Digital circuit arrangements for ambient noise-reduction

Patent metadata

Field	Value
Publication number	US-9558729-B2
Application number	US-201314142329-A
Country	US
Kind code	B2
Filing date	Dec 27, 2013
Priority date	Apr 12, 2006
Publication date	Jan 31, 2017
Grant date	Jan 31, 2017

Abstract

A digital circuit arrangement for an ambient noise-reduction system affording a higher degree of noise reduction than has hitherto been possible. The arrangement converts the analog signals into N-bit digital signals at sample rate f 0 , 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 f 0 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.

First claim

The invention claimed is:

1. A sound reproduction system configured as a feedforward noise reduction system, comprising:
a sound source for generating a desired signal,
a microphone for receiving analog signals indicative of ambient noise,
an analog-digital converter for converting said analog signals into digital signals;
a digital filter, for performing a filtering operation on said digital signals to generate a digital noise cancellation signal;
a digital-analog converter for converting said digital noise cancellation signal into an analog noise cancellation signal; and
a speaker for outputting said analog noise cancellation signal and said desired signal;
wherein a delay imparted by the digital filter is adjustable, such that a delay of an electronic path containing the microphone, the analog-digital converter, the digital filter, the digital-analog converter and the speaker can be made equal to a delay of an acoustic path from the microphone to the speaker.

2. A sound reproduction system as claimed in claim 1 wherein the adjustment controller comprises a shift register.

3. A sound reproduction system as claimed in claim 1 wherein the adjustment controller comprises a chain of D-type flip-flops.

4. A sound reproduction system as claimed in claim 1 wherein the adjustment controller comprises a cyclic memory buffer with read and write addresses which are offset by the required delay.

5. A sound reproduction system as claimed in claim 1 wherein the digital signals are N-bit digital signals at a sample rate f 0 , wherein N is no greater than 8, and f 0 is substantially greater than the Nyquist sampling rate.

6. A sound reproduction system as claimed in claim 1 wherein the digital noise cancellation signal is at a sample rate f 0 , wherein f 0 is substantially greater than the Nyquist sampling rate.

7. A sound reproduction system as claimed in claim 1 further comprising a delta sigma modulator, for receiving the digital noise cancellation signal.

8. A sound reproduction system as claimed in claim 7 further comprising a digital-analog converter connected to the delta-sigma modulator.

9. A sound reproduction system according to claim 1 wherein said digital filtering incorporates a high-pass filter to remove low frequency ambient noise signals.

10. A sound reproduction device as claimed in claim 1 further comprising a second speaker for outputting said converted analog signals and said desired signal, wherein
the sound source, analog-digital converter, digital filter, adjustment controller and digital-analog converter are contained within a portable electronic device; and
the speaker, second speaker and microphone are contained within earphones connected to the portable electronic device.

11. A sound reproduction system as claimed in claim 10 wherein the earphones are wired directly to the portable electronic device.

12. A sound reproduction system as claimed in claim 10 wherein the earphones are connected via wireless links to the portable electronic device.

13. A sound reproduction system as claimed in claim 1 wherein the system is contained within a telephone handset.

Assignees

• Wolfson Microelectronics Plc
• Cirrus Logic Inc

Inventors

• Clemow Richard

Classifications

• G10K2210/3051

  Sampling, e.g. variable rate, synchronous, decimated or interpolated · CPC title

• H04R1/1083 Primary

  Reduction of ambient noise (active noise reduction per se G10K11/175; protective devices for the ear, e.g. providing acoustic protection A61F11/06) · CPC title

• G10K11/16 Primary

  Methods or devices for protecting against, or for damping, noise or other acoustic waves in general (G10K11/36 takes precedence) · CPC title

• G10K2210/1081

  Earphones, e.g. for telephones, ear protectors or headsets · CPC title

• G10K2210/3217

  Collocated sensor and cancelling actuator, e.g. "virtual earth" designs · CPC title