Abstract:
An active noise reduction system is provided for receiving an audio input signal and a noise interference signal and calculating an audio broadcasting signal according to a Feedback Filtered-X Least-Mean-Square (FFXLMS) algorithm, wherein the FFXLMS algorithm optimizes a (convergence factor) μ so as to decrease the numbers of divisions operated by the active noise reduction system and increase the operation speed of the active noise reduction system.

Description:
[0001]    This application claims the benefit of Taiwan Patent Application Serial No. 099104372, filed on Feb. 11, 2010, the subject matter of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a noise reduction system, and more particularly relates to an active noise reduction system. 
       BACKGROUND OF THE INVENTION 
       [0003]    Due to the rapid development in the electronic technology, almost everyone has an MP3 player. Moreover, most mobile phones are installed with functions of an MP3 player so that users can enjoy music wherever and whenever they want. However, the MP3 player is mostly used when users commute or go out; as a result, music is often badly affected by background noises outside and the quality of music is also degraded. 
         [0004]    Most users raise the volume of the music up to decrease the ratio between noise and music so as to ignore the background noise, which makes them stay in an environment of high decibel such that the users hearing can be damaged. 
         [0005]    Therefore, many researches are doing researches in order to decrease the level of noise. In general, methods for decreasing noises are in two groups—one is passive noise reduction and the other is active noise reduction. The method of passive noise reduction absorbs and blocks noises away by means of concrete materials. For example, in the prior art, hard materials of the outer shell of a conventional earpiece are used for sound isolation while soft materials of the inner part of the conventional earpiece are used for sound absorption. 
         [0006]    The method of active noise reduction counterbalances noises by digital processing, i.e., outputting a waveform of which the frequency is the same as that of noises, and the phase is different but the amplitude is the same as that of noises. There have been many algorithms being developed for the method of active noise reduction. To control an active noise reduction system by means of adaptive algorithms is commonly known. Among the adaptive algorithms, the Least-Mean-Square (LMS) algorithm is put into use the most extensively. 
         [0007]    However, it is found that when applying the LMS algorithm in the prior art, the secondary pathway is included among the noise source, speaker, and microphone for receiving the noise. A system may become instable if the effect of a function of a transfer of the secondary pathway is neglected when the conventional LMS algorithm is applied. Thus, a Feedback Filtered-X Least-Mean-Square algorithm is developed for an active noise reduction system in the prior art. 
         [0008]    Referring to  FIGS. 1 and 2 , wherein  FIG. 1  is a schematic drawing of an active noise reduction system while  FIG. 2  is a drawing of a structure of an active noise reduction system using a FFXLMS algorithm. An active noise reduction system  100  in the prior art receives an audio input signal a(n) and a noise interference signal d(n), and calculates an audio broadcasting signal y(n) according to the FFXLMS algorithm, wherein the active noise reduction system  100  includes a microphone for receiving the noise interference signal d(n) and a speaker for broadcasting the audio broadcasting signal y(n). Furthermore, the audio input signal a(n) is sent from an electronic device into the active noise reduction system  100 . 
         [0009]    The active noise reduction system  100  includes: 
         [0010]    A first operation unit  11  for receiving the audio input signal a(n) and the audio broadcasting signal y(n), a first reference signal y 2 ( n ) being analyzed with an analytic function of y 2 ( n )=y(n)+a(n); 
         [0011]    A first error operation unit  12  for receiving the noise interference signal d(n) and the audio broadcasting signal y(n), a first error signal e(n) being analyzed with an analytic function of 
         [0000]    
       
         
           
             
               
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         [0012]    a second error operation unit  13  for receiving the first error signal e(n) and the audio input signal a(n), a second error signal e 2 ( n ) being analyzed with an analytic function of 
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         [0013]    a second operation unit  14  for receiving the second error signal e 2 ( n ) and the audio broadcasting signal y(n), a first noise prediction signal x(n) being analyzed with an analytic function of 
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         [0014]    a first adaptive operation unit  15  for receiving the first noise prediction signal x(n), the audio broadcasting signal y(n) being analyzed with an analytic function of 
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         [0000]    and 
         [0015]    a second adaptive operation unit  16  for receiving the first noise prediction signal x(n), a second noise prediction signal x′(n) being analyzed with analytic function of 
         [0000]    
       
         
           
             
               
                 
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         [0016]    wherein S′m and Wl are functions of a Least-Mean-Square algorithm, the definition of S′m being S′m(n+1)=S′m(n)+μa(n−m)e 2 ( n ), m=0, 1, . . . , M−1, and the definition of Wl being Wl(n+1)=Wl(n)+μx′(n−l)e 2 ( n ), l=0, 1, . . . , L−1, wherein μ is a convergence factor. 
         [0017]    However, since the convergence factor μ is usually a floating point number smaller than 1, the conventional active noise reduction system  100  needs a more powerful controller such as a Digital Signal Processing (DSP) or an Application-Specific Integrated Circuit (ASIC); otherwise, it takes too much time in running a floating point operation virtually. 
         [0018]    For example, referring to  FIG. 3 , a table showing work cycle needed by a microcontroller unit (MCU) controller for basic floating point operations and basic integral number operations if the active noise reduction system  100  using the FFXLMS algorithm is utilized and the controller is a microcontroller unit (MCU), which is simpler and cheaper. 
         [0019]    Take a MCU of a working frequency of 40 MHz for example. One of its working cycles is 0.025 uS. The shortest working cycle needed by the basic floating point operation is a multiplication, which is 109*0.025 uS=2.75 uS. If a sampling frequency of 10 kHz is applied in the active noise reduction system  100 , each operation loop has merely an operation time of 1/10kHz=100 uS; that is, the MCU can merely run the floating point operation for 36 times. 
         [0020]    If the active noise reduction system  100  is an active noise reduction system of n orders, the second error operation unit  13 , second operation unit  14 , and the second adaptive operation unit  16 , which use the function of S′m for calculation, need n times of integer division in each operation loop for calculating the convergence factor μ in the function of S′m. 
         [0021]    The first adaptive operation unit  15 , which uses the function of Wl for calculation, needs n times of integer division for calculating the convergence factor μ in the function of Wl in each operation loop. 
         [0022]    It is known from the above that only lower order of LMS algorithm S′m and Wl can be adopted for calculating when a simple controller such as an MCU is applied in the active noise reduction system  100  using the FFXLMS algorithm. However, the noise reduction of the active noise reduction system  100  is not good. 
         [0023]    Based on the above facts, since the convergence factor μ is usually a floating point number smaller than 1, the conventional active noise reduction system  100  using the FFXLMS algorithm needs to run a huge amount of floating point number operations. Although a more powerful controller such as a DSP or an ASIC can directly run a floating point number operation, it is more expensive and the costs of the active noise reduction system  100  increases. 
         [0024]    On the contrary, although a cheaper controller such as a MCU can lower the costs, this kind of controller cannot directly run a floating point number operation—floating point numbers needed are indirectly generated through a division. As a result, the operation takes too long and only lower order of LMS algorithm can be used, which results in the bad quality of noise reduction of the active noise reduction system  100 . 
       SUMMARY OF THE INVENTION 
       [0025]    The present invention relates to a noise reduction system, and more particularly relates to an active noise reduction system. The active noise reduction system is to reduce the number of operations on floating point number of the FFXLMS algorithm such that when the active noise reduction system uses a controller which cannot run an operation of floating point number, the operation time of the controller used is reduced. 
         [0026]    The present invention provides an active noise reduction system for receiving an audio input signal and a noise interference signal and calculating an audio broadcasting signal according to a FFXLMS algorithm, wherein the active noise reduction system optimizes the convergence factor μ of the FFXLMS algorithm such that the active noise reduction system runs less divisions and increases operation speed. 
         [0027]    Compared with the active noise reduction system of the prior art, the active noise reduction system of the present invention optimizes the convergence factor μ of the FFXLMS algorithm such that the active noise reduction system runs less floating point number operations. Therefore, when the active noise reduction system uses a controller which cannot run an operation of floating point number, the operation time of the controller used is reduced, and the order of LMS is increased so that the noise reduction is improved. 
         [0028]    In other words, the active noise reduction system of the present invention uses a controller which cannot run an operation of floating point number, and lower the costs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    The accompanying drawings are included to provide a further understanding of the present invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principals of the present invention. 
           [0030]      FIG. 1  is a schematic drawing of an active noise reduction system of the prior art. 
           [0031]      FIG. 2  is a drawing of a structure of an active noise reduction system of the prior art using an FFXLMS algorithm. 
           [0032]      FIG. 3  is a table showing work cycle needed by an MCU controller for basic floating point operations and basic integral number operations. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]    The present invention relates to a noise reduction system, and more particularly relates to an active noise reduction system. Two preferable embodiments of the present invention are as following. It is a common understanding for persons having ordinary skill in the art that these preferable embodiments are examples of the present invention and should not limit the invention itself. 
         [0034]    Referring to  FIG. 2 , the active noise reduction system  100  of the present invention optimizes the convergence factor μ of the FFXLMS algorithm. A first embodiment of the active noise reduction system  100  is used for receiving an audio input signal a(n) and a noise interference signal d(n) and calculating an audio broadcasting signal y(n) according to the FFXLMS algorithm, wherein when the audio input signal a(n)=0, the active noise reduction system  100  performs a noise reduction merely on the noise interference signal d(n). 
         [0035]    The active noise reduction system  100  of the present invention includes a first operation unit  11 , a first error operation unit  12 , a second error operation unit  13 , a second operation unit  14 , a first adaptive operation unit  15 , and a second adaptive operation unit  16 . 
         [0036]    The first operation unit  11  is used for receiving the audio input signal a(n) and the audio broadcasting signal y(n), and a first reference signal y 2 ( n ) is analyzed with an analytic function of y 2 ( n )=y(n)+a(n), wherein the first operation unit  11  is an adder. 
         [0037]    The first error operation unit  12  is used for receiving the noise interference signal d(n) and the audio broadcasting signal y(n), and a first error signal e(n) is analyzed with an analytic function of 
         [0000]    
       
         
           
             
               
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         [0000]    wherein the first error operation unit  12  includes at least a subtractor, at least an adder, and at least a multiplier. 
         [0038]    The second error operation unit  13  is used for receiving the first error signal e(n) and the audio input signal a(n), and a second error signal e 2 ( n ) is analyzed with an analytic function of 
         [0000]    
       
         
           
             
               
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         [0000]    wherein the second error operation unit  13  includes at least an adder and at least a multiplier. 
         [0039]    The second operation unit  14  is used for receiving the second error signal e 2 ( n ) and the audio broadcasting signal y(n), and a first noise prediction signal x(n) is analyzed with an analytic function of 
         [0000]    
       
         
           
             
               
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         [0000]    wherein the second operation unit  14  includes at least an adder and at least a multiplier 
         [0040]    The first adaptive operation unit  15  is used for receiving the first noise prediction signal x(n), and the audio broadcasting signal y(n) is analyzed with an analytic function of 
         [0000]    
       
         
           
             
               
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         [0000]    wherein μ is a convergence factor of the LMS algorithm and the first adaptive operation unit  15  includes at least an adder and at least a multiplier. 
         [0041]    The second adaptive operation unit  16  is used for receiving the first noise prediction signal x(n), and a second noise prediction signal x′(n) is analyzed with analytic function of 
         [0000]    
       
         
           
             
               
                 
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         [0000]    wherein the second adaptive operation unit  16  includes at least an adder and at least a multiplier. 
         [0042]    The S′m and Wl are functions of the LMS algorithm. The definition of S′m is S′m(n+1)=S′m(n)+μa(n−m)e 2 ( n ) and the definition of Wl is  Wl(n+1) =  Wl(n) +x′(n−l)e 2 ( n ). 
         [0043]    According to the active noise reduction system  100  of the present invention, the first adaptive operation unit  15  uses the analytic function of 
         [0000]    
       
         
           
             
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         [0000]    If the active noise reduction system  100  is an active noise reduction system of n orders, the first adaptive operation unit  15 , which uses the function of  Wl  for calculation, merely needs one time of integer division for calculating the convergence factor μ in each operation loop. 
         [0044]    In comparison, the first adaptive operation unit  15  needs n times of integer division for calculating the convergence factor μ in the function of Wl in each operation loop in the conventional active noise reduction system  100  using conventional FFXLMS algorithm, in terms of an active noise reduction system of n orders, while only one integer division is needed according to the first embodiment of the present invention. If an MCU controller is applied in the active noise reduction system  100 , each operation loop saves the time for integer division for n−1 times. 
         [0045]    According to a second embodiment of the present invention, the analytic function of the second error operation unit  13  is 
         [0000]    
       
         
           
             
               
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         [0000]    the analytic function of the second operation unit  14  is 
         [0000]    
       
         
           
             
               
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         [0000]    the analytic function of the first adaptive operation unit  15  is 
         [0000]    
       
         
           
             
               
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         [0000]    and the analytic function of the second adaptive operation unit  16  is 
         [0000]    
       
         
           
             
               
                 
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         [0000]    wherein S′m and Wl are functions of the LMS algorithm, the definition of S′m being  S′m(n+1) =  S′m(n) +a(n−m)e 2 ( n ) and the definition of Wl being Wl(n+1)=Wl(n)+μx′(n−l)e 2 ( n ), 
         [0046]    It is obvious that, if the active noise reduction system  100  is an active noise reduction system of n orders, the second error operation unit  13 , the second operation unit  14 , and the second adaptive operation unit  16 , which use the function of  Sm′  for calculation, need only one time of integer division in each operation loop for calculating the convergence factor μ. In other words, all operation units which use the function of  Sm′  for calculation merely need to run integer division for three times in order to calculate the convergence factor μ. 
         [0047]    In comparison, the second error operation unit  13 , the second operation unit  14 , and the second adaptive operation unit  16  need n times of integer division in each operation loop for calculating the convergence factor μ in the function of S′m in the conventional active noise reduction system  100  using conventional FFXLMS algorithm, in terms of an active noise reduction system of n orders, while only three integer divisions are needed according to the second embodiment of the present invention. If a MCU controller is applied in the active noise reduction system  100 , each operation loop saves the time for integer division for n−3 times. 
         [0048]    Furthermore, according to the present invention, all analytic functions using the convergence factor μ can be optimized in the conventional active noise reduction system  100  using conventional FFXLMS algorithm. Thus, when compared with the prior art, the active noise reduction system  100  of the present invention can save the time for integer division for at least (n−1)+(n−3)=2n−4 times. 
         [0049]    Based on the above facts, the active noise reduction system  100  of the present invention optimizes the convergence factor μ of the FFXLMS algorithm such that the active noise reduction system runs less floating point number operations. Therefore, when the active noise reduction system  100  uses a controller which cannot run an operation of floating point number, the operation time of the controller used is reduced, and the order of LMS is increased so that the noise reduction is improved. 
         [0050]    In other words, the active noise reduction system  100  uses a controller which cannot run an operation of floating point number, and lower the costs. 
         [0051]    Although the present invention has been described with reference to the preferred embodiments thereof, it is apparent to those skilled in the art that a variety of modifications and changes may be made without departing from the scope of the present invention which is intended to be defined by the appended claims.