NOISE CANCELLATION SYSTEM WITH LOWER RATE EMULATION

• US 20100310086A1
• Filed: 12/12/2008
• Published: 12/09/2010
• Est. Priority Date: 12/21/2007
• Status: Active Grant

• Current Assignee: Cirrus Logic Incorporated
• Original Assignee: Cirrus Logic Incorporated
• Inventors: Magrath, Anthony James, Clemow, Richard
• Granted Patent: US 8,908,876 B2
• Time in Patent Office: Days
• US Class Current: 381/71.11
• CPC Class Codes: G10K 11/17823 Reference signals, e.g. ambient acoustic environment
• CPC Class Codes: G10K 11/1783 handling or detecting of non-standard events or conditions, e.g. changing operating modes under specific operating conditions
• CPC Class Codes: G10K 11/17854 the filter being an adaptive filter
• CPC Class Codes: G10K 11/17855 for improving speed or power requirements
• CPC Class Codes: G10K 11/17873 using a reference signal without an error signal, e.g. pure feedforward
• CPC Class Codes: G10K 11/17875 using an error signal without a reference signal, e.g. pure feedback
• CPC Class Codes: G10K 11/17885 additionally using a desired external signal, e.g. pass-through audio such as music or speech
• CPC Class Codes: G10K 2210/1081 Earphones, e.g. for telephones, ear protectors or headsets
• CPC Class Codes: G10K 2210/3026 Feedback
• CPC Class Codes: G10K 2210/3027 Feedforward
• CPC Class Codes: G10K 2210/3028 Filtering, e.g. Kalman filters or special analogue or digital filters
• CPC Class Codes: G10K 2210/3051 Sampling, e.g. variable rate, synchronous, decimated or interpolated
• CPC Class Codes: H04R 3/002 Damping circuit arrangements for transducers, e.g. motional feedback circuits

Abstract

There is provided a noise cancellation system, comprising: an input for a digital signal, the digital signal having a first sample rate; a digital filter, connected to the input to receive the digital signal; a decimator, connected to the input to receive the digital signal and to generate a decimated signal at a second sample rate lower than the first sample rate; and a processor. The processor comprises: an emulation of the digital filter, connected to receive the decimated signal and to generate an emulated filter output; and a control circuit, for generating a control signal on the basis of the emulated filter output. The control signal is applied to the digital filter to control a filter characteristic thereof.

Claims

1. A noise cancellation system, comprising:

an input for a digital signal, the digital signal having a first sample rate;

a digital filter, connected to the input to receive the digital signal;

a decimator, connected to the input to receive the digital signal and to generate a decimated signal at a second sample rate lower than the first sample rate; and

a processor, wherein the processor comprises;

an emulation of the digital filter, connected to receive the decimated signal and to generate an emulated filter output; and

a control circuit, for generating a control signal on the basis of the emulated filter output,

wherein the control signal is applied to the digital filter to control a filter characteristic thereof.

2. A noise cancellation system as claimed in claim 1, wherein the processor comprises:

a source input, for receiving a wanted signal; and

an adder, for forming a sum of the emulated filter output and the wanted signal,

wherein the control circuit is configured to generate the control signal on the basis of a comparison between said sum of the emulated filter output and the wanted signal and a threshold value.

3. A noise cancellation system as claimed in claim 1, wherein the processor comprises a smoothing filter for smoothing said control signal to reduce noise in the noise cancellation system.

4. A noise cancellation system as claimed in claim 1, wherein the processor further comprises a warping filter for generating the control signal.

5. A noise cancellation system as claimed in claim 1, wherein the emulation of the digital filter comprises a lower order approximation of the digital filter.

6. A noise cancellation system as claimed in claim 5, wherein the digital filter comprises a sixth order IIR filter, and the emulation of the digital filter comprises a second order approximation of the digital filter.

7. A noise cancellation system as claimed in claim 1, wherein the digital filter comprises a fixed part and an adaptive part.

8. A noise cancellation system as claimed in claim 7, wherein the emulation of the digital filter comprises an emulation of the adaptive part of the digital filter.

9. A noise cancellation system as claimed in claim 1, wherein the digital filter comprises a fixed part and an adaptive part, with the fixed part of the digital filter being connected to the input to receive the digital signal, and the adaptive part of the digital filter being connected to the fixed part of the digital filter to receive the input signal filtered by the fixed part of the digital filter;

wherein the decimator is connected to the fixed part of the digital filter to receive the input signal filtered by the fixed part of the digital filter; and

wherein the emulation of the digital filter comprises an emulation of the adaptive part of the digital filter.

10. A noise cancellation system as claimed in claim 1, wherein the filter characteristic is a cut-off frequency of the digital filter.

11. A noise cancellation system as claimed in claim 1, wherein the digital signal is a signal representing frequencies in the audio range.

12. A noise cancellation system as claimed in claim 1, wherein the noise cancellation system is a feedforward noise cancellation system.

13. A noise cancellation system as claimed in claim 1, wherein the noise cancellation system is a feedback noise cancellation system.

14. An integrated circuit, comprising:

a noise cancellation system as claimed in claim 1.

15. A mobile phone, comprising:

an integrated circuit as claimed in claim 14.

16. A pair of headphones, comprising:

an integrated circuit as claimed in claim 14.

17. A pair of earphones, comprising:

an integrated circuit as claimed in claim 14.

18. A headset, comprising:

an integrated circuit as claimed in claim 14.

19. A method of cancelling ambient noise, comprising:

receiving a digital signal, the digital signal having a first sample rate;

filtering said signal with a digital filter;

generating a decimated signal from said digital signal, the decimated signal having a second sample rate lower than the first sample rate;

emulating the digital filter using said decimated signal, generating an emulated filter output; and

controlling a filter characteristic of the digital filter on the basis of the emulated filter output.

20. A method as claimed in claim 19, further comprising:

receiving a wanted signal;

forming a sum of the emulated filter output and the wanted signal; and

controlling the filter characteristic of the digital filter on the basis of a comparison between said sum of the emulated filter output and the wanted signal and a threshold value.)

21. A method as claimed in claim 19, further comprising:

generating a control signal for controlling the filter characteristic of the digital filter; and

smoothing said control signal to reduce noise in the noise cancellation system.

22. A method as claimed in claim 19, wherein said emulating the digital filter comprises approximating the digital filter with a lower order filter.

23. A method as claimed in claim 22, wherein the digital filter comprises a sixth order NR filter, and the emulation of the digital filter comprises a second order approximation of the digital filter.

24. A method as claimed in claim 19, wherein the digital filter comprises a fixed part and an adaptive part.

25. A method as claimed in claim 24, wherein said emulating of the digital filter comprises emulating the adaptive part of the digital filter.

26. A method as claimed in claim 19, wherein the digital filter comprises a fixed part and an adaptive part, with the fixed part of the digital filter receiving the digital signal, and the adaptive part of the digital filter receiving the input signal filtered by the fixed part of the digital filter;

wherein the decimator receives the input signal filtered by the fixed part of the digital filter; and

wherein emulating the digital filter comprises emulating the adaptive part of the digital filter.

27. A method as claimed in claim 19, wherein the filter characteristic is a cut-off frequency of the digital filter.

28. A method as claimed in claim 19, wherein the digital signal is a signal representing frequencies in the audio range.