Automatic polarity adaptation for ambient noise cancellation

1. A sound reproducing device having:

a loudspeaker arranged to produce sound from an audio signal provided by an audio signal source;

a microphone positioned to pick up ambient noise and generate a microphone signal comprising said noise;

an ambient noise cancellation (ANC) system arranged to receive the microphone signal from said microphone and to generate anti-noise corresponding to the ambient noise in said microphone signal; and

an automatic polarity adaptation (AAP) system arranged to monitor said ANC system and, when a decision criterion is fulfilled, to cause a switch in polarity for the generated anti-noise,

wherein the generated anti-noise is added to said audio signal provided by said audio signal source to form a resulting audio signal which is fed to said loudspeaker, and wherein said AAP system is arranged to cause said switch in polarity for the generated anti-noise by reversing the polarity of said resulting audio signal;

said AAP system comprising:

control means coupled to monitor said ANC system;

switch means coupled to be controllable by said control means and having an output coupled to said loudspeaker; and

inverter means coupled to receive said resulting audio signal and having an output coupled to an input of said switch means,

wherein the control means is arranged, when said decision criterion is fulfilled, to issue a command or control signal to said switch means to enable said switch means to reverse the polarity of said resulting audio signal and deliver it to said loudspeaker.

2. A sound reproducing device as defined in claim 1 , said AAP system being arranged to monitor said ANC system by monitoring said microphone signal, wherein said decision criterion is that the sum of the ambient noise and the generated anti-noise in said microphone signal exceeds a predetermined limit.

3. A sound reproducing device as defined in claim 1 , said ANC system comprising at least one signal filter and being arranged to perform a feedback loop adaptation process in which filter parameters of said signal filter are adapted in order to minimize the sum of the ambient noise and the generated anti-noise in said microphone signal, wherein said decision criterion is that the feedback loop adaptation process does not converge as expected.

4. A sound reproducing device as defined in claim 1 ,

wherein the AAP system is further arranged, when said decision criterion is fulfilled, to store information in a memory, said stored information indicating that the loudspeaker is mounted with a reversed polarity which requires a switch in polarity for the generated anti-noise in order for said ANC system to work properly.

5. A sound reproducing device as defined in claim 4 ,

wherein at least one of said AAP system or said ANC system is further arranged to use the information stored in said memory in the future for causing said switch in polarity based on said stored information.

6. A portable communication apparatus, comprising a sound reproducing device as defined in claim 1 .

7. An ambient noise cancellation method, involving:

picking up ambient noise to generate a microphone signal;

providing ambient noise cancellation (ANC) to generate anti-noise corresponding to the ambient noise in said microphone signal;

monitoring the ambient noise cancellation;

determining that a decision criterion is fulfilled; and

causing a switch in polarity for the generated anti-noise,

wherein the generated anti-noise is added to an audio signal provided by an audio signal source to form a resulting audio signal which is fed to a loudspeaker, and wherein said switch in polarity for the generated anti-noise reverses the polarity of said resulting audio signal;

the causing the switch in polarity for the generated anti-noise including:

monitoring said anti-noise cancellation;

when said decision criterion is fulfilled, issuing a command or control signal to enable the reversing of the polarity of said resulting audio signal; and

delivering the resulting audio signal to said loudspeaker.