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The invention relates to adaptive directional systems, and more particularly to a method and apparatus for producing adaptive directional signals. The invention may be applied to the provision of audio frequency adaptive directional microphone systems for devices such as hearing aids and mobile tele

The invention relates to adaptive directional systems, and more particularly to a method and apparatus for producing adaptive directional signals. The invention may be applied to the provision of audio frequency adaptive directional microphone systems for devices such as hearing aids and mobile telephones. The method involves constructing the adaptive directional signal (46) from a weighted sum of a first signal (42A) having an omni-directional polar pattern and a second signal (42B) having a bi-directional polar pattern, wherein the weights are calculated to give the combined signal a constant gain in a predetermined direction and to minimize the power of the combined signal. The method has particular application in producing signals in digital hearing aids, the predetermined direction being in the forward direction with respect to the wearer.

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1. A method executed by a processor for producing a combined adaptive directional signal, the method comprising: constructing the combined adaptive directional signal from a weighted sum of a first signal weight of a first sound signal having an omni-directional polar pattern and a second signal wei

1. A method executed by a processor for producing a combined adaptive directional signal, the method comprising: constructing the combined adaptive directional signal from a weighted sum of a first signal weight of a first sound signal having an omni-directional polar pattern and a second signal weight of a second sound signal having a bi-directional polar patternwherein the first signal weight and the second signal weights are calculated to give the combined signal a constant gain in a predetermined direction and to minimize power of the combined signal, wherein the weights are calculated by the processor in a non-iterative manner, and wherein the signal weights are calculated by solving the following equation: a=∑y2-∑xy∑x2-2⁢∑xy+∑y2wherea=weight for the first signal(1−a)=weight for the second signalx=first signal sampley=second signal sample. 2. A method according to claim 1, wherein the first and second signals are sampled, the signal weights being calculated for successive sets of said first and second signals samples. 3. A method according to claim 1, wherein the first and second signals are sampled, the signal weights being calculated for successive sets of said first and second signals samples, and the signal weights are calculated continuously by calculating x2, y2, and xy for each sample and adding them to an appropriate running sum. 4. A method according to claim 3, wherein a leaky integrator is used to perform the running sum in order to address issues of numerical overflow. 5. A method in accordance with claim 1, wherein the second signal having a bi-directional polar pattern is derived from a first omni-directional microphone and from a second omni-directional microphone, and wherein the first signal having an omni-directional polar pattern is derived from one of the first and second omni-directional microphones. 6. A method according to claim 1, wherein said signal weights are calculated so as to construct an omni-directional combined signal when a total power in said first signal is below a certain value and value a defaults to a value of 1.0 in the event that Σx2 is less than a prescribed minimum value. 7. A method according to claim 5, wherein the omni-directional microphones comprise a front microphone and a rear microphone, and said predetermined direction is the forward direction along the microphone axis. 8. A method according to claim 7, wherein the second signal is provided by the difference between signals produced by the front and rear microphones, without the use of a delay element. 9. A method according to claim 8, further comprising processing the second signal by means of an integrator element or an integrator-like filter before constructing the combined signal, thereby compensating for the attenuation of low frequencies and phase shifts introduced in the subtraction of the two omni-directional signals. 10. A method according to claim 8, further comprising amplifying the signals produced by one or more of the front and the rear microphone before constructing the bi-directional signal, to ensure an equivalent gain between the microphones. 11. A method according to claim 1, wherein said first and second signals are frequency domain samples. 12. A method according to claim 11, further comprising calculating and applying the weights to several independent subsets of frequency domain samples, to give different directional responses at different frequencies and/or to allow selective suppression of different frequencies. 13. A method according to claim 1, comprising applying a frequency weighting function to said first and second signal before calculating said signal weights. 14. An apparatus for producing a combined adaptive directional signal, the apparatus comprising: an analog-to-digital converter for producing a first sound signal having an omni-directional polar pattern and, a second sound signal having a bi-directional polar pattern;a summation device for constructing the adaptive directional signal from a weighted sum of a first signal weight of the first signal and a second signal weight of the second signal, wherein the first signal weight and the second signal weight are calculated to give the combined signal a constant gain in a predetermined direction and to minimize power of the combined signal; andmeans for calculating the weights by solving the following equation: a=∑y2-∑xy∑x2-2⁢∑xy+∑y2where:a=weight for the first signal(1−a)=weight for the second signalx=first signal sampley=second signal sample. 15. An apparatus according to claim 14, further comprising a first omni-directional microphone and a second omni-directional microphone, wherein the second signal having a bi-directional polar pattern is derived from the first and second omni-directional microphones and wherein the first signal having an omni-directional polar pattern is derived from one of the first and second omni-directional microphones. 16. An apparatus according to claim 14, including means for calculating said signal weights for a series of frames, each frame having a predetermined length consisting of N first signal samples and N second signal samples. 17. An apparatus according to claim 14, including a filter for filtering or smoothing the series of weights to minimize frame-to-frame variation in the calculated weights. 18. An apparatus according to claim 14, including means for calculating said weights continuously for samples of said first and second signals. 19. An apparatus according to claim 14, further comprising leaky integrator to perform a running sum on said first and second signal samples in order to address issues of numerical overflow system memory. 20. An apparatus according to claim 14, further comprising means for calculating said signal weights so as to construct an omni-directional combined signal when a total power in said first signal is below a certain value. 21. An apparatus according to claim 15, wherein the two omni-directional microphones comprise a front microphone and a rear microphone, and wherein said predetermined direction is the forward direction along an axis of the microphones. 22. An apparatus according to claim 21, further comprising means for providing said second signal from the difference between signals produced by the front and rear microphones, without the use of a delay element. 23. An apparatus according to claim 21, further comprising integrator element or an integrator-like filter for processing the second signal before constructing the combined signal, thereby compensating for attenuation of low frequencies and phase shifts introduced in the provision of the second signal. 24. An apparatus according to claim 21, further comprising means for amplifying the signals produced by the front and/or the rear microphone before the step of constructing the bi-directional signal, to ensure an equivalent gain between the microphones. 25. A method according to claim 1, wherein said signal weights are calculated for a series of frames, each frame having a predetermined length comprising of N first signal samples and N second signal samples. 26. A method according to claim 25, wherein N=64. 27. A method according to claim 25, further including filtering or smoothing the series of weights to minimise frame-to-frame variation in the calculated weights.