Patent Publication Number: US-8116467-B2

Title: Method for manufacturing array microphones and system for categorizing microphones

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to array microphones, and more particularly to signal delays between component microphones of an array microphone. 
     2. Description of the Related Art 
     An array microphone is a device comprising an array of microphones. Referring to  FIG. 1 , a block diagram of an apparatus  100  comprising an array microphone  110  is shown. When a sound propagates to the array microphone  110 , each of the microphones  102  and  103  receives the same sound to respectively generate an audio signal. The array microphone  110  therefore generates a plurality of audio signals S 1  and S 1 ′ corresponding to the sound. Because the microphones have a location difference therebetween, the sound propagates to the microphones with different phases, and the audio signals S 1  and S 1 ′ have phase difference therebetween due to phase difference of received sounds. After the audio signals S 1  and S 1 ′ are amplified and converted from analog to digital to respectively obtain audio signals S 3  and S 3 ′, the digital signal processor  108  can generate an audio signal S 4  reflecting a sound component coming from a specific direction according to the phase difference between the audio signals S 3  and S 3 ′. 
     The phase difference between the audio signals S 1  and S 1 ′ generated by the array microphone  110  are crucial for synthesis of the audio signal S 4 . The phase difference between the audio signals S 1  and S 1 ′ must faithfully reflect the phase difference between the sounds received by the microphones  102  and  103 . When the microphones  102  and  103  generate the signals S 1  and S 1 ′ with different delay, the delay difference causes the signals S 1  and S 1 ′ to have additional phase difference therebetween, referred to as an intrinsic phase difference between the microphones  102  and  103 . The intrinsic phase difference is then combined with the phase difference of the received sound to generate audio signals S 1  and S 1 ′ with the distorted phase difference, resulting in an erroneously synthesized signal S 4  which cannot correctly reflect the sound component coming from the specific direction. Thus, a method for manufacturing an array microphone with smaller intrinsic phase difference between its component microphones is required. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a method for manufacturing array microphones. First, signal delays of a plurality of microphones are measured. The microphones are then categorized into a plurality of categories according to the signal delays. A plurality of array microphones are then assembled with a number of component microphones selected from the same categories. 
     The invention provides a system for categorizing microphones. In one embodiment, the system comprises a front speaker, a sound card, and a computer. Wherein the front speaker, plays a front sound in front of a tested microphone selected from the microphones to be categorized and a reference microphone. The sound card then records a tested signal generated by the tested microphone in response to the front sound and a reference signal generated by the reference microphone in response to the front sound. Finally, the computer calculates a signal delay between the tested signal and the reference signal, and classifies the tested microphone as one of a plurality of categories according to the signal delay. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       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 block diagram of an apparatus comprising an array microphone; 
         FIG. 2  is a flowchart of a method for manufacturing an array microphone with small intrinsic phase difference between component microphones thereof according to the invention; 
         FIG. 3A  is a block diagram of a system categorizing microphones according to the invention; 
         FIG. 3B  is a block diagram of another system categorizing microphones according to the invention; 
         FIG. 4  is a schematic diagram of a software structure of the computer of  FIG. 3A ; 
         FIG. 5  is a flowchart of a method for categorizing a plurality of microphones according to the invention; 
         FIG. 6  is a flowchart of a method for classifying a tested microphone according to the invention; and 
         FIGS. 7A˜7E  respectively show embodiments of delay ranges corresponding to the categories according to 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. 
     Referring to  FIG. 2 , a flowchart of a method  200  for manufacturing an array microphone with small intrinsic phase difference between component microphones thereof according to the invention is shown. First, signal delays of a plurality of microphones are measured (step  202 ). In one embodiment, the microphones are omni-directional microphones. The microphones are then categorized into a plurality of categories according to the signal delays thereof (step  204 ). For example, microphones with similar signal delays are categorized as the same category. After the microphones are categorized, microphones belonging to the same category therefore have similar signal delays. To fabricate an array microphone, a number of component microphones are first selected from the same categories (step  206 ), and the selected component microphones are gathered to assemble the array microphone (step  208 ). Because the component microphones are selected from the same category and have almost equal signal delays, the delay difference or phase difference between the component microphones are small. Thus, one array microphone with a small phase difference is obtained, and other microphones can be repeatedly fabricated according to steps  206  and  208  until all microphones are exhausted (step  210 ). 
     Referring to  FIG. 3A , a block diagram of a system  300  categorizing microphones according to the invention is shown. The system  300  comprises a computer  302 , a sound card  304 , an amplifier  306 , a switch  308 , a power biasing circuit  310 , and an anechoic chamber  320 . Inside the anechoic chamber  320  are a front speaker  322 , a side speaker  324 , a reference microphone  332 , and a tested microphone  334 . The front speaker  322  is placed in front of the reference microphone  332  and the tested microphone  334  and at the same distance d 1  from the reference microphone  332  and the tested microphone  334 . In one embodiment, the distance d 1  is 20 cm, and the distance d 2  between the reference microphone  332  and the tested microphone  334  is 10.5 mm. The side speaker  324  is placed at a lateral angle from the reference microphone  332  and the tested microphone  334 . In one embodiment, the side speaker  324  is at distance equal to the distance d 1  from the reference microphone  332  and the tested microphone  334 . 
     The computer  302  is a core of the system  300  and controls the sound card  304  and the switch  308 . The power biasing circuit  310  provides the two microphones  332  and  334  with operating voltages. The two microphones  332  and  334  are coupled to two receiving channels of the sound card  304 . Thus, the sound card  304  can record the audio signals S A  and S B  generated by the reference microphone  332  and the tested microphone  334 . In addition, the sound card  304  can also play a sound signal. After the amplifier  306  amplifies the sound signal, the computer  302  controls the switch  308  to pass the sound signal to the front speaker  322  or the side speaker  324 . The front speaker  322  then plays the sound signal S C  as a front sound in front of the microphones  332  and  334 . Otherwise, the side speaker  324  plays the sound signal S D  as a side sound. Referring to  FIG. 3B , a block diagram of another system  350  categorizing microphones according to the invention is shown. The system  350  is almost the same as the system  300  except for the anechoic chamber  320  is replaced with a standing wave pipe  370  in which there is only one front speaker  372 . No side speaker is in the standing wave pipe  370  of  FIG. 3B , and the switch  308  is therefore removed from the system  350 . Because the system  350  is roughly the same as the system  300 , the following embodiments of the invention are illustrated with the system  330 . 
     Referring to  FIG. 4 , a schematic diagram of a software structure of the computer  302  of  FIG. 3A  is shown. The software  400  of the computer  302  comprises a high level software  402  and a low level software  404 . The high level software  402  comprises a system configuration unit  412 , a calibration unit  414 , a sound card setting unit  416 , and a microphone categorization tool  418 . The low level software  404  comprises a microphone test library  422 , an algorithm library  424 , and a sound card control library  426 . The system  300  may comprise multiple sound cards  304 , and the system configuration unit  412  is responsible for selecting a sound card  304  for signal recording and selecting a sound card  304  for sound playing. The calibration unit  414  is responsible for start-up calibration. The sound card setting unit  416  stores sound card settings. The microphone categorization tool  418  then classifies the tested microphone  334  as one of the categories with the low level software  404 . In addition, the microphone categorization tool  418  is a user interface for showing categorization result. 
     Referring to  FIG. 5 , a flowchart of a method  500  for categorizing a plurality of microphones according to the invention is shown. The system  300  categorizes the microphones into a plurality of categories according the method  500 . The method  500  is divided into a calibration stage  532  comprising steps  502  and  504 , a measurement stage  534  comprising steps  505 ˜ 512 , and a categorization stage comprising step  514 . First, the computer  302  calibrates sound volumes played by the front speaker  322  and the side speaker  324  to a standard volume (step  502 ). In addition, because the tested microphone  334  and the reference microphone  332  are respectively coupled to one receiving channel of the sound card  304 , the computer  302  calibrates an intrinsic signal delay between the two recording channels of the sound card  304  (step  504 ). The signal delay between the two receiving channels therefore does not affect the categorization result after calibration. 
     A user then selects a tested microphone  332  from a plurality of microphones (step  505 ) and installs the tested microphone  332  in the anechoic chamber  320  as shown in  FIG. 3A . The computer  302  then controls the sound card  304  to generate a sound signal S C  passed to the front speaker  322 , which then plays the sound signal S C  as a front sound (step  506 ). The reference microphone  332  and the tested microphone  334  then respectively generate audio signals S A  and S B  in response to the front sound. The sound card  304  then records the audio signals S A  and S B  as a reference signal and a tested signal and passes the recorded signals to the computer  302  (step  506 ). The computer  302  then calculates a first signal delay between the tested signal and the reference signal (step  508 ). Thus, a signal delay corresponding to the tested microphone  334  is obtained. 
     In one embodiment, the computer  302  calculates the first signal delay corresponding to the tested microphone  334  on the basis of a sub-band analysis. The computer  302  first filters the tested signal with a set of filters with un-overlapping pass-bands to obtain sub-band components of the tested signal. In one embodiment, the pass-bands of the filters are a first sub-band SB 1  with a frequency range from 120˜500 Hz, a second sub-band SB 2  with a frequency range from 500˜1800 Hz, a third sub-band SB 3  with a frequency range from 1800˜4 kHz, and a fourth sub-band SB 4  with a frequency range from 4 k˜8 kHz. The computer  302  then filters the reference signal with the same set of filters to obtain sub-band components of the reference signal. The sub-band components of the tested signal are then respectively compared with corresponding sub-band components of the reference signal to obtain a set of sub-band delays D 1 , D 2 , D 3 , and D 4 , wherein the sub-band delays D 1 , D 2 , D 3 , and D 4  respectively correspond to the sub-bands SB 1 , SB 2 , SB 3 , and SB 4 . 
     The computer  302  then controls the sound card  304  to generate a sound signal S D  passed to the side speaker  324 , which then plays the sound signal S D  as a side sound (step  510 ). The reference microphone  332  and the tested microphone  334  then respectively generate audio signals S A  and S B  in response to the side sound. The sound card  304  then records the audio signals S A  and S B  as a second reference signal and a second tested signal and passes the recorded signals to the computer  302  (step  510 ). The computer  302  then calculates a second signal delay between the second tested signal and the second reference signal (step  512 ). In one embodiment, the computer  302  calculates the second signal delay corresponding to the tested microphone  334  on the basis of a sub-band analysis. Thus, another set of sub-band delays D 1 ′, D 2 ′, D 3 ′, and D 4 ′ respectively corresponding to the sub-bands SB 1 , SB 2 , SB 3 , and SB 4  are obtained. 
     After the first signal delay and the second signal delay is calculated, the computer classifies the tested microphone as one of the plurality of categories according to the first signal delay and the second signal delay (step  514 ). In one embodiment, each of the categories has a corresponding delay range defining a range of the signal delay of the tested microphone. The computer  302  then compares the measured signal delay with the plurality of delay ranges corresponding to the categories. When the measured signal delay meets the delay range corresponding to a target category selected from the categories, the computer  302  classifies the tested microphone as the target category. Another microphone is then selected from the microphones as a next tested microphone to replace the current tested microphone until all microphones has been classified (step  516 ). Thus, all microphones are classified and can be used to assemble array microphones in steps  206  and  208  of the method  200 . 
     In one embodiment, the first signal delay of the tested microphone comprises a set of sub-band delays D 1 , D 2 , D 3 , and D 4  respectively corresponding to the sub-bands SB 1 , SB 2 , SB 3 , and SB 4 , and the second signal delay of the tested microphone comprises a set of sub-band delays D 1 ′, D 2 ′, D 3 ′, and D 4 ′ respectively corresponding to the sub-bands SB 1 , SB 2 , SB 3 , and SB 4 . The computer  302  can then classify the tested microphone according to the sub-band delays D 1 , D 2 , D 3 , and D 4 . Referring to  FIG. 6 , a flowchart of a method  600  for classifying a tested microphone according to the invention is shown. The first sub-band delays D 1 , D 2 , D 3 , and D 4  and second sub-band delays D 1 ′, D 2 ′, D 3 ′, and D 4 ′ are first respectively obtained in steps  602  and  604 . The computer  302  then compares the first sub-band delays with a first threshold range (step  606 ). If first sub-band delays exceed the first threshold range, the tested microphone is marked as a failed one, which is abandoned and not used for assembling an array microphone (step  622 ). Accordingly, the computer  302  also compares the second sub-band delays with a second threshold range (step  608 ). If second sub-band delays exceed the second threshold range, the tested microphone is abandoned and not used for assembling an array microphone (step  622 ). 
     The computer  302  then compares the sub-band delays D 1 , D 2 , D 3 , and D 4  with the plurality of delay ranges corresponding to the categories. In one embodiment, the delay ranges are defined according to the first sub-band SB 1  and the second sub-band SB 2 , and only the sub-band delays D 1  and D 2  are therefore compared. Referring to  FIGS. 7A˜7E , embodiments of delay ranges corresponding to the categories according to the invention are shown. The delay ranges have a unit of a sampling period. Taking the embodiment of  FIG. 7B  for example, the microphones are categorized into categories A, B, C, and D. The computer  302  first compares the measured sub-band delays D 1  and D 2  with the delay range corresponding to the category A (step  610 ). The delay range corresponding to the sub-band SB 1  is (−0.2, 0.1), and the delay range corresponding to the sub-band SB 2  is (−0.1, 0). If the sub-band delay D 1  is within the delay range (−0.2, 0.1) and the sub-band delay D 2  is within the delay range (−0.1, 0), the tested microphone is classified as the category A (step  612 ). Otherwise, the measured sub-band delays D 1  and D 2  are compared with delay ranges of other categories until a target category is found. Finally, the categorization result is shown on a screen of the computer  302  to notify the user. 
     The invention provides a method for manufacturing array microphones. Signal delays of microphones are first measured. The microphones are then categorized into a plurality of categories according to the measured signal delays, wherein microphones of one category have similar signal delays. Component microphones of an array microphone are then selected from the same category. Thus, a delay difference or a phase difference between the component microphones of the array microphone is small to improve the performance of the array microphone. 
     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.