Audio beamforming

1. An audio beamforming apparatus comprising:

a receiving circuit for receiving signals from an at least two-dimensional microphone array comprising at least three microphones;

a reference circuit for generating at least three reference beams from the microphone signals;

combining circuit for generating an output signal corresponding to a desired beam pattern by combining the reference beams in response to a first direction of a desired sound source and a direction estimate for an interfering sound source;

an estimation circuit for generating the direction estimate by:

determining a first angle corresponding to a local minimum for a power measure of the output signal in a first angle interval,

determining a second angle corresponding to a local minimum for a power measure of the output signal in a second angle interval, and

determining the direction estimate as an angle selected from a set of angles corresponding to local minima for a power measure of the output signal, the set of angles comprising at least the first angle and the second angle; and

wherein the combining circuit is arranged to determine combination parameters for the combining of the reference beams to provide a notch in an angle corresponding to the direction estimate of an interfering point source and a maximization of a directivity cost measure of the interfering point source, the directivity cost measure being indicative of a ratio between a gain in the first direction and an average gain.

2. The apparatus as claimed in claim 1 , wherein the estimation circuit is arranged to select the direction estimate as one of the first angle and the second angle in response to a gradient of a power measure of the output signal as a function of the direction estimate for an angle separating the first angle interval and the second angle interval.

3. The apparatus as claimed in claim 1 , wherein the first angle interval comprises angles from 0 to w and the second angle interval comprises angles from π to 2π.

4. The apparatus as claimed in claim 3 , wherein the estimation circuit is arranged to select the direction estimate as one of the first angle and the second angle in response to a gradient of a power measure of the output signal as a function of the direction estimate for an angle of π.

5. The apparatus as claimed in claim 1 , wherein the combining circuit comprises a sidelobe canceller.

6. The apparatus as claimed in claim 5 , wherein the sidelobe canceller is arranged to generate the output signal as a weighted combination of at least a primary signal, a first noise reference signal and a second noise reference signal.

7. The apparatus as claimed in claim 6 , wherein the combining circuit is arranged to calculate weights for the first and second noise reference signals in response to the direction estimate and a maximization of the directivity cost measure.

8. The apparatus as claimed in claim 5 , wherein the estimation circuit is arranged to determine at least one of the first and second angles by a gradient search applied to a sidelobe canceller corresponding to the sidelobe canceller of the combining circuit and having an angle input variable.

9. The apparatus as claimed in claim 8 , wherein an update value for the angle input variable is determined as a function of an output signal of the sidelobe canceller for a current phase value of the angle input variable, and a first and second noise reference signal of the sidelobe canceller for the current phase value.

10. The apparatus as claimed in claim 9 , wherein the first and second noise reference signals are weighted as a function of the current phase value.

11. The apparatus as claimed in claim 9 , wherein the estimation circuit is arranged to determine a power estimate for at least one of the first and second noise reference signals and to perform a normalization of the update value as a function of the power estimate.

12. The apparatus as claimed in claim 1 , wherein the at least two-dimensional microphone array comprises at least four microphones, and the apparatus comprises a circuit for combining signals from at least two of the at least four microphones prior to generating the reference beams.

13. The apparatus as claimed in claim 1 , wherein said apparatus further comprises the at least two-dimensional microphone array, the at least two-dimensional microphone array comprising directional microphones having a maximum response in a direction outwardly of a perimeter of the at least two-dimensional microphone array.

14. A method of audio beamforming comprising:

receiving signals from an at least two-dimensional microphone array comprising at least three microphones;

generating at least three reference beams from the microphone signals;

generating an output signal corresponding to a desired beam pattern by combining the reference beams in response to a first direction of a desired sound source and a direction estimate for an interfering sound source;

generating the direction estimate by:

determining a first angle corresponding to a local minimum for a power measure of the output signal in a first angle interval,

determining a second angle corresponding to a local minimum for a power measure of the output signal in a second angle interval, and

determining the direction estimate as an angle selected from a set of angles corresponding to local minima for a power measure of the output signal, the set of angles comprising at least the first angle and the second angle; and

wherein the combining of the reference beams comprises determining combination parameters for the combining of the reference beams to provide a notch in an angle corresponding to the direction estimate and a maximization of a directivity cost measure, the directivity cost measure being indicative of a ratio between a gain in the first direction and an energy averaged gain.

15. A non-transitory computer readable storage medium encoded with a computer program having steps for causing a processor to carry out the method of claim 14 .