Realisation of controller transfer function for active noise cancellation

1. An active noise cancellation apparatus comprising:

a sensing microphone;

a speaker; and

a peak filter, a notch filter, and a shelving filter, each filter being operable over a different frequency range, the center frequency of the frequency range of the notch filter being greater than the center frequency of the frequency range of the peak filter and less than the center frequency of the frequency range of the shelving filter, and at least one of the peak, notch, or shelving filters having at least one adjustable parameter whereby the at least one filter can be adjusted such that the peak, notch, and shelving filters cumulatively realise a required active noise cancellation control law transfer function; and

wherein the speaker receives a signal comprising an output of the sensing microphone modified by the control law transfer function.

2. Apparatus as claimed in claim 1 wherein each of the peak, notch, and shelving filters includes at least one adjustable parameter.

3. Apparatus as claimed in claim 2 wherein the adjustable parameter is amplitude.

4. Apparatus as claimed in claim 2 wherein the adjustable parameter is bandwidth.

5. Apparatus as claimed in claim 2 , wherein the Q of the peak filter and the notch filter is adjustable.

6. Apparatus as claimed in claim 1 wherein the peak filter and the notch filter comprise parametric filters.

7. Apparatus as claimed in claim 1 wherein the adjustable parameter is dynamically adjustable.

8. Apparatus as claimed in claim 7 further comprising an adjustment controller adapted to monitor a noise signal from the sensing microphone and to adjust the at least one adjustable parameter.

9. Apparatus as claimed in claim 8 wherein a plurality of parameters are adjustable, and the adjustment controller determines which parameter or parameters of one or more of the peak, notch, and shelving filters requires adjustment.

10. Apparatus as claimed in claim 9 , wherein the adjustment controller comprises a digital signal processor.

11. Apparatus as claimed in claim 7 further comprising an adjustable gain stage.

12. Apparatus as claimed in claim 1 , further comprising an adjustable gain stage.

13. Apparatus as claim in claim 1 wherein the peak, notch, and shelving filters are analog.

14. Apparatus as claim in claim 1 wherein the filters are configured in a cascaded arrangement of peak filter, notch filter, and shelving filter.

15. Apparatus as claimed in claim 1 , wherein the signal modified by the control law transfer function is adapted to cancel the noise signal sensed by the microphone.

16. Apparatus as claimed in claim 1 , wherein the peak filter and notch filter have centre frequencies of approximately 300 Hz and 8 kHz respectively.

17. Apparatus as claimed in claim 1 , wherein the signal generated by the control law transfer function is adapted to cancel noise not generated by the speaker.

18. A method of active noise cancellation for apparatus including a speaker, a sensing microphone, a peak filter, a notch filter, and a shelving filter, each of the plurality of filters being operable over a different frequency range, the center frequency of the frequency range of the notch filter being greater than the center frequency of the frequency range of the peak filter and less than the center frequency of the frequency range of the shelving filter and at least one of the filters having at least one adjustable parameter whereby the at least one filter can be adjusted, the method comprising:

monitoring a noise signal detected by the sensing microphone;

using the monitored signal to determine the noise cancellation performance of the apparatus;

dynamically adjusting the adjustable parameter dependent on the determined performance such that the plurality of filters cumulatively realise a control law transfer function to implement active noise cancellation; and

actuating the speaker with an output of the sensing microphone modified by the control law transfer function.

19. A method as claimed in claim 18 , wherein a plurality of parameters are adjustable and the method includes determining which parameter or parameters of one or more of the plurality of filters requires adjustment.

20. A method as claimed in claim 18 , further comprising adjusting the gain of the apparatus.

21. Method as claim in claim 18 wherein the peak, notch, and shelving filters are analog.

22. A method as claimed in claim 18 , wherein the signal modified by the control law transfer function is adapted to cancel the noise signal sensed by the microphone.

23. Apparatus for realising an active noise cancellation control law transfer function between a sensing microphone and a speaker, the apparatus comprising a plurality of filters, each filter being operable over a different frequency range, at least one of the plurality of filters having at least one adjustable parameter whereby the at least one filter can be adjusted such that the plurality of filters cumulatively realise an active noise cancellation control law transfer function, and an adjustment controller operable to monitor an externally generated noise signal detected by the sensing microphone and to dynamically adjust the at least one adjustable parameter whereby the plurality of filters cumulatively realise a control law transfer function required to implement active noise cancellation.

24. Apparatus as claimed in claim 23 , wherein a plurality of parameters are adjustable, and the adjustment controller determines which parameter or parameters of one or more of the plurality of filters requires adjustment.

25. Apparatus as claimed in claim 24 , wherein the adjustment controller comprises a digital signal processor.

26. Apparatus as claimed in claim 24 , further comprising an adjustable gain stage.

27. Apparatus as claimed in claim 23 , wherein the signal modified by the control law transfer function is adapted to cancel the noise signal sensed by the microphone.