Reduced delay digital active noise cancellation

1. A method for generating an anti-noise signal for a digital active noise cancellation circuit in a digital audio device, the method comprising: receiving an analog noise signal; converting the analog noise signal into a digital noise signal by an oversampled, sigma-delta, A/D converter; transferring the digital noise signal to a digital decimation filter operable to down-sample the digital noise signal; selectively bypassing at least a portion of the digital decimation filter by transferring the digital noise signal to a separate digital intermediate filter; and processing the digital noise signal in the digital intermediate filter to generate a digital anti-noise signal.

2. The method of claim 1 further comprising the steps of: transferring the digital anti-noise signal into a digital interpolation filter operable to up-sample the digital anti-noise signal; selectively bypassing at least a portion of the digital interpolation filter; and converting the digital anti-noise signal into an analog anti-noise signal by a sigma-delta, D/A converter.

3. The method of claim 2 wherein the digital noise signal is transferred directly from the A/D converter to the digital intermediate filter and the digital anti-noise signal is transferred directly from the digital intermediate filter to the D/A converter.

4. The method of claim 3 wherein the digital interpolation filter comprises a plurality of FIR filters, an up-sampler, and a sigma-delta modulator.

5. The method of claim 4 wherein the digital noise signal is transferred directly from the A/D converter to the digital intermediate filter and the digital anti-noise signal is transferred directly from the digital intermediate filter to the sigma-delta modulator.

6. The method of claim 4 wherein the digital noise signal is transferred directly from the A/D converter to the digital intermediate filter and the digital anti-noise signal is transferred directly from the digital intermediate filter to the up-sampler.

7. The method of claim 2 wherein the digital decimation filter comprises a CIC filter and a plurality of FIR filters.

8. The method of claim 7 wherein the digital noise signal is transferred from a stage of the CIC filter to the digital intermediate filter and the digital anti-noise signal is transferred directly to the D/A converter.

9. The method of claim 7 wherein the digital noise signal is transferred from one of the FIR filters to the digital intermediate filter and the digital anti-noise signal is transferred directly to the D/A converter.

10. The method of claim 2 wherein the digital interpolation filter comprises a plurality of FIR filters, an up-sampler, and a sigma-delta modulator and wherein the digital decimation filter comprises a CIC filter and a plurality of FIR filters.

11. The method of claim 1 further comprising the steps of: combining the digital anti-noise signal with a digital audio input signal to generate a digital combined audio signal; transferring the digital combined audio signal into a digital interpolation filter operable to up-sample the digital combined audio signal; selectively bypassing at least a portion of the digital interpolation filter; and converting the digital combined audio signal into an analog combined audio signal by a sigma-delta, D/A converter.

12. A digital audio device enabled for active noise cancellation comprising: a digital active noise cancellation circuit operatively coupled to the digital audio circuit comprising: an oversampled, sigma-delta, A/D converter; a digital decimation filter; a separate digital intermediate filter; a digital interpolation filter; and a sigma-delta, D/A converter; wherein the digital active noise cancellation circuit is operative to perform the steps of: receiving the analog noise signal; converting the analog noise signal into a digital noise signal by an oversampled, sigma-delta, analog-to-digital converter; transferring the digital noise signal to a digital decimation filter operable to down-sample the digital noise signal; selectively bypassing at least a portion of the digital decimation filter by transferring the digital noise signal to a digital intermediate filter; and processing the digital anti-noise signal in the digital intermediate filter to generate a digital anti-noise signal; and a digital audio circuit operative to combine an audio signal with an analog anti-noise signal and to amplify the combined signals through a speaker.

13. The device of claim 12 wherein the digital active noise cancellation circuit is further operative to perform the steps of: transferring the digital anti-noise signal into a digital interpolation filter operable to up-sample the digital anti-noise signal; selectively bypassing at least a portion of the digital interpolation filter; and converting the digital anti-noise signal into an analog anti-noise signal by a sigma-delta, D/A converter.

14. The device of claim 13 wherein the digital anti-noise circuit is further operative to transfer the digital noise signal directly from the A/D converter to the digital intermediate filter and to transfer the digital anti-noise signal directly from the digital intermediate filter to the D/A converter.

15. The device of claim 13 wherein the digital interpolation filter comprises a plurality of FIR filters, an up-sampler, and a sigma-delta modulator.

16. The device of claim 15 wherein the digital noise signal is transferred directly from the A/D converter to the digital intermediate filter and the digital anti-noise signal is transferred directly from the digital intermediate filter to the sigma-delta modulator.

17. The device of claim 15 wherein the digital noise signal is transferred directly from the A/D converter to the digital intermediate filter and the digital anti-noise signal is transferred directly from the digital intermediate filter to the up-sampler.

18. The device of claim 13 wherein the digital decimation filter comprises a CIC filter and a plurality of FIR filters.

19. The device of claim 18 wherein the digital noise signal is transferred from a stage of the CIC filter to the digital intermediate filter and the digital anti-noise signal is transferred directly to the D/A converter.

20. The device of claim 18 wherein the digital noise signal is transferred from one of the FIR filters to the digital intermediate filter and the digital anti-noise signal is transferred directly to the D/A converter.

21. The device of claim 13 wherein the digital interpolation filter comprises a plurality of FIR filters, an up-sampler, and a sigma-delta modulator and wherein the digital decimation filter comprises a CIC filter and a plurality of FIR filters.

22. The device of claim 12 wherein the digital active noise cancellation circuit is further operative to perform the steps of: combining the digital anti-noise signal with a digital audio input signal to generate a digital combined audio signal; transferring the digital combined audio signal into a digital interpolation filter operable to up-sample the digital combined audio signal; selectively bypassing at least a portion of the digital interpolation filter; and converting the digital combined audio signal into an analog combined audio signal by a sigma-delta, D/A converter.

23. A digital active noise cancellation circuit device comprising: an oversampled, sigma-delta, A/D converter; a digital decimation filter; a separate digital intermediate filter; a digital interpolation filter; and a sigma-delta, D/A converter; wherein the digital active-noise cancellation circuit device is operative to perform the steps of: receiving the analog noise signal; converting the analog noise signal into a digital noise signal by an oversampled, sigma-delta, analog-to-digital converter; transferring the digital noise signal to a digital decimation filter operable to down-sample the digital noise signal; selectively bypassing at least a portion of the digital decimation filter by transferring the digital noise signal to a digital intermediate filter; and processing the digital noise signal in the digital intermediate filter to generate a digital anti-noise signal.

24. The device of claim 23 wherein the digital active noise cancellation circuit device is further operative to perform the steps of: transferring the digital anti-noise signal into a digital interpolation filter operable to up-sample the digital anti-noise signal; selectively bypassing at least a portion of the digital interpolation filter; and converting the digital anti-noise signal into an analog anti-noise signal by a sigma-delta, D/A converter.

25. The device of claim 24 wherein the digital anti-noise circuit is further operative to transfer the digital noise signal directly from the A/D converter to the digital intermediate filter and to transfer the digital anti-noise signal directly from the digital intermediate filter to the D/A converter.

26. The device of claim 24 wherein the digital interpolation filter comprises a plurality of FIR filters, an up-sampler, and a sigma-delta modulator.

27. The device of claim 26 wherein the digital noise signal is transferred directly from the A/D converter to the digital intermediate filter and the digital anti-noise signal is transferred directly from the digital intermediate filter to the sigma-delta modulator.

28. The device of claim 26 wherein the digital noise signal is transferred directly from the A/D converter to the digital intermediate filter and the digital anti-noise signal is transferred directly from the digital intermediate filter to the up-sampler.

29. The device of claim 24 wherein the digital decimation filter comprises a CIC filter and a plurality of FIR filters.

30. The device of claim 24 wherein the digital interpolation filter comprises a plurality of FIR filters, an up-sampler, and a sigma-delta modulator and wherein the digital decimation filter comprises a CIC filter and a plurality of FIR filters.

31. The device of claim 23 wherein the digital active noise cancellation circuit device is further operative to perform the steps of: combining the digital anti-noise signal with a digital audio input signal to generate a digital combined audio signal; transferring the digital combined audio signal into a digital interpolation filter operable to up-sample the digital combined audio signal; selectively bypassing at least a portion of the digital interpolation filter; and converting the digital combined audio signal into an analog combined audio signal by a sigma-delta, D/A converter.