Noise cancellation

1. A method for correcting a gain of a noise cancellation system, the method comprising:

generating an ambient noise signal, representing ambient noise;

generating a noise cancellation signal, by applying the ambient noise signal to a feedforward filter, wherein the feedforward filter comprises a high-pass filter having an adjustable cut-off frequency, and by applying a controllable gain;

applying the noise cancellation signal to a loudspeaker, to generate a sound to at least partially cancel the ambient noise; and

generating an error signal, representing unwanted sound in the region of the loudspeaker, wherein the method further comprises:

comparing a phase of the ambient noise signal to a phase of the error signal, and controlling said gain on the basis of a result of said comparison, and taking into consideration any changes to the phase of the noise cancellation signal when controlling said gain.

2. A method as claimed in claim 1 , wherein the step of taking into consideration any changes to the phase of the noise cancellation signal comprises adding any changes to the phase to the phase of the ambient noise signal before performing the comparison.

3. A method as claimed in claim 1 , wherein the step of taking into consideration any changes to the phase of the noise cancellation signal comprises subtracting any changes to the phase from the phase of the error signal before performing the comparison.

4. A method as claimed in claim 1 , wherein the step of taking into consideration any changes to the phase of the noise cancellation signal adjusting a threshold with which a phase difference between the phase of the ambient noise signal and the phase of the error signal is compared.

5. A method as claimed in claim 1 , further comprising taking account of a phase difference uncorrected by said feedforward filter when performing said comparison.

6. A method as claimed in claim 1 , comprising adjusting the cut-off frequency of the high-pass filter based on a determination as to whether the ambient noise includes wind noise.

7. A method as claimed in claim 1 , comprising adjusting the cut-off frequency of the high-pass filter based on a determination as to whether a level of the noise cancellation signal will overdrive the loudspeaker.

8. A method as claimed in claim 1 , comprising adjusting the cut-off frequency of the high-pass filter based on a determination as to whether a level of the noise cancellation signal, combined with a level of a wanted sound signal, will overdrive the loudspeaker.

9. A method as claimed in claim 1 , comprising comparing the phase of the ambient noise signal to the phase of the error signal in each of a plurality of analysis frequency bands, and controlling said gain on the basis of a result of said comparisons.

10. A computer program product, embodied on a non-transitory computer readable medium, comprising code for causing a computer to perform a method for correcting a gain of a noise cancellation system, the method comprising:

receiving an ambient noise signal, representing ambient noise;

generating a noise cancellation signal by applying the ambient noise signal to a feedforward filter, wherein the feedforward filter comprises a high-pass filter having an adjustable cut-off frequency, and by applying a controllable gain;

applying the noise cancellation signal to a loudspeaker, to generate a sound to at least partially cancel the ambient noise; and

generating an error signal, representing unwanted sound in the region of the loudspeaker, wherein the method further comprises:

comparing a phase of the ambient noise signal to a phase of the error signal, and controlling said gain on the basis of a result of said comparison, and taking into consideration any changes to the phase of the noise cancellation signal when controlling said gain.

11. A computer program product embodied on a non-transitory computer readable medium as claimed in claim 10 wherein the code is adapted to cause a digital signal processor to carry out the method.

12. A sound reproduction device, comprising a noise cancellation system, wherein the noise cancellation system is adapted to:

receive an ambient noise signal, representing ambient noise;

generate a noise cancellation signal by applying the ambient noise signal to a feedforward filter, wherein the feedforward filter comprises a high-pass filter having an adjustable cut-off frequency, and by applying a controllable gain;

apply the noise cancellation signal to a loudspeaker, to generate a sound to at least partially cancel the ambient noise; and

generate an error signal, representing unwanted sound in the region of the loudspeaker, wherein the noise cancellation system is further adapted to:

compare a phase of the ambient noise signal to a phase of the error signal, and control said gain on the basis of a result of said comparison, and taking into consideration any changes to the phase of the noise cancellation signal when controlling said gain.

13. A sound reproduction device as claimed in claim 12 , comprising a handset, having a loudspeaker, a noise microphone, for detecting ambient noise, and an error microphone, positioned in front of the loudspeaker for detecting unwanted sound entering an ear canal of a user, when the handset is being held to an ear of the user.

14. A sound reproduction system, comprising a sound reproduction device as claimed in claim 12 and at least one earphone.

15. A sound reproduction system as claimed in claim 14 , wherein the earphone comprises a loudspeaker, a noise microphone, for detecting ambient noise, and an error microphone, positioned in front of the loudspeaker for detecting unwanted sound entering an ear canal of a wearer, when the earphone is being worn normally.

16. A sound reproduction system as claimed in claim 15 , wherein the earphone is a supra-aural earphone.

17. A sound reproduction system as claimed in claim 15 , wherein the earphone is an intra-concha earphone.