Noise cancellation system with gain control based on noise level

1. A noise cancellation system, for generating a noise cancellation signal to be added to a wanted signal to mitigate the effects of ambient noise, said system comprising:

an input, for receiving an input signal representing ambient noise;

a detector, for detecting a magnitude of said input signal; and

a voice activity detector, for detecting voiceless periods when said input signal does not contain a signal representing a voice,

wherein said detector is adapted to detect the magnitude of said input signal during said voiceless periods, and

wherein the system is adapted to operate in a first mode when said detected magnitude of said input signal is above a threshold value, and a second mode when said detected magnitude of said input signal is below the threshold value, the first mode comprising:

generating a noise cancellation signal with a first magnitude for at least partially cancelling the ambient noise, the noise cancellation signal with said first magnitude being added to the wanted signal;

the second mode comprising:

generating a noise cancellation signal with a second magnitude that is less than the first magnitude, the noise cancellation signal with said second magnitude being added to the wanted signal.

2. A noise cancellation system as claimed in claim 1 , wherein the voice activity detector is adapted to generate a plurality of samples of said input signal.

3. A noise cancellation system as claimed in claim 2 , wherein said plurality of samples is divided into a plurality of frames, each frame comprising one or more of said plurality of samples.

4. A noise cancellation system as claimed in claim 3 , wherein the ambient noise is taken to be the magnitude of the input signal during a frame of the plurality of frames having the lowest average magnitude.

5. A noise cancellation system as claimed in claim 1 , wherein said system is adapted to transition from the first mode to the second mode when the magnitude of the input signal falls below the first threshold value, and wherein said system is adapted to transition from the second mode to the first mode when the magnitude of the input signal rises above a second threshold value, said second threshold value being greater than said first threshold value.

6. A noise cancellation system as claimed in claim 1 , further comprising:

an adaptive gain element, for applying a first gain value in order to generate said noise cancellation signal with said first magnitude, and for applying a second gain value in order to generate said noise cancellation signal with said second magnitude.

7. A noise cancellation system as claimed in claim 6 , wherein said second gain value decreases linearly with the magnitude of the input signal.

8. A noise cancellation system as claimed in claim 1 , wherein said second magnitude is zero.

9. An integrated circuit, comprising:

a noise cancellation system as claimed in claim 1 .

10. A mobile phone, comprising:

an integrated circuit as claimed in claim 9 .

11. A pair of headphones, comprising:

an integrated circuit as claimed in claim 9 .

12. A pair of earphones, comprising:

an integrated circuit as claimed in claim 9 .

13. A headset, comprising:

an integrated circuit as claimed in claim 9 .

14. A noise cancellation system, for generating a noise cancellation signal to be added to a wanted signal to mitigate the effects of ambient noise, said system comprising:

an input, for receiving a signal representing ambient noise;

a detector, for detecting a magnitude of said ambient noise signal; and

a voice activity detector, adapted to generate a plurality of samples of said input signal, for detecting voiceless periods when said input signal does not contain a signal representing a voice,

wherein said plurality of samples is divided into a plurality of frames, each frame comprising one or more of said plurality of samples,

wherein the ambient noise is taken to be the magnitude of the input signal during a frame of the plurality of frames having the lowest average magnitude,

wherein said detector is adapted to detect the magnitude of said ambient noise signal during said voiceless periods,

wherein the system is adapted to operate in a first mode when said detected magnitude of said ambient noise signal is above a threshold value, and a second mode when said detected magnitude of the ambient noise signal is below the threshold value, the first mode comprising:

generating a noise cancellation signal for at least partially cancelling the ambient noise, the noise cancellation signal being added to the wanted signal;

the second mode comprising:

switching off the noise cancellation system.

15. A noise cancellation system as claimed in claim 14 , wherein said system is adapted to transition from the first mode to the second mode when the magnitude of the input signal falls below the first threshold value, and wherein said system is adapted to transition from the second mode to the first mode when the magnitude of the input signal rises above a second threshold value, said second threshold value being greater than said first threshold value.

16. A method of controlling a noise cancellation system, wherein said noise cancellation system is for generating a noise cancellation signal to be added to a wanted signal to mitigate the effects of ambient noise, said system comprising: an input, for receiving an input signal representing ambient noise; a detector, for detecting a magnitude of said input signal; and a voice activity detector, for detecting voiceless periods when said input signal does not contain a signal representing a voice, wherein said detector is adapted to detect the magnitude of said input signal during said voiceless periods, the method comprising: operating in a first mode when said detected magnitude of said input signal is above a threshold value; while operating in said first mode, generating a noise cancellation signal for at least partially cancelling the ambient noise, the noise cancellation signal with said first magnitude being added to the wanted signal; and operating in a second mode when said detected magnitude of said input signal is below the threshold value; and while operating in said second mode, generating a noise cancellation signal with a second magnitude that is less than the first magnitude, the noise cancellation signal with said second magnitude being added to the wanted signal.

17. A method as claimed in claim 16 , wherein the voice activity detector generates a plurality of samples of said input signal.

18. A method as claimed in claim 17 , wherein said plurality of samples is divided into a plurality of frames, each frame comprising one or more of said plurality of samples.

19. A method as claimed in claim 18 , wherein the ambient noise is taken to be the magnitude of the input signal during a frame of the plurality of frames having the lowest average magnitude.

20. A method as claimed in claim 16 , further comprising:

transitioning from the first mode to the second mode when the magnitude of the input signal falls below the first threshold value, and

transitioning from the second mode to the first mode when the magnitude of the input signal rises above a second threshold value,

wherein said second threshold value is greater than said first threshold value.

21. A method of controlling a noise cancellation system,

wherein said noise cancellation system for generating a noise cancellation signal to be added to a wanted signal to mitigate the effects of ambient noise, said system comprising:

an input, for receiving an input signal representing ambient noise;

a detector, for detecting a magnitude of said input signal; and

a voice activity detector that generates a plurality of samples of said input signal, for detecting voiceless periods when said input signal does not contain a signal representing a voice,

wherein said plurality of samples is divided into a plurality of frames, each frame comprising one or more of said plurality of samples,

wherein the ambient noise is taken to be the magnitude of the input signal during a frame of the plurality of frames having the lowest average magnitude,

wherein said detector is adapted to detect the magnitude of said input signal during said voiceless periods,

the method comprising:

operating in a first mode when said detected magnitude of said input signal is above a threshold value;

while operating in said first mode, generating a noise cancellation signal for at least partially cancelling the ambient noise, the noise cancellation signal being added to the wanted signal; and

operating in a second mode when said detected magnitude of said input signal is below the threshold value; and

while operating in said second mode, switching off the noise cancellation system.

22. A method as claimed in claim 21 , further comprising:

transitioning from the first mode to the second mode when the magnitude of the input signal falls below the first threshold value, and

transitioning from the second mode to the normal mode when the magnitude of the input signal rises above a second threshold value,

wherein said second threshold value is greater than said first threshold value.