Digtal Circuit Arrangements for Ambient Noise-Reduction

• US 20090046867A1
• Filed: 04/11/2007
• Published: 02/19/2009
• Est. Priority Date: 04/12/2006
• Status: Active Grant

• Current Assignee: Cirrus Logic Incorporated
• Original Assignee: Wolfson Microelectronics plc (Cirrus Logic Incorporated)
• Inventors: Clemow, Richard
• Granted Patent: US 8,165,312 B2
• Time in Patent Office: Days
• US Class Current: 381/71.600
• CPC Class Codes: G10K 11/16 Methods or devices for protecting against, or for damping, noise or other acoustic waves in general G10K11/36 takes precedence
• CPC Class Codes: G10K 11/178 by electro-acoustically regenerating the original acoustic waves in anti-phase
• CPC Class Codes: G10K 11/17853 of the 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 2210/3051 Sampling, e.g. variable rate, synchronous, decimated or interpolated
• CPC Class Codes: G10K 2210/3217 Collocated sensor and cancelling actuator, e.g. "virtual earth" designs
• CPC Class Codes: H04R 1/1083 Reduction of ambient noise active noise reduction per se G10K11/175; protective devices for the ear, e.g. providing acoustic protection A61F11/06
• CPC Class Codes: H04R 3/02 for preventing acoustic reaction , i.e. acoustic oscillatory feedback specially adapted for hearing aids H04R25/453

Abstract

The invention provides a digital circuit arrangement for an ambient noise-reduction system affording a higher degree of noise reduction than has hitherto been possible, through the use of a low latency signal processing chain consisting of analogue-to-digital conversion, digital processing and digital-to-analogue conversion. The arrangement converts the analogue signals into N-bit digital signals at sample rate f₀, and then subjects the converted signals to digital filtering. The value of N in some embodiments is 1 but, in any event, is no greater than 8, and f₀ may be 64 times the Nyquist sampling rate but, in any event, is substantially greater than the Nyquist sampling rate. This permits digital processing to be used without incurring group delay problems that rule out the use of conventional digital processing in this context. Furthermore, adjustment of the group delay can readily be achieved, in units of a fraction of a micro-second, providing the ability to fine tune the group delay for feed-forward applications.

Claims

1. A digital circuit arrangement for receiving analogue electrical signals indicative of ambient noise and for performing a filtering operation upon said analogue signals;

the arrangement comprising means for converting said analogue signals into N-bit digital signals at sample rate f₀, digital means for effecting said filtering operation and digital delta-sigma modulator means;

where N is no greater than 8 and f₀ is substantially greater than the Nyquist sampling rate.

2. An arrangement according to claim 1, wherein N is 5 or less.

3. An arrangement according to claim 1, wherein N equals 1.

4. An arrangement according to claim 1, wherein f₀ is at least 8 times the Nyquist sampling rate.

5. An arrangement according to claim 1, wherein f₀ is 64 times the Nyquist sampling rate.

6. An arrangement according to claim 1, adapted such that the delay imparted to signals processed thereby is adjustable.

7. An arrangement according to claim 6, further comprising at least one of:

shift register means, flip-flop means or memory buffer means adapted to adjust said delay.

8. An arrangement according to claim 1, further comprising an output digital-to-analogue converter configured to directly drive a loudspeaker via switching transistors.

9. An arrangement according to claim 1, wherein said digital filtering means comprises at least one HR filter.

10. An arrangement according to claim 1, wherein said digital filtering means incorporates a high-pass filter to remove low frequency ambient noise signals.

11. An arrangement according to claim 1, wherein said digital filtering means incorporates a filter adapted to roll off the noise cancelling effect at higher frequencies.

12. An arrangement according to claim 1, wherein a plurality of filters are combined into a single, more complex, filter function.

13. A noise reduction system comprising microphone means to generate electrical signals indicative of ambient noise, an arrangement according to any preceding claim configured to operate upon said electrical signals, and a loudspeaker disposed to project sound into the ear of a listener;

the circuit arrangement being adapted to supply driving signals to the loudspeaker.

14. A system according to claim 13 configured as a feedback noise reduction system.

15. A system according to claim 13 configured as a feed-forward noise reduction system.

16. A system according to claim 13, wherein the filter of the arrangement is supplied with first and second inputs and includes a mixing function;

said first input being derived from an external microphone forming part of a feed-forward system and said second input being derived from an internal microphone forming part of a feedback system.

17. A system according to claim 13, further comprising means for receiving further sound signals intended to be heard by the listener, and for conveying said further sound signals, or signals derived therefrom, to the listener.

18. A system according to claim 17, wherein said further sound signals comprise music signals.

19. A system according to claim 17, wherein said further sound signals comprise speech signals from a mobile telephone.

20. A system according to claim 13, wherein the digital circuit arrangement comprise further digital processing means adapted to permit adjustment of the degree of noise reduction.

21. A system according to claim 13, wherein the filter incorporates filter means adapted to compensate for a non-flat low-frequency amplitude and phase loudspeaker response in the 20-500 Hz region.

22. A system according to claim 13, further comprising circuit means adapted to reduce the average number of output pulse transitions per second.

23. An arrangement according to claim 1, wherein the digital circuit arrangement is implemented as an application-specific integrated circuit.

24. A system according to claim 13, wherein the digital circuit arrangement is implemented as an application-specific integrated circuit.