Noise cancellation system

1. A method of controlling a noise cancellation system, for use in an audio device, the method comprising:

generating an ambient noise signal representative of ambient noise;

filtering and applying gain to the ambient noise signal to generate a noise cancellation signal;

passing the noise cancellation signal to a speaker; and

generating an error signal from an error microphone,

wherein the gain applied to the ambient noise signal is controlled based on a gain increment/decrement value derived from the error signal,

and the method further comprising:

determining from the gain increment/decrement value or the applied gain whether or not the audio device is in an off-ear position, and

controlling the noise cancellation system differently based on said determination as to whether or not the audio device is in the off-ear position.

2. A method as claimed in claim 1 , comprising:

determining based on the error signal and the ambient noise signal whether the gain applied to the ambient noise signal should be increased or decreased from a current gain value, within a range defined by an upper limiting value and a lower limiting value.

3. A method as claimed in claim 2 , comprising:

determining based on a phase difference between the error signal and the ambient noise signal whether the gain applied to the ambient noise signal should be increased or decreased from the current gain value.

4. A method as claimed in claim 2 , comprising adjusting the gain to be applied to the ambient noise signal to a predetermined off-ear value between the upper limiting value and the lower limiting value in response to a determination that the audio device is in the off-ear position.

5. A method as claimed in claim 4 , comprising adjusting the gain to be applied to the ambient noise signal to the off-ear value by a series of steps.

6. A method as claimed in claim 2 , comprising determining that the audio device is in the off-ear position in response to a succession of determinations that the gain to be applied to the ambient noise signal should be increased.

7. A method as claimed in claim 6 , comprising determining that the audio device is in the off-ear position in response to a succession of determinations that the gain to be applied to the ambient noise signal should be increased, following a determination that the gain has reached the upper limiting value.

8. A noise cancellation system, for use with an audio device, the noise cancellation system comprising:

a first input for receiving an ambient noise signal representative of ambient noise;

a filter circuit for filtering and applying gain to the ambient noise signal to generate a noise cancellation signal;

an output for the noise cancellation signal;

a second input for receiving an error signal from an error microphone; and

a controller for controlling the gain applied to the ambient noise signal based on a gain increment/decrement value derived from the error signal,

and the controller being further adapted to:

determine from the gain increment/decrement value or the applied gain whether or not the audio device is in an off-ear position, and

control the noise cancellation system differently based on said determination as to whether or not the audio device is in the off-ear position.

9. A noise cancellation system as claimed in claim 8 , wherein the controller is adapted to:

determine based on the error signal and the ambient noise signal whether the gain applied to the ambient noise signal should be increased or decreased from a current gain value, within a range defined by an upper limiting value and a lower limiting value.

10. A noise cancellation system as claimed in claim 9 , wherein the controller is adapted to:

determine based on a phase difference between the error signal and the ambient noise signal whether the gain applied to the ambient noise signal should be increased or decreased from the current gain value.

11. A noise cancellation system as claimed in claim 9 , wherein the controller is adapted to adjust the gain to be applied to the ambient noise signal to an off-ear value between the upper limiting value and the lower limiting value in response to a determination that the audio device is in the off-ear position.

12. A noise cancellation system as claimed in claim 11 , wherein the controller is adapted to adjust the gain to be applied to the ambient noise signal to the off-ear value by a series of steps.

13. A noise cancellation system as claimed in claim 9 , wherein the controller is adapted to determine that the audio device is in the off-ear position in response to a succession of determinations that the gain to be applied to the ambient noise signal should be increased.

14. A noise cancellation system as claimed in claim 13 , wherein the controller is adapted to determine that the audio device is in the off-ear position in response to a succession of determinations that the gain to be applied to the ambient noise signal should be increased, following a determination that the gain has reached the upper limiting value.

15. An audio system, comprising:

an audio device, comprising a first microphone, for generating an ambient noise signal representative of ambient noise; a speaker; an error microphone located close to the speaker; and

a noise cancellation system as claimed in any of claim 8 , wherein the first microphone is connected to the first input of the noise cancellation system, the error microphone is connected to the second input of the noise cancellation system, and the speaker is connected to the output of the noise cancellation system.

16. An audio system as claimed in claim 15 , wherein the audio system comprises a handset device.

17. An audio system as claimed in claim 16 , wherein the audio system comprises a mobile phone handset.