Research On Null Broadening Algorithm For Differential Microphone Arrays Beamforming

A differential microphone array has the following number of advantages in comparison with an additive array.1)Small size.2)It can form frequency-invariant beampattern,it is effective not only for high frequencies,but for low frequencies as well.Therefore,it is more suitable to process broadband speech signals.3)For a given number of sensors,it has the potential to attain maximum directional gain.Nowadays,differential microphone arrays have received much interest in the field of audio and speech signal processing.However,differential microphone arrays are more sensitive to the microphone mismatches and vulnerable to white noise amplification,especially for low frequencies and high-order arrays.Beamforming is one of the important technologies in array signal processing.Its essence is spatial domain filtering,which enhances the desired signal and suppresses interference and noise.In general,beamforming for differential microphone arrays forms a narrow null in the direction of the interference.In the actual environment,the changing jamming situation or the array platform motion may cause the interference to move out of the null,and then the interference cannot be effectively suppressed,so it is necessary to form a broad null.At present,the derivative constraints method is used to realize the null broadening for differential microphone arrays.However this method has the following two disadvantages: the width of the null is not adjustable;it is not suitable for the first-order differential microphone arrays.Therefore,this thesis studies the null broadening algorithm based on differential microphone arrays beamforming,and discusses the influence of microphone mismatches on the performance of differential arrays null broadening algorithm.The specific work is as follows:1.In this thesis,a null broadening beamforming algorithm based on virtual interference method for robust differential microphone arrays is proposed,which combines the virtual interference method with the LCMV beamforming algorithm for robust differential microphone arrays.It is assumed that there are multiple virtual interferences uniform distributed in the vicinity of the original point interference source.Beamforming can form a trough in the sector of virtual interference sources,thereby increasing the width of the null.Compared with the existing derivative constraints method,the proposed method is not only suitable for the first-order differential microphone arrays,but also for the high-order differential microphone arrays.Furthermore,the width of the pattern null can be flexibly controlled by adjusting the value of the parameters,i.e.the extended interference width and the interference to noise ratio.2.Although differential microphone arrays can achieve superdirective beamforming,they are more sensitive to the microphone mismatches.In practical applications,the microphone mismatches may cause the mainlobe orientation to be undesirably reversed,which will seriously affect the array performance.In this thesis,on the basis of the virtual interference method realizing the differential microphone arrays null broadening,we study the mainlobe misorientation problem for the first-order null broadening differential microphone arrays in the presence of microphone gain,phase errors and self-noise respectively.First of all,there are detailed analyses when the microphone mismatches are random but the boundaries are known.It is easy to appear that the mainlobe misorientation usually occurs when frequency is low,or the values of the extended interference width and the interference to noise ratio are large.Secondly,some design strategies are proposed to ensure the array is always oriented around the desired direction.For different signal frequencies,the range of the extended interference width and the interference to noise ratio are given to avoid the reversal of the mainlobe direction.Finally,the theoretical analysis is verified by simulation experiments.