Noise reduction method and apparatus

• US 6,230,123 B1
• Filed: 12/03/1998
• Issued: 05/08/2001
• Est. Priority Date: 12/05/1997
• Status: Expired due to Term

• Current Assignee: Blackberry Limited
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Cavander, Hans, Mekuria, Fisseha
• Primary Examiner(s): Dorvil, Richemond
• Assistant Examiner(s): Nolan, Daniel A.
• Application Number:
  US09/205,342
• Time in Patent Office: 887 Days
• Field of Search: 704/226, 704/200, 704/201, 704/228, 704/227, 704/233, 704/231, 704/500, 704/503
• US Class Current: 704/226
• CPC Class Codes: G10L 2025/783 based on threshold decision
• CPC Class Codes: H03G 3/32 the control being dependent upon ambient noise level or sound level
• CPC Class Codes: H03G 3/341 Muting when no signals or only weak signals are present H03G3/344, H03G3/345 take precedence

Abstract

Apparatuses for noise reduction and noise processing methods for reducing noise in audio signals are presented. The noise level of an input signal at an input terminal is measured and the noise-to-signal ratio is established. A reduced voice activity detector is used to determine whether the input signal comprises speech or not. If the measured noise level exceeds a threshold level a switch connects the input signal to means for noise reduction. However, if the measured noise level does not exceed the threshold level, i.e. when noise reduction is not needed, the switch disconnects the means for noise reduction and the input signal is passed unchanged. Power is saved by powering off the means for noise reduction when it is not needed.

Claims

1. An apparatus for reducing noise comprising:

an input terminal for receiving an input signal;

a noise measurer connected to the input terminal and operable to measure a noise level of the input signal;

a comparator for receiving the measured noise level from the noise measurer and operable to generate a first control signal by comparing the measured noise level with a first predetermined noise level;

a switch connected to the input terminal and controlled by the first control signal generated by the comparator; and

a noise reducer connected to the switch and operable to reduce noise within the input signal when said noise reducer is activated by the switch, wherein said first control signal is operable to control the switch so as to activate the noise reducer when the measured noise level is higher than the first predetermined noise level, and said first control signal is operable to control the switch so as to deactivate the noise reducer when the measured noise level is lower than the first predetermined noise level.

2. The apparatus of claim 1, further comprising an analog-to-digital converter for digitizing said input signal.

3. The apparatus of claim 2, wherein said noise measurer includes a voice activity detector and means for establishing a noise-to-signal ratio corresponding to the measured noise level.

4. The apparatus of claim 3, wherein said noise-to-signal ratio is calculated according to:

$R=\frac{\frac{1}{N}·<br><br>\underset{n=1}{\overset{N}{\sum <br><br>}}<br><br>v<br><br>\left(n\right)}{\frac{1}{N}·<br><br>\underset{n=1}{\overset{N}{\sum <br><br>}}<br><br>x<br><br>\left(n\right)}$

where R is the noise-to-signal ratio, v(n) is the input signal at sample n when said voice activity detector does not detect voice, x(n) is the input signal at sample n when said voice activity detector detects voice and N is a predetermined number of samples.

5. The apparatus of claim 3, wherein said voice activity detector includes a reduced voice activity detector including an auto-correlation function detector and an energy detector.

6. The apparatus of claim 1, wherein said noise measurer, said comparator, said switch and said noise reducer are implemented by a digital signal processor.

7. The apparatus of claim 1, wherein said comparator is further operable to generate a second control signal by comparing the measured noise level with a second predetermined noise level higher than said first predetermined noise level;

and

said switch is further controlled by said second control signal generated by the comparator; and

wherein said apparatus further comprises;

a filter connected to the switch and operable to filter noise within the input signal when said filter is activated by the switch, wherein said second control signal is operable to control the switch so as to activate the filter when the measured noise level is higher than the second predetermined noise level during which the noise reducer is also deactivated even when the measured noise level is higher than the first predetermined noise level.

8. The apparatus of claim 7, wherein said filter includes a low pass filter.

9. The apparatus of claim 1, wherein said apparatus is implemented within a mobile communication device.

10. The apparatus of claim 1, wherein said apparatus is implemented within a communications system.

11. A noise processing method for reducing noise comprising the steps of:

measuring a noise level of an input signal;

determining if the measured noise level is lower than a first predetermined noise level; and

providing the input signal as an output signal when the measured noise level is lower than said first predetermined noise level and, otherwise, performing noise reduction on the input signal and providing the so processed input signal as the output signal.

12. The noise processing method of claim 11 further including the step of deactivating the noise reduction when the measured noise level is lower than said first predetermined noise level.

13. The noise processing method of claim 11 further including the step of digitizing the input signal.

14. The noise processing method of claim 11 further including the step of establishing a noise-to-signal ratio corresponding to the measured noise level.

15. A noise processing method for reducing noise comprising the steps of:

measuring a noise level of an input signal;

determining if the measured noise level is lower than a first predetermined noise level or higher than a second predetermined noise level, said second predetermined noise level being higher than said first predetermined noise level; and

providing the input signal as an output signal when the measured noise level is lower than said first predetermined noise level and, otherwise, performing noise reduction on the input signal and providing the so processed input signal as the output signal when the measured noise level is higher than or equal to said first predetermined noise level but lower than said second predetermined noise level and, otherwise, performing low pass filtering on said input signal and providing the so filtered input signal as the output signal when the measured noise level is higher than or equal to said second predetermined noise level.

16. The noise processing method of claim 15 further including the step of deactivating the noise reduction when the measured noise level is lower than said first predetermined noise level and higher than said second predetermined noise level.

17. The noise processing method of claim 15 further including the step of deactivating the low pass filtering when the measured noise level is lower than said second predetermined noise level.

18. The noise processing method of claim 15 further including the step of digitizing the input signal.

19. The noise processing method of claim 15 further including the step of establishing a noise-to-signal ratio corresponding to the measured noise level.