Noise cancellation system and signal processing method for an ear-mountable playback device

1. A noise cancellation system for an ear-mountable playback device having a speaker, a feedforward microphone configured to predominantly sense ambient sound and an error microphone configured to sense ambient sound and sound being output from the speaker, the noise cancellation system comprising

a filter chain for coupling the feedforward microphone to the speaker, the filter chain comprising a series connection or parallel connection of a coarse filter and a fine filter; and

a noise control processor;

wherein

the fine filter is formed of a set of sub-filters;

each of the sub-filters has a predefined frequency range;

the predefined frequency range of each of the sub-filters together forms an effective overall frequency range of the fine filter; and

the noise control processor is configured to

calculate an error signal based on a first noise signal sensed by the feedforward microphone and on a second noise signal sensed by the error microphone;

perform an adaptation of coarse filter parameters of the coarse filter based on the error signal; and

perform a limited adaptation of fine filter parameters of each of the sub-filters based on the error signal, wherein limits of the limited adaptation comprise the predefined frequency ranges of the sub-filters and at least one of a gain limit and a Q factor limit.

2. The noise cancellation system according to claim 1 ,

wherein the predefined frequency range of each of the sub-filters is adjacent to or at least partially overlap with the predefined frequency range of at least one other sub-filter of the set of sub-filters.

3. The noise cancellation system according to claim 1 ,

wherein the set of sub-filters comprises between 6 and 12 sub-filters, in particular between 8 and 10 sub-filters.

4. The noise cancellation system according to claim 1 ,

wherein the effective overall frequency range of the fine filter is from 80 Hz to 2000 Hz, in particular from 80 Hz to 1000 Hz.

5. The noise cancellation system according to claim 1 ,

wherein each sub-filter is one of a peak filter and a notch filter.

6. The noise cancellation system according to claim 1 ,

wherein each sub-filter is a minimum-phase filter.

7. The noise cancellation system according to claim 1 ,

wherein the limited adaptation of the sub-filters is based on an error minimization algorithm, in particular a least-mean-squares, LMS, algorithm.

8. The noise cancellation system according to claim 1 ,

wherein the limited adaptation of the sub-filters comprises an adaptation of a gain, a center frequency and a Q factor of at least one of the sub-filters.

9. The noise cancellation system according to claim 1 ,

wherein the limited adaptation of the sub-filters comprises directly adapting the fine filter parameters of at least one of the sub-filters and checking the limits of the limited adaptation for the adapted fine filter parameters.

10. The noise cancellation system according to claim 1 ,

wherein the noise control processor is configured to perform the coarse adaptation in advance of or at a different adaptation rate to the limited adaptation.

11. The noise cancellation system according to claim 1 ,

wherein the noise control processor is configured to perform the coarse adaptation by adapting a gain factor and/or a cut-off frequency of the coarse filter.

12. The noise cancellation system according to claim 1 , further comprising a feedback noise filter coupling the error microphone to the speaker.

13. An ear-mountable playback device, in particular headphone (HP) or handset, comprising a noise cancellation system according to claim 1 , the speaker, the feedforward microphone and the error microphone located in proximity to the speaker.

14. An audio player comprising a noise cancellation system according to claim 1 .

15. The noise cancellation system according to claim 1 , wherein the gain limit limits a gain range of the respective sub-filter, and the Q factor limit limits a Q factor range of the respective sub-filter.

16. A signal processing method for an ear-mountable playback device having a speaker, a feedforward microphone configured to predominantly sense ambient sound and an error microphone configured to sense ambient sound and sound being output from the speaker, wherein the feedforward microphone is coupled to the speaker via a filter chain, the filter chain comprising a series connection or parallel connection of a coarse filter and a fine filter, wherein the fine filter is formed of a set of sub-filters, each of the sub-filters has a predefined frequency range, and the predefined frequency range of each of the sub-filters together forms an effective overall frequency range of the fine filter, the method comprising

calculating an error signal based on a first noise signal sensed by the feedforward microphone and on a second noise signal sensed by the error microphone;

performing an adaptation of coarse filter parameters of the coarse filter based on the error signal; and

performing a limited adaptation of fine filter parameters of each of the sub-filters based on the error signal, wherein limits of the limited adaptation comprise the predefined frequency ranges of the sub-filters and at least one of a gain limit and a Q factor limit.

17. The method according to claim 16 , wherein the gain limit limits a gain range of the respective sub-filter, and the Q factor limit limits a Q factor range of the respective sub-filter.