Abstract:
A broadside small array microphone beamforming unit comprises a first omni-directional microphone to generate a signal X 1 ( t ), a second omni-directional microphone to generate a signal X 2 ( t ), a first delay unit delaying the signal X 1 ( t ) to generate a signal X 1 ( t −T), a second delay unit delaying the signal X 2 ( t ) to generate a signal X 2 ( t −T), a first substrator subtracting the signal X 1 ( t −T) from the signal X 2 ( t ) to generate a signal R(t)=X 2 ( t )−X 1 ( t −T), a second substrator subtracting the signal X 2 ( t −T) from the signal X 1 ( t ) to generate a signal L(t)=X 1 ( t )−X 2 ( t −T), a third delay unit delaying the signal R(t) to generate a signal R′(t)=R(t−D), a gain function unit convoluting the signal L(t) with a gain function G(t) to generate a signal L′(t)=L(t)*G(t−i), and a substrator subtracting the signal L′(t) from the signal R′(t) to generate a signal B′(t)=R′(t)−L′(t).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to broadside small array microphone beamforming unit, and in particular to low noise adjustable beams for broadside small array microphone beamforming unit. 
     2. Description of the Related Art 
     Many communication system and voice recognition devices are designed for use in noisy environments. Examples of such applications include communication and/or voice recognition in cars or mobile environments (e.g., on street). For these applications, the microphones in the system pick up not only the desired voice but also noise as well. The noise can degrade the quality of voice communication and speech recognition performance if it is not dealt with in an effective manner. 
     Noise suppression is often required in many communication systems and voice recognition devices to suppress noise to improve communication quality and voice recognition performance. Noise suppression may be achieved using various techniques, which may be classified as single microphone techniques and array microphone techniques. 
     Single microphone noise reduction techniques typically use spectral subtraction to reduce the amount of noise in a noisy speech signal. With spectral subtraction based techniques, the power spectrum of the noise is estimated and then subtracted from the power spectrum of the noisy speech signal. The phase of the resultant enhanced speech signal is maintained equal to the phase of the noisy speech signal so that the speech signal is minimally distorted. The spectral subtraction based techniques are effective in reducing stationary noise but are not very effective in reducing non-stationary noise. Moreover, even for stationary noise reduction, these techniques can cause distortion in the speech signal at low signal-to-noise ratio (SNR). 
     Array microphone noise reduction technique use multiple microphones that are placed at different locations and are separated from each other by some minimum distance to form a beam. Conventionally, the beam is used to pick up speech that is then used to reduce the amount of noise picked speech that is then used to reduce the amount of noise picked up outside of the beam. Thus, the array microphone techniques can suppress non-stationary noise. Multiple microphones, however, also create more noise due to the number of microphones. 
     Thus, effective suppression of noise in communication system and voice recognition devices is desirable. 
     BRIEF SUMMARY OF THE INVENTION 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     An embodiment of a broadside small array microphone beamforming unit for adjusting a beam direction and reducing internal noise in a reference channel is provided. The broadside small array microphone beamforming unit comprises a first omni-directional microphone responding to input to generate a first signal X 1 ( t ), a second omni-directional microphone responding to input to generate a second signal X 2 ( t ), a first delay unit delaying the first signal X 1 ( t ) by a period T to generate a third signal X 1 ( t −T), a second delay unit delaying the second signal X 2 ( t ) by the period T to generate a fourth signal X 2 ( t −T), a first substrator subtracting the third signal X 1 ( t −T) from the second signal X 2 ( t ) to generate a fifth signal R(t)=X 2 ( t )−X 1 ( t −T), a second substrator subtracting the fourth signal X 2 ( t −T) from the first signal X 1 ( t ) to generate a sixth signal L(t)=X 1 ( t )−X 2 ( t −T), a third delay unit delaying the fifth signal R(t) by D samples to generate a seventh signal R′(t)=R(t−D), a gain function unit convoluting the sixth signal L(t) with a gain function G(t) to generate an eighth signal L′(t)=L(t)*G(t−i) and a substrator subtracting the eighth signal L′(t) from the seventh signal R′(t) to generate a ninth signal B′(t)=R′(t)−L′(t). 
     An embodiment of a broadside small array microphone beamforming unit for adjusting a beam direction and reducing internal noise in a reference channel is provided. The broadside small array microphone beamforming unit comprises a first voice activity detector VAD 1  detecting the correlation between a first signal A(t) and a second signal B′(t) to generate a correlated signal V 1 ( t ), a second voice activity detector VAD 2  detecting the non-correlation between the first signal A(t) and the second signal B′(t) to generate a non-correlated signal V 2 ( t ), a first delay unit delaying the second signal B′(t) by D 1  samples to generate a third signal B′(t−D 1 ), a second delay unit delaying the second signal B′(t) by D 2  samples to generate a fourth signal B′(t−D 2 ), a first adaptive filter suppressing correlated components and leaving non-correlated components between the first signal A(t) and the third signal B′(t−D 1 ) to generate a fifth signal C(t) according to the correlated signal V 1 ( t ) and a second adaptive filter suppressing non-correlated components between the fourth signal B′(t−D 2 ) and the fifth signal C(t) to generate a sixth signal B″(t) according to the non-correlated signal V 2 ( t ). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram of a beamforming mechanism for a broadside small array microphone according to an embodiment of the invention; 
         FIG. 2  is a schematic diagram of a reference channel beamforming unit according to an embodiment of the invention; 
         FIG. 3  is a schematic diagram of a reference channel beamforming unit according to another embodiment of the invention; 
         FIG. 4  is a schematic diagram of a main channel beamforming unit according to another embodiment of the invention; and 
         FIG. 5  is a schematic diagram of a reference channel beamforming unit according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG. 1  is a schematic diagram of a beamforming mechanism for a broadside small array microphone according to an embodiment of the invention. As shown in  FIG. 1 , two omni-directional microphones  10  and  20  are co-disposed and separated to form two channels, a reference channel and main channel, for beamforming. The sum of the two signals generated by the two omni-directional microphones  10  and  20  is used as the main channel with omni-directional lobe  60 . A signal generated by one of microphones  10  and  20  can be used as the main channel. Omni-directional microphones  10  and  20  can form two directional microphones with single main lobes  40  and  50 , with one directional microphone with single lobe  40  or  50  pointed to the left and the other to the right. The two directional microphones with single main lobes can further form a bi-directional microphone as the reference channel. Signal source  30  is located at the cross point of the two single main lobes  40  and  50  or the null of the bi-directional microphone. In this invention, the bi-directional microphone is used as a reference and one of the omni-directional microphones is used as main channel to form a narrow beam facing the signal source  30 . 
     During formation of bi-directional microphones with single main lobes by using omni-directional microphones, extra noise is generated in the reference channel, particularly at low frequencies. This couples noise to the main channel to affect voice quality and degrade noise suppression in beamforming. In addition, the null of the bi-directional microphone determines the beam direction. In this case, the beam is fixed, which may not be suitable for some applications. In the invention, the beam is adjustable for specific applications. 
       FIG. 2  is a schematic diagram of reference channel beamforming unit  200  according to an embodiment of the invention. Two omni-directional microphones  211  and  212  form two directional microphones with single main lobes, one pointing left and the other right. Omni-directional microphones  211  and  212  are at different positions separated by distance d 1 , respectively generating signals X 1 ( t ) and X 2 ( t ) according to input voice. Delay unit  213  receives signal X 1 ( t ) and delays signal X 1 ( t ) by period T to generate signal X 1 ( t −T). Delay unit  214  receives signal X 2 ( t ) and delay signal X 2 ( t ) by period T to generate signal X 2 ( t −T). Substrator  215  subtracts signal X 1 ( t −T) from X 2 ( t ) to generate signal R(t)=X 2 ( t )−X 1 ( t −T). Signal R(t) is the signal for the directional microphone pointing right. Substrator  216  subtracts signal X 2 ( t −T) from X 1 ( t ) to generate signal L(t)=X 1 ( t )−X 2 ( t −T). Signal L(t) is the signal for the directional microphone pointing left. The polar patterns of these two directional microphones are determined by delay time T. Substrator  217  subtracts signal L(t) from R(t) to get reference channel signal B(t)=R(t)−L(t) for the bi-directional microphone. However, the null of the directional microphones is fixed, i.e., the direction of the polar patterns is vertical to the line link two microphones. Moreover, forming the bi-directional microphone in this way will cause more noise because the internal noise of the two microphones is independent, i.e., the internal noise cannot be cancelled in the process to form the bi-directional microphone. In addition, due to the low frequency component loss in the bi-directional microphone formation, low frequency component requires boosting. In such case, the low frequency noise will also be boosted accordingly and therefore the SNR at low frequencies becomes much lower. 
       FIG. 3  is a schematic diagram of reference channel beamforming unit  300  according to another embodiment of the invention. Reference channel beamforming unit  300  in  FIG. 3  is modified from reference channel beamforming unit  200  in  FIG. 2  for adjusting the beam direction to certain range in order to avoid suppression of the desired voice. Two omni-directional microphones  311  and  312  form two directional microphones with single main lobes, one pointing left and the other right. Omni-directional microphones  311  and  312  at different positions are separated by distance d 1  and respectively generate signals X 1 ( t ) and X 2 ( t ) according to input voice. Delay unit  313  receives signal X 1 ( t ) and delays signal X 1 ( t ) by period T to generate signal X 1 ( t −T). Delay unit  314  receives signal X 2 ( t ) and delay signal X 2 ( t ) by period T to generate signal X 2 ( t −T). Substrator  315  subtracts signal X 1 ( t −T) from X 2 ( t ) to generate signal R(t)=X 2 ( t )−X 1 ( t −T). Signal R(t) is the signal for the directional microphone pointing right. D-sample delay unit  317  delay signal R(t) by D samples to get signal R′(t)=R(t−D). Gain function unit  318  convolutes signal L(t) with a gain function G(t) to generate signal L′(t)=L(t)*G(t−i). Substrator  319  subtracts signal L′(t) from R′(t) to generate reference channel signal B′(t)=R′(t)−L′(t). The gain function G(i) is updated by signal B′(t) by any adaptive filtering algorithm. In one embodiment of the invention, the gain function G(i) is adjusted according to reference channel signal B′(t) to minimize signal B′(t). In another embodiment of the invention, some constrains are also added into the gain function G(t), to limit variations, i.e., Th 1 ( i )&lt;∥G(t−i)∥&lt;Th 2 ( i ). Th(i) is a constrain function, for example, for D=1, three taps of G(t−i), Th 1 ( i )=[0.1, 0.5, 0.1], and Th 2 ( i )=[0.2, 1.5, 0.2]. 
       FIG. 4  is a schematic diagram of main channel beamforming unit  400  according to another embodiment of the invention. Omni-directional microphones  311  and  312  respectively generate signals X 1 ( t ) and X 2 ( t ). Adder  320  adds signal X 1 ( t ) and signal X 2 ( t ) to generate main channel signal A(t). In another embodiment, signal generated by one of two omni-directional microphones  311  or  312  is used as the main channel (not shown in  FIG. 4 ). 
       FIG. 5  is a schematic diagram of reference channel beamforming unit  500  according to another embodiment of the invention. Reference channel beamforming unit  500  reduces internal noise in the formed bi-directional microphone to improve reference channel signal B″(t) for beamforming. Main channel signal A(t) is sent to adaptive filter  501 , voice activity detectors VAD 1  and VAD 2 . Reference channel signal B′(t) is sent to delay units  503  and  504  and voice activity detectors VAD 1  and VAD 2 . Delay unit  503  delays reference channel signal B′(t) by D 1  samples to generate signal B′(t−D 1 ) and then sent signal B′(t−D 1 ) to adaptive filter  501 . Delay unit  504  delays reference channel signal B′(t) by D 2  samples to generate signal B′(t−D 2 ) and then sent signal B′(t−D 2 ) to adaptive filter  502 . In one embodiment of the invention, delay sample D 2  is larger than delay sample D 1 . Voice activity detectors VAD 1  and VAD 2  detect the correlation between reference signal B′(t) and main channel signal A(t). For example, VAD 1 =1 means the presence of the correlated signals between the main channel signal A(t) and reference channel signal B′(t). Adaptive filter  501  receives main channel signal A(t) and signal B′(t−D 1 ) and filters the two signals to provide signal C(t) which suppresses correlated components and leaves non-correlated components between main channel signal A(t) and signal B′(t−D 1 ) according to correlated signal V 1 ( t ). Constraint  1  is added to adaptive filter  501  to reduce residual desired voice. The specific constraint in Constraint  1  is |C(t)|&lt;|B′(t−D 1 )|. Since the internal noise of the two microphones is non-correlated and most voice is correlated, the internal noise can be kept and voice is suppressed in signal C(t). Both signal C(t) and signal B″(t−D 2 ) are sent to adaptive filter  502 . Adaptive filter  502  is controlled by voice activity detector VAD 2 . Here voice activity detector VAD 2  indicates the presence of non-correlated noise only. Constraint  2  is added to adaptive filter  502  to limit the over adaptation to improve noise suppression. The specific constraint in Constraint  2  is W(i)=W(i)/∥W(i)∥. Adaptive filter  502  filters signal C(t) and signal B″(t−D 2 ) to provide reference channel signal B″(t) with suppressed internal non-correlated noise. 
     The invention provides a reference channel beamforming unit to reduce internal noise in a reference channel, reducing noise coupling and enhancing beamforming performance, particularly at low frequencies, and introduces a parameter T to adjust the beam direction for a certain range, enhancing flexibility and reducing degradation of the desired sound. 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.