Abstract:
The invention provides a system for calibrating phase and gain mismatches of an array microphone. The array microphone is installed in a voice interface device and comprises a plurality of microphones. The system comprises a loudspeaker and a computing equipment. The loudspeaker plays a segment of sound to be received by the array microphone. The computing equipment controlls the voice interface device which converts the segment of sound to a plurality of audio signals with the microphones of the array microphone, records the audio signals outputted by the voice interface device at bypass mode without any signal processing, calculates delays between the audio signals, and instructs the voice interface device to adjust phase mismatches between the audio signals according to the delays.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to array microphones, and more particularly to production line calibration of voice interface devices including array microphones. 
         [0003]    2. Description of the Related Art 
         [0004]    A single microphone only capable of receive sound from all directions with uniform gain is referred to as an omni-directional microphone. An omni-directional microphone used to receive a target voice from a single direction, simultaneously receives other surrounding noises coming from other directions. Thus, surrounding noise captured with the target voice degrades voice quality. 
         [0005]    An array microphone including a plurality of microphones, prevents the described deficiency of an omni-directional microphone by receiving a target sound at different locations. Thus there are small differences between the phases and amplitudes of signals received by the microphones, caused by receiving sound at different locations. Thus, the array microphone can identify the target sound coming from a specific direction according to the phase and amplitude differences, and suppress surrounding noise coming from other directions. Such an array microphone is referred to as a “directional microphone”, because it is capable of capturing sound from a specific direction. 
         [0006]    For this reason, the phase and amplitude differences of audio signals received by the microphones in an array microphone are crucial for the extraction of the target sound. The phase and amplitude differences, however, are not always caused by the differences in sound received by the microphones at different locations. The component mismatches between the microphones and the input circuits thereof also induce the phase and amplitude differences of the audio signals. For example, the capacitance difference between diaphragms of different microphones may cause a delay in the audio signals, and the resistance difference of the input circuits of the microphones may cause gain difference in the audio signals. If such phase and amplitude differences are used to extract the target sound coming from a specific direction, the derived target sound may be erroneous. Hence, the phase and amplitude differences induced by component mismatches significantly affect the performance of an array microphone. It is very difficult, however, to fabricate an array microphone with identical microphones. Thus, a method for calibrating phase and gain mismatches during fabrication of an array microphone is desirable. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The invention provides a system for calibrating phase and gain mismatches of an array microphone. The array microphone is installed in a voice interface device and comprises a plurality of microphones. The system comprises a loudspeaker and a computing equipment. The loudspeaker plays a segment of sound to be received by the array microphone. The computing equipment controls the voice interface device which converts the segment of sound to a plurality of audio signals with the microphones of the array microphone, records the audio signals outputted by the voice interface device at bypass mode without any signal processing, calculates delays between the audio signals, and instructs the voice interface device to adjust phase mismatches between the audio signals according to the delays. 
         [0008]    The invention also provides a method for calibrating phase and gain mismatches of an array microphone. The array microphone is installed in a voice interface device and comprises a plurality of microphones. First, a segment of sound to be received by the array microphone is played. The voice interface device is then controlled to bypass audio signals converted from the sound by the microphones of the array microphone. The audio signals output by the voice interface device are then recorded. Correlation coefficients based on correlation of the audio signals is then calculated. Delays between the audio signals are then determined according to the correlation coefficients. Finally, the voice interface device is instructed to adjust phase mismatches between the audio signals according to the delays. 
         [0009]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0011]      FIG. 1  is a block diagram of a system for calibrating phase and gain mismatches of array microphones according to the invention; 
           [0012]      FIG. 2  is a flowchart of a method for calibrating phase and gain mismatches of array microphones according to the invention; 
           [0013]      FIG. 3  is a flowchart of a system calibrating the gain and phase mismatches of a voice interface device according to the invention; 
           [0014]      FIG. 4  is a flowchart of another system calibrating the gain and phase mismatches of a voice interface device according to the invention; and 
           [0015]      FIG. 5  is a flowchart of a phase and gain mismatch calibration method on the basis of sub-band analysis according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    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. 
         [0017]      FIG. 1  is a block diagram of a system  102  for calibrating phase and gain mismatches of array microphones according to the invention. The system  102  includes a computing equipment  106  and a loudspeaker  108 , and is used to calibrate the array microphone  110  of a voice interface device  104  during production of the voice interface device  104  on a production line. For example, the voice interface device  104  may be a Bluetooth earphone, a GPS hands-free speakerphone, or a hands-free car kit, or cellphone or PC, etc. The voice interface device  104  includes an array microphone  110 , which further comprises two omni-directional microphones,  112  and  114 , separated by a distance d. The computing equipment  106  may be a computer or a microcontroller. 
         [0018]    In addition to the microphone array  110 , the voice interface device  100  also includes two microphone input circuits  122  and  132 , two analog to digital converters  124  and  134 , a digital signal processor  126 , a memory  128 , a digital I/O interface  142 , and a control I/O interface  144 . The omni-directional microphones  112  and  114  first respectively convert a received sound to audio signals X 1  and Y 1 . The audio signals X 1  and Y 1  are then respectively amplified and filtered by the microphone input circuits  122  and  132  to obtain the audio signals X 2  and Y 2 , which are further converted to digital audio signals X 3  and Y 3  by analog to digital converters  124  and  134 . 
         [0019]    The digital signal processor  126  can then process the audio signals X 3  and Y 3  to obtain the audio signals X 4  and Y 4  according to instructions of the computing equipment  106 . The computing equipment  106  is connected to the voice interface device  104  via two interfaces: the digital I/O interface  142  and the control I/O interface  144 . The audio signals X 4  and Y 4  can be transmitted to the computing equipment  106  through the digital I/O interface  142 . The computing equipment  106  sends instructions to control the digital signal processor  126  via the control I/O interface  144 . Although the array microphone  110  includes only two omni-directional microphones, the system  102  can be used to calibrate a voice interface device  104  including a microphone array containing more than two omni-directional microphones. 
         [0020]    To illustrate the calibration process of the system  100 , a method  200  for calibrating phase and gain mismatches of array microphones according to the invention is provided in  FIG. 2 . The computing equipment  106  functions according to method  200  to calibrate the voice interface device  100 . First, the computing equipment  106  controls the loudspeaker  108  to play a segment of sound in step  202 , wherein the loudspeaker  108  is put at the same distances to the two microphones  112  and  114 . At the same time, the computing equipment  106  also sets the digital signal processor  126  as a bypass mode in step  204 . When the loudspeaker  108  plays the sound, the microphones  112  and  114  respectively converts the sound to audio signals X 1  and Y 1 , and the audio signals X 1  and Y 1  are then processed by the microphone input circuits and the analog to digital converters to form audio signals X 3  and Y 3 . In bypass mode, the digital signal processor  126  directly bypasses the audio signals X 3  and Y 3  to be output to the computing equipment  106  as the audio signals X 4  and Y 4 . Thus, the audio signals X 4  and Y 4  only comprise phase and gain mismatches induced by the microphones  112  and  114 , the input circuits  122  and  132 , and the analog to digital converters  124  and  134 , and can be recorded by the computing equipment  106  for further analysis in step  206 . 
         [0021]    The recorded audio signals X 4  and Y 4  are then analyzed by the computing equipment  106  in two different analysis paths. One analysis path  210  is to determine the phase mismatch between the audio signals X 4  and Y 4 , and the other analysis path  220  is to determine the gain mismatch between the audio signals X 4  and Y 4 . With regard to phase mismatching, because the sampling rate of analog to digital converters  124  and  134  may be lower, the computing equipment  106  first interpolates the audio signals in step  210  to increase the sampling rate of the audio signals fitting the requirement for delay calculation with enough precision. The interpolated audio signals are then used to calculate cross-correlation coefficients in step  214 . A delay between the samples of the audio signals can then be determined according to the correlation coefficients in step  216 . Because the loudspeaker  108  is separated by the same distance from microphones  112  and  114 , the sound is delayed by the same amount prior to reception by the microphones, thus, no phase mismatching exists between the audio signals. Thus, the delay between the audio signals is caused completely by component mismatch of the microphones themselves, the input circuits thereof, and the ADCs. A set of predetermined delay values may be stored in the memory  128  in advance, and a delay index can be determined in step  218  to select a delay value nearest the delay calculated in step  216  from the set of delay values. Thus, after the delay index is delivered to the digital signal processor  126 , the digital signal processor  126  can then delay the samples of the audio signals X 3  or Y 3  according to the delay index, and the audio signals X 4  and Y 4  without phase mismatching. 
         [0022]    The gain mismatch is determined in the analysis path  220 . The computing equipment  106  first measuring the powers of the audio signals X 4  and Y 4  in step  222 . The measured powers are then smoothed in step  224  to obtain average powers of the audio signals. Because the loudspeaker  108  is separated from the microphones  112  and  114  by the same distance, the sound suffers the same amount of attenuation before being received by the microphones, thus, no amplitude mismatching exists between the audio signals. Thus, the power difference between the audio signals is caused completely by component mismatching of the microphones, the input circuits thereof, and the ADCs. A gain value can then be determined according to the smoothed powers in step  226 . After the gain value is delivered to the digital signal processor  126 , the digital signal processor  126  can then amplify the samples of the audio signals X 3  or Y 3  according to the gain value to compensate for the gain mismatch, and the audio signals X 4  and Y 4  without gain mismatching is obtained. 
         [0023]    Moreover, the delay and the gain calculated in steps  218  and  226  can be further used to determine a set of filtering coefficients for compensating the phase and gain mismatches of the audio signals X 3  and Y 3 . The filtering coefficients can be stored in the memory  128 , and the digital signal processor  126  then filters the audio signals X 3  and Y 3  according to the filtering coefficients to obtain the audio signals X 4  and Y 4  without phase and gain mismatches. In one embodiment, multiple sets of filtering coefficients are stored in the memory  128  in advance, and the computing equipment  106  simply determines a filtering coefficient index which selects an appropriate set of filtering coefficients from the multiple sets of filtering coefficients, and the digital signal processor  126  can then filter the audio signals X 3  and Y 3  according to the filtering coefficient index to remove the phase and gain mismatches. 
         [0024]      FIG. 3  is a flowchart of a system  302  calibrating the gain and phase mismatches of a voice interface device  304  according to the invention. Two adjustment circuits  323  and  333  are added to the voice interface device  304 . After the delay and gain are determined in the step  216  and  226  of  FIG. 2 , the adjustment circuits  323  and  333  can directly delay the audio signals X 2  and Y 2  and amplifies the audio signals X 2  and Y 2  according to the computer instructions C 2  and C 3 , thus obtaining audio signals X 2 ′ and Y 2 ′ without phase and gain mismatches. 
         [0025]      FIG. 4  is a flowchart of another system  402  calibrating the gain and phase mismatches of a voice interface device  404  according to the invention. The analog to digital converters  424  and  434  of the voice interface device  404  are converts the audio signals X 2  and Y 2  with a high sampling rate to obtain the audio signals X 3  and Y 3 . Two sampling adjustment circuits  423  and  433  are added to the voice interface device  404 . After the delay is determined in the step  216  of  FIG. 2 , the sampling adjustment circuits  423  and  433  directly delay the audio signals X 3  and Y 3  according to the computer instructions C 2  and C 3 , thus, audio signals X 3 ′ and Y 3 ′, without phase mismatches, are obtained. 
         [0026]      FIG. 5  is a flowchart of a phase and gain mismatch calibration method  500  on the basis of sub-band analysis according to the invention. Method  500  is roughly similar to method  200  of  FIG. 2 , except for step  508 . A sub-band analysis is performed on the audio signals in step  508 , and the delay and gain are determined on the basis of the sub-band analysis of step  508 . Thus, a sub-band calibration can be performed to remove the phase and gain mismatches. Although the sub-band calibration  500  requires more computation and is more complicated, the sub-band calibration  500  can remove phase and gain mismatches with better precision. 
         [0027]    The invention provides a method for calibrating phase and gain mismatches of an array microphone. Because the phase and gain mismatches are calibrated when array microphones are fabricated, signals generated by the array microphones will not comprise the delay and attenuation caused by component mismatches of the microphones and the input circuits thereof. Thus, beam-forming can be precisely performed to extract in-band sounds coming from specific directions and suppress the out-of-band noise, and the performance of the voice interface devices including the array microphones is enhanced. 
         [0028]    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.