gain G_i, adjusting for a minimum gain, and scaling the reduced-echo signal with the computed gain.

20. The apparatus of claim 19 wherein the loudspeaker and the microphone comprise a hands-free telephone.

21. The apparatus of claim 19 further comprising a near-end speech detector comprising:

an echo energy detector for determining an echo path energy ratio (EPR) as an energy ratio between the output signal and the input signal;

an echo canceller output detector for determining an echo canceller energy ratio (ECER) as an energy ratio between an output of the echo cancellation circuit and the output signal;

a comparator circuit for simultaneously comparing (i) EPR to a first predetermined threshold level and (ii) ECER to a second predetermined threshold level; and

a detector circuit responsive to the comparator circuit for indicating that near-end speech is present when EPR and ECER simultaneously respectively exceed the first and second predetermined threshold levels.