Patent Publication Number: US-11024280-B2

Title: Signal processing apparatus, method, and program

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit under 35 U.S.C. § 120 as a continuation application of U.S. application Ser. No. 16/340,764, filed on Apr. 10, 2019, which claims the benefit under 35 U.S.C. § 371 as a U.S. National Stage Entry of International Application No. PCT/JP2017/027075, filed in the Japan Patent Office on Jul. 26, 2017, which claims priority to Japanese Patent Application No. 2016-203429, filed in the Japan Patent Office on Oct. 17, 2016, each of which is incorporated by reference herein in its entirety. U.S. patent application Ser. No. 16/340,764 issued as U.S. Pat. No. 10,748,519 on Aug. 18, 2020. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a signal processing apparatus, a method, and a program. 
     BACKGROUND ART 
     In recent years, a noise cancelling (noise reduction) technology has been widely developed. The noise reduction technology is broadly divided into two methods including a Feedforward (FF) method and a Feedback (FB) method. In many cases, these methods are appropriately used in accordance with a targeted noise. 
     On the other hand, there exists an idea of combining these noise reduction technologies. For example, Patent Literature 1 described below discloses a technology of enhancing a noise reduction effect in headphones by combining the FF method and the FB method.
     Patent Literature 1: JP 2008-116782A   

     DISCLOSURE OF INVENTION 
     Technical Problem 
     Nevertheless, in the combination of the FF method and the FB method as disclosed in Patent Literature 1 described above, a noise reduction effect sometimes becomes limitative. For example, as for a noise containing a component varying in characteristic, a noise reduction effect can be limitative. 
     In view of the foregoing, the present disclosure proposes a structure that can effectively reduce a noise containing a component varying in characteristic. 
     Solution to Problem 
     According to the present disclosure, there is provided a signal processing apparatus including a signal processing section configured to generate a first noise reduction signal using an adaptive filter on the basis of a signal output from a first input apparatus, and cause the generated first noise reduction signal to be output by a first output apparatus, and generate a second noise reduction signal using a fixed filter on the basis of a signal output from a second input apparatus, and cause the generated second noise reduction signal to be output by a second output apparatus. 
     Moreover, according to the present disclosure, there is provided a method executed by a processor, the method including generating a first noise reduction signal using an adaptive filter on the basis of a signal output from a first input apparatus, and causing the generated first noise reduction signal to be output by a first output apparatus; and generating a second noise reduction signal using a fixed filter on the basis of a signal output from a second input apparatus, and causing the generated second noise reduction signal to be output by a second output apparatus. 
     Moreover, according to the present disclosure, there is provided a recording medium having a program recorded thereon, the program causing a computer to function as: a signal processing section configured to generate a first noise reduction signal using an adaptive filter on the basis of a signal output from a first input apparatus, and cause the generated first noise reduction signal to be output by a first output apparatus, and generate a second noise reduction signal using a fixed filter on the basis of a signal output from a second input apparatus, and cause the generated second noise reduction signal to be output by a second output apparatus. 
     Advantageous Effects of Invention 
     As described above, according to the present disclosure, a structure that can effectively reduce a noise containing a component varying in characteristic is provided. Note that the effects described above are not necessarily limitative. With or in the place of the above effects, there may be achieved any one of the effects described in this specification or other effects that may be grasped from this specification. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram for describing a noise targeted by a noise cancelling system according to an embodiment of the present disclosure. 
         FIG. 2  is a diagram for describing an effect caused by the noise cancelling system according to the present embodiment. 
         FIG. 3  is a block diagram illustrating an example of an internal configuration of the noise cancelling system according to the present embodiment. 
         FIG. 4  is a diagram for describing noise reduction processing of an AFF method that is performed by the noise cancelling system according to the present embodiment. 
         FIG. 5  is a diagram for describing noise reduction processing of a FB method that is performed by the noise cancelling system according to the present embodiment. 
         FIG. 6  is a diagram for describing noise reduction processing of a FB method that is performed by the noise cancelling system according to the present embodiment. 
         FIG. 7  is a diagram for describing noise reduction processing of a FB method that is performed by the noise cancelling system according to the present embodiment. 
         FIG. 8  is a diagram for describing a first arrangement of the noise cancelling system according to the present embodiment. 
         FIG. 9  is a diagram for describing a second arrangement of the noise cancelling system according to the present embodiment. 
         FIG. 10  is a diagram for describing a third arrangement of the noise cancelling system according to the present embodiment. 
         FIG. 11  is a diagram for describing a fourth arrangement of the noise cancelling system according to the present embodiment. 
         FIG. 12  is a diagram for describing a fifth arrangement of the noise cancelling system according to the present embodiment. 
         FIG. 13  is a diagram for describing a sixth arrangement of the noise cancelling system according to the present embodiment. 
         FIG. 14  is a block diagram illustrating an example of a schematic configuration of a vehicle system. 
     
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION 
     Hereinafter, (a) preferred embodiment(s) of the present disclosure will be described in detail with reference to the appended drawings. Note that, in this specification and the appended drawings, structural elements that have substantially the same function and structure are denoted with the same reference numerals, and repeated explanation of these structural elements is omitted. 
     Note that the description will be given in the following order. 
     1. Overview 
     2. Basic Configuration Example
         2.1. Internal Configuration Example   2.2. Noise Reduction Processing of AFF Method   2.3. Noise Reduction Processing of FB Method       

     3. Variations of Configuration
         3.1. First Arrangement   3.2. Second Arrangement   3.3. Third Arrangement   3.4. Fourth Arrangement   3.5. Fifth Arrangement   3.6. Sixth Arrangement   3.7. Switching of Arrangement       

     4. Hardware Configuration Example 
     5. Conclusion 
     1. OVERVIEW 
     A signal processing apparatus (hereinafter, also referred to as a noise cancelling system (i.e. noise reduction system)) according to an embodiment of the present disclosure can be implemented in various forms. Hereinafter, as an example, a case where the noise cancelling system is implemented as an in-vehicle type will be described. First of all, a noise targeted by an in-vehicle noise cancelling system will be described with reference to  FIG. 1 . 
       FIG. 1  is a diagram for describing a noise targeted by the noise cancelling system according to the present embodiment. In each graph in  FIG. 1 , a horizontal axis indicates frequency and a vertical axis indicates amplitude. A noise  10  of a running vehicle has a noise characteristic including a first noise  11  with a narrow frequency bandwidth and a second noise  12  with a wide frequency bandwidth. The first noise  11  is a peaky noise emerging at a frequency of constant number times of the number of revolutions of an internal combustion engine (i.e. engine), for example. The second noise  12  is a broad noise including a road noise, a wind noise, and the like, for example. For example, the frequency bandwidth of the first noise  11  may have a range of about ±5% with respect to a center frequency. In addition, the frequency bandwidth of the second noise  12  may be about 50 Hz to 300 Hz in consideration of a range that the noise cancelling system can actually reduce (i.e. cancel). It should be appreciated that these values of the targeted noise can vary in accordance with the design/mounting of the noise cancelling system. 
     The first noise  11  varies in frequency in accordance with a variation in the number of revolutions of the engine. Thus, for reducing the first noise  11 , application of noise reduction processing of not a simple FF method but an Adaptive Feedforward (AFF) method is desirable. For example, a noise  31  obtainable after noise reduction processing of the AFF method that uses a filter having a first characteristic  21  is applied is a noise obtained by reducing the first noise  11  from the noise  10 . Even in a case where the frequency of the first noise  11  varies in accordance with a variation in the number of revolutions of the engine, or the like, the noise reduction processing of the AFF method can continuously reduce the first noise  11  by causing a characteristic of the filter to follow the variation. Nevertheless, a component of the second noise  12  still remains in the noise  31 . At the time of vehicle running, influences of not only an engine noise but also a road noise, a wind noise, and the like are large. Thus, the application of only the noise reduction processing of the AFF method is insufficient. 
     On the other hand, for reducing the second noise  12 , application of the noise reduction processing of the FB method is desirable. The noise reduction processing of the FB method can uniformly reduce various noises by a certain degree of amount. Nevertheless, a noise biased toward a pinpointed frequency, such as an engine noise, remains outstanding without being sufficiently reduced. For example, a noise  32  obtainable after the noise reduction processing of the FB method that uses a filter having a second characteristic  22  is applied is a noise obtained by reducing the second noise  12  from the noise  10 . Nevertheless, a component of the first noise  11  still remains in the noise  32 . Thus, the application of only the noise reduction processing of the FB method is insufficient. 
     In view of the foregoing, in the present embodiment, a noise cancelling system that uses the noise reduction processing of the AFF method and the noise reduction processing of the FB method in combination is proposed.  FIG. 2  is a diagram for describing an effect caused by the noise cancelling system according to the present embodiment. In each graph in  FIG. 2 , a horizontal axis indicates frequency, a vertical axis indicates amplitude, and the noise  10 , the first characteristic  21 , and the second characteristic  22  are as described above with reference to  FIG. 1 . As illustrated in  FIG. 2 , a noise  33  obtainable after filter reduction processing of the AFF method that uses a filter having the first characteristic  21  and filter reduction processing of the FB method that uses a filter having the second characteristic  22  are applied is a noise obtained by reducing both of the first noise  11  and the second noise  12  from the targeted noise  10 . 
     2. BASIC CONFIGURATION EXAMPLE 
     Hereinafter, a basic configuration example of the noise cancelling system according to the present embodiment will be described with reference to  FIGS. 3 to 7 . 
     &lt;2.1. Internal Configuration Example&gt; 
       FIG. 3  is a block diagram illustrating an example of an internal configuration of the noise cancelling system according to the present embodiment. As illustrated in  FIG. 3 , a noise cancelling system  100  according to the present embodiment includes a microphone  110 , a speaker  120 , a sensor section  130 , a storage section  140 , and a control section  170 . Note that the structural elements are connected via an in-vehicle network such as a controller area network (CAN). 
     (1) Microphone  110   
     The microphone  110  is an input apparatus that collects (i.e. inputs) an ambient sound. The microphone  110  outputs a signal indicating a sound collection result, to the control section  170 . The microphone  110  may include a microphone amplifier that amplifies a collected signal. In addition, the microphone  110  may output an analog signal, or may include an analog digital converter (ADC) and output a digital signal converted from an analog signal. 
     As illustrated in  FIG. 3 , the microphone  110  includes an AFF microphone  110 A and a FB microphone  110 B. The AFF microphone  110 A is a first input apparatus that performs sound collection at least for the noise reduction processing of the AFF method. The FB microphone  110 B is a second input apparatus that performs sound collection at least for the noise reduction processing of the FB method. 
     (2) Speaker  120   
     The speaker  120  is an output apparatus that outputs (i.e. reproduces) a sound to the periphery. The speaker  120  outputs a sound on the basis of a signal output from the control section  170 . The speaker  120  may include an amplifier that amplifies a signal. In addition, the speaker  120  may include a digital analog converter (DAC) and output a sound on the basis of an analog signal converted from a digital signal. 
     As illustrated in  FIG. 3 , the speaker  120  includes an AFF speaker  120 A and a FB speaker  120 B. The AFF speaker  120 A is a first output apparatus that performs an output for the AFF method. The FB speaker  120 B is a second output apparatus that performs an output for the FB method. 
     (3) Sensor Section  130   
     The sensor section  130  has a function of detecting various types of information. The sensor section  130  according to the present embodiment performs sensing for obtaining a reference signal for the noise reduction processing of the AFF method. For example, the sensor section  130  can be implemented as a tachometer for detecting the number of revolutions of the engine, or a microphone for collecting an engine noise. 
     (4) Storage Section  140   
     The storage section  140  has a function of temporarily or permanently storing information for an operation of the noise cancelling system  100 . 
     (5) Control Section  170   
     The control section  170  has a function of controlling an overall operation of the noise cancelling system  100 . As illustrated in  FIG. 3 , the control section  170  includes a signal processing section  150  and a setting section  160 . 
     The signal processing section  150  has a function of applying various types of signal processing to an input signal and outputting the resultant signal. The setting section  160  has a function of setting an operation mode of the signal processing section  150 . 
     Hereinafter, the signal processing section  150  will be described in detail. The signal processing section  150  according to the present embodiment uses the noise reduction processing of the AFF method and the noise reduction processing of the FB method in combination. Specifically, the signal processing section  150  generates a first noise reduction signal using an adaptive filter on the basis of a signal output from the AFF microphone  110 A. Then, the signal processing section  150  causes the generated first noise reduction signal to be output by the AFF speaker  120 A. In addition, the signal processing section  150  generates a second noise reduction signal using a fixed filter on the basis of a signal output from the FB microphone  110 B. Then, the signal processing section  150  causes the generated second noise reduction signal to be output by the FB speaker  120 B. In this manner, because the signal processing section  150  uses, in combination, the noise reduction processing of two methods different in characteristic, it becomes possible to reduce both noises targeted by the respective methods. Note that the adaptive filter is a filter for which a filter coefficient is sequentially generated. In addition, the fixed filter is a filter for which a filter coefficient is set in a fixed manner. 
     The first noise reduction signal targets a noise with a narrow frequency bandwidth, and the second noise reduction signal targets a noise with a wide frequency bandwidth. In other words, the first noise reduction signal targets a noise with a narrower frequency bandwidth as compared with the second noise reduction signal. In the example illustrated in  FIG. 1 , the first noise reduction signal targets the first noise  11  and the second noise reduction signal targets the second noise  12 . 
     Specifically speaking about a noise to be targeted, the first noise reduction signal may target a noise corresponding to the number of revolutions of a power source that performs a revolving motion, and the second noise reduction signal may target a noise corresponding to a movement performed by a movable object using the power source. In the case of an in-vehicle type, the power source is an internal combustion engine or a motor, the movable object is a vehicle, and the noise corresponding to a movement includes a road noise, a wind noise, and the like, for example. The signal processing section  150  uses, in combination, the noise reduction processing of the AFF method that excels at reduction of a variable peaky noise such as an engine noise, and the noise reduction processing of the FB method that excels at reduction of a broad noise such as a road noise and a wind noise. It thereby becomes possible for the noise cancelling system  100  to reduce more noises in a wide bandwidth at the time of actual running of the vehicle. 
     It should be appreciated that the noise cancelling system  100  can be implemented in a form other than the in-vehicle type. For example, a variety of movable objects can be considered such as a two-wheel vehicle, an electric bicycle, an airplane, a drone, or a ship. In addition, a variety of power sources can be considered such as a thermal engine such as an external-combustion engine, an electrical machinery, or a fluid machinery. In addition, the noise cancelling system  100  needs not be mounted on a movable object. For example, the noise cancelling system  100  may be mounted on a fixedly-installed object such as a digital signage, or may be mounted on headphones, a smartphone, or the like. 
     Hereinafter, the noise reduction processing of the AFF method and the noise reduction processing of the FB method that are performed by the signal processing section  150  will be sequentially described. 
     &lt;2.2. Noise Reduction Processing of AFF Method&gt; 
     Hereinafter, the noise reduction processing of the AFF method will be described with reference to  FIG. 4 . 
       FIG. 4  is a diagram for describing the noise reduction processing of the AFF method that is performed by the noise cancelling system  100  according to the present embodiment. As illustrated in  FIG. 4 , the processing involves the AFF microphone  110 A, the AFF speaker  120 A, the sensor section  130 , an adaptive algorithm (Adaptive algorithm)  151 , and a Feedforward filter (FF filter)  152 . Note that the adaptive algorithm  151  and the FF filter  152  are included in the signal processing section  150 . 
     The sensor section  130  acquires information indicating the number of revolutions of the engine of the vehicle. Hereinafter, the information will also be referred to as an rpm signal. In the noise reduction processing of a typical AFF method, the rpm signal is defined as a noise. Note that the sensor section  130  may collect an engine noise in place of or together with the number of revolutions of the engine, and regards the information as an rpm signal. 
     The AFF microphone  110 A outputs a signal indicating a sound collection result. For example, the AFF microphone  110 A collects a noise in a state of being reduced by the first noise reduction signal output from the AFF speaker  120 A. The AFF microphone  110 A is also referred to as an error microphone, and a signal output from the AFF microphone  110 A is also referred to as an error microphone signal. A position of the error microphone is also referred to as a cancel target point (or a control point (Control Point)), and a noise cancellation effect at the position is the largest. 
     The adaptive algorithm  151  sets a filter coefficient of the FF filter  152 . More specifically, the adaptive algorithm  151  calculates the filter coefficient using an adaptive algorithm on the basis of the reference signal output from the sensor section  130 , and the error microphone signal output from the AFF microphone  110 A. Then, the adaptive algorithm  151  sequentially rewrites the filter coefficient of the FF filter  152  to the calculated filter coefficient. Note that examples of an adaptive algorithm that can be used include least mean square (LMS) and the like. 
     The FF filter  152  generates the first noise reduction signal. More specifically, first of all, the FF filter  152  calculates a reference signal (i.e. noise) on the basis of the rpm signal output from the sensor section  130 . Subsequently, the FF filter  152  generates the first noise reduction signal by applying (i.e. convolving) the filter coefficient set by the adaptive algorithm  151 , to the reference signal. 
     The AFF speaker  120 A outputs the first noise reduction signal output from the FF filter  152 . 
     Here, a positional relationship of the AFF microphone  110 A and the AFF speaker  120 A will be described. As illustrated in  FIG. 4 , the AFF microphone  110 A is installed on a ceiling inner side of the vehicle (i.e. vehicle inner side), for example, and the AFF speaker  120 A is installed on a door inner side of the vehicle (i.e. vehicle inner side), for example. The speaker installed on the door inner side may be a speaker for music reproduction. In other words, the signal processing section  150  uses a speaker for music reproduction as the AFF speaker  120 A. Note that, typically, a speaker for music reproduction that can be used as the AFF speaker  120 A has a larger caliber as compared with a speaker installed on a headrest of a seat that can be used as the FB speaker  120 B, which will be described later. In a case where a speaker for music reproduction that has a large caliber is used for noise reduction processing, it is possible to reduce a noise more effectively especially in a low bandwidth. Note that the music here is concept including any sound other than a noise reduction signal, such as radio voice, television voice, or navigation voice, aside from generally-said music. 
     &lt;2.3. Noise Reduction Processing of FB Method&gt; 
     Hereinafter, the noise reduction processing of the FB method will be described with reference to  FIGS. 5 to 7 . 
       FIGS. 5 to 7  are diagrams for describing the noise reduction processing of the FB method that is performed by the noise cancelling system  100  according to the present embodiment. As illustrated in  FIG. 5 , the processing involves the FB microphone  110 B, the FB speaker  120 B, and a Feedback filter (FB filter)  153 . Note that the FB filter  153  is included in the signal processing section  150 . 
     The FB microphone  110 B outputs a signal indicating a sound collection result. For example, the FB microphone  110 B collects a noise in a state of being reduced by the second noise reduction signal output from the FB speaker  120 B. A position of the FB microphone  110 B is also referred to as a cancel target point, and a noise cancellation effect at the position is the largest. 
     The FB filter  153  generates the second noise reduction signal. More specifically, the FB filter  153  generates the second noise reduction signal by applying a filter coefficient set in a fixed manner, to a signal output from the FB microphone  110 B. 
     The FB speaker  120 B outputs the first noise reduction signal output from the FF filter  152 . 
     Here, a positional relationship of the FB microphone  110 B and the FB speaker  120 B will be described in more detail with reference to  FIG. 6 . As illustrated in  FIG. 6 , the FB microphone  110 B and the FB speaker  120 B are installed near an ear of a user on a headrest  41  of a seat  40  in the vehicle, for example. By the FB microphone  110 B and the FB speaker  120 B being installed near the ear, it becomes possible to effectively reduce a noise delivered to the ear of the user. 
     It is desirable that a relative positional relationship of the FB microphone  110 B and the FB speaker  120 B is fixed. The relative positional relationship here is concept including a relative coordinate and a relative direction. This is because it is difficult to cause a filter coefficient to follow a change in relative positional relationship since a filter coefficient is set in a fixed manner in the noise reduction processing of the FB method. 
     For this reason, it is desirable that the signal processing section  150  uses, as the FB microphone  110 B and the FB speaker  120 B, a microphone and a speaker, respectively, that are provided on one component (i.e. unseparated component) among one or more components forming the seat. In the example illustrated in  FIG. 6 , among components such as the headrest  41 , a seating face  42 , and an armrest that form the seat  40 , a microphone and a speaker that are provided on the headrest  41  are used as the FB microphone  110 B and the FB speaker  120 B, respectively. Thus, even if reclining or the like of the seat  40  is performed, for example, a relative positional relationship of the FB microphone  110 B and the FB speaker  120 B remains unchanged. Nevertheless, for example, when reclining or the like of the seat  40  is performed, absolute positions of the FB microphone  110 B and the FB speaker  120 B change. Note that the absolute position is concept including an absolute coordinate and an absolute direction in a movable object. For example, an absolute position in a vehicle means a position in a coordinate system having an origin corresponding to any position (e.g. engine, etc.) of the vehicle. 
     Note that, in a case where a relative positional relationship of the FB microphone  110 B and the FB speaker  120 B is variable, a filter coefficient to be set for the FB filter  153  may be switched in accordance with a change in the relative positional relationship. For example, in a case where the FB microphone  110 B is provided on the seating face  42  and the FB speaker  120 B is provided on the headrest  41 , a filter coefficient to be set for the FB filter  153  may be switched in accordance with a height of the headrest  41 . Nevertheless, the FB method has a lower degree of freedom in the setting of a filter coefficient, as compared with the AFF method of adaptively generating a filter coefficient, because the FB method merely selects a filter coefficient appropriately from filter coefficients prepared in advance. 
     Installation positions of the FB microphone  110 B and the FB speaker  120 B are not limited to those in the example illustrated in  FIG. 6 . Among one or more components forming a seat, a microphone and a speaker that are provided on any one component can be used as the FB microphone  110 B and the FB speaker  120 B, respectively. For example, as illustrated in  FIG. 7 , in the seat  40 , a microphone and a speaker that are provided not on the headrest  41  but on the seating face  42  may be used as the FB microphone  110 B and the FB speaker  120 B, respectively. In the example illustrated in  FIG. 7 , a FB microphone  110 B- 1  and a FB speaker  120 B- 1  are installed at positions on the seating face  42  that correspond to the left shoulder of the user, and a FB microphone  110 B- 2  and a FB speaker  120 B- 2  are installed at positions on the seating face  42  that correspond to the right shoulder of the user. Also in this example, even if reclining or the like of the seat  40  is performed, for example, a relative positional relationship of the FB microphone  110 B- 1  or  110 B- 2  and the FB speaker  120 B- 1  or  120 B- 2  remains unchanged. Nevertheless, for example, when reclining or the like of the seat  40  is performed, absolute positions of the FB microphone  110 B- 1  or  110 B- 2  and the FB speaker  120 B- 1  or  120 B- 2  change. 
     3. VARIATIONS OF CONFIGURATION 
     A variety of arrangements of the AFF microphone  110 A and the AFF speaker  120 A, and the FB microphone  110 B and the FB speaker  120 B can be considered. For example, arrangements listed in the table described below can be considered. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                 Relative  
               
               
                   
                   
                   
                   
                 positional  
               
               
                   
                   
                 Absolute  
                 Absolute  
                 relationship  
               
               
                   
                   
                 position  
                 position  
                 of  
               
               
                 Arrangement  
                 AFF/  
                 of  
                 of  
                 microphone  
               
               
                 name  
                 FB  
                 microphone  
                 speaker  
                 and speaker 
               
               
                   
               
             
            
               
                 First  
                 AFF  
                 Fixed  
                 Fixed  
                 Fixed  
               
               
                 arrangement  
                 FB  
                 Variable  
                 Variable  
                 Fixed  
               
               
                 Second  
                 AFF  
                 Variable  
                 Fixed  
                 Variable  
               
               
                 arrangement  
                 FB  
                   
                 Variable  
                 Fixed  
               
               
                 Third  
                 AFF  
                 Fixed  
                 Variable  
                 Variable  
               
               
                 arrangement  
                 FB  
                 Variable  
                   
                 Fixed  
               
               
                 Fourth  
                 AFF  
                 Variable  
                 Variable  
                 Fixed  
               
               
                 arrangement  
                 FB  
                   
                   
                   
               
               
                 Fifth  
                 AFF  
                 Fixed  
                 Fixed  
                 Fixed  
               
               
                 arrangement  
                 FB  
                   
                   
                   
               
               
                 Sixth  
                 AFF  
                 Fixed  
                 Fixed  
                 Fixed/  
               
               
                 arrangement  
                   
                 Variable  
                 Variable  
                 variable  
               
               
                   
                 FB  
                 Variable  
                 Variable  
                 Fixed 
               
               
                   
               
            
           
         
       
     
     It should be noted that a relative positional relationship of the FB microphone  110 B and the FB speaker  120 B is fixed in any arrangement. As described above, this is because it is difficult to cause a filter coefficient to follow a change in the relative positional relationship since a filter coefficient is set in a fixed manner in the noise reduction processing of the FB method. On the other hand, a relative positional relationship of the AFF microphone  110 A and the AFF speaker  120 A may be fixed or may be variable. This is because the noise reduction processing of the AFF method can easily cause a filter coefficient to follow a change in the relative positional relationship by adaptively generating a filter coefficient. 
     Hereinafter, each of the arrangements listed in the table described above will be described in detail. Note that, among the processes of the adaptive algorithm  151 , the FF filter  152 , and the FB filter  153 , the description of portions similar to the portions described above will be omitted. 
     &lt;3.1. First Arrangement&gt; 
       FIG. 8  is a diagram for describing a first arrangement of the noise cancelling system  100  according to the present embodiment. According to the arrangement illustrated in  FIG. 8 , the noise cancelling system  100  concurrently performs the noise reduction processing of the AFF method and the noise reduction processing of the FB method while separately using devices required for each processing. Hereinafter, the arrangement of a microphone and a speaker will be described in detail. 
     The signal processing section  150  uses, as the AFF microphone  110 A and the AFF speaker  120 A, a microphone and a speaker, respectively, that have a fixed relative positional relationship. In addition, the signal processing section  150  uses, as the AFF microphone  110 A and the AFF speaker  120 A, a microphone and a speaker, respectively, that have fixed absolute positions. In the example illustrated in  FIG. 8 , the AFF microphone  110 A is installed on a ceiling inner side and the AFF speaker  120 A is installed on a door inner side. For example, even if reclining or the like of a seat is performed, absolute positions and a relative positional relationship of the AFF microphone  110 A and the AFF speaker  120 A remain fixed. On the other hand, the signal processing section  150  uses, as the FB microphone  110 B and the FB speaker  120 B, a microphone and a speaker, respectively, that have a fixed relative positional relationship. In addition, the signal processing section  150  uses, as the FB microphone  110 B and the FB speaker  120 B, a microphone and a speaker, respectively, that have variable absolute positions. In the example illustrated in  FIG. 8 , the FB microphone  110 B and the FB speaker  120 B are installed on a headrest. For example, when reclining or the like of the seat is performed, the absolute positions change while a relative positional relationship of the FB microphone  110 B and the FB speaker  120 B remaining fixed. 
     According to such an arrangement, it becomes possible for the noise cancelling system  100  to reduce a variable peaky noise such as an engine noise and a broad noise such as a road noise and a wind noise. 
     &lt;3.2. Second Arrangement&gt; 
       FIG. 9  is a diagram for describing a second arrangement of the noise cancelling system  100  according to the present embodiment. According to the arrangement illustrated in  FIG. 9 , the noise cancelling system  100  performs the noise reduction processing of the AFF method and the noise reduction processing of the FB method using a common microphone. Hereinafter, the arrangement of a microphone and a speaker will be described in detail. 
     The signal processing section  150  uses, as the AFF microphone  110 A and the AFF speaker  120 A, a microphone and a speaker, respectively, that have a variable relative positional relationship. In the example illustrated in  FIG. 9 , the AFF microphone  110 A is installed on a headrest and the AFF speaker  120 A is installed on a door inner side. For example, when reclining or the like of a seat is performed, an absolute position of the AFF microphone  110 A changes, and relative positions of the AFF microphone  110 A and the AFF speaker  120 A change. On the other hand, the signal processing section  150  uses, as the FB microphone  110 B and the FB speaker  120 B, a microphone and a speaker, respectively, that have a fixed relative positional relationship. In the example illustrated in  FIG. 9 , the FB microphone  110 B and the FB speaker  120 B are installed on a headrest. For example, when reclining or the like of the seat is performed, absolute positions change while a relative positional relationship of the FB microphone  110 B and the FB speaker  120 B remaining fixed. 
     Here, the signal processing section  150  uses, as the AFF microphone  110 A and the FB microphone  110 B, the same microphone having a variable absolute position. In the example illustrated in  FIG. 9 , the signal processing section  150  uses one microphone installed on the headrest, as the AFF microphone  110 A and also as the FB microphone  110 B. A signal output from the microphone used as the AFF microphone  110 A and the FB microphone  110 B is input to the adaptive algorithm  151  and used for the generation of the first noise reduction signal, and input to the FB filter  153  and used for the generation of the second noise reduction signal. 
     In such an arrangement, because the number of microphones to be physically arranged can be decreased by one as compared with the first arrangement, a configuration becomes simpler and system design become easier. In addition, in this arrangement, because there is one cancel target point, an unnecessary diffraction system becomes inexistent and oscillation becomes less likely to be generated. Furthermore, even in a case where reclining or the like of the seat is performed, because at least the relative positional relationship of the FB microphone  110 B and the speaker  120 B is fixed, the noise cancelling system  100  can stably perform the noise reduction processing of the FB method. 
     &lt;3.3. Third Arrangement&gt; 
       FIG. 10  is a diagram for describing a third arrangement of the noise cancelling system  100  according to the present embodiment. According to the arrangement illustrated in  FIG. 10 , the noise cancelling system  100  performs the noise reduction processing of the AFF method and the noise reduction processing of the FB method using a common speaker. Hereinafter, the arrangement of a microphone and a speaker will be described in detail. 
     The signal processing section  150  uses, as the AFF microphone  110 A and the AFF speaker  120 A, a microphone and a speaker, respectively, that have a variable relative positional relationship. In the example illustrated in  FIG. 10 , the AFF microphone  110 A is installed on a ceiling inner side and the AFF speaker  120 A is arranged on a headrest. For example, when reclining or the like of a seat is performed, an absolute position of the AFF speaker  120 A changes, and relative positions of the AFF microphone  110 A and the AFF speaker  120 A change. On the other hand, the signal processing section  150  uses, as the FB microphone  110 B and the FB speaker  120 B, a microphone and a speaker, respectively, that have a fixed relative positional relationship. In the example illustrated in  FIG. 10 , the FB microphone  110 B and the FB speaker  120 B are installed on the headrest. For example, when reclining or the like of the seat is performed, absolute positions change while a relative positional relationship of the FB microphone  110 B and the FB speaker  120 B remaining fixed. 
     Here, the signal processing section  150  uses, as the AFF speaker  120 A and the FB speaker  120 B, the same speaker having a variable absolute position. In the example illustrated in  FIG. 10 , the signal processing section  150  uses one speaker installed on the headrest, as the AFF speaker  120 A and also as the FB speaker  120 B. The speaker used as the AFF speaker  120 A and the FB speaker  120 B outputs a signal indicating an addition result of the first noise reduction signal and the second noise reduction signal that is obtained by an adder  154 . 
     Typically, a speaker for music reproduction is installed on a door inner side. With regard to this, according to this arrangement, the speaker for music reproduction that is installed on the door inner side is no longer used as the AFF speaker  120 A. Thus, because the speaker for music reproduction does not output the first noise reduction signal, it becomes possible to eliminate pressure (e.g. overflow) on a dynamic range that is attributed to the first noise reduction signal. In addition, it becomes possible to separate a system of an amplifier between the first noise reduction signal and a signal for music reproduction. Note that this effect can be similarly caused also in a case where the AFF speaker  120 A is installed aside from a speaker for music reproduction, without commonalizing a speaker. 
     In addition, in this arrangement, because the number of speakers to be physically arranged can be decreased by one as compared with the first arrangement, an effect similar to the second arrangement is caused. In other words, in this arrangement, a configuration becomes simpler and system design becomes easier as compared with the first arrangement. Furthermore, even in a case where reclining or the like of the seat is performed, because at least the relative positional relationship of the FB microphone  110 B and the speaker  120 B is fixed, the noise cancelling system  100  can stably perform the noise reduction processing of the FB method. 
     &lt;3.4. Fourth Arrangement&gt; 
       FIG. 11  is a diagram for describing a fourth arrangement of the noise cancelling system  100  according to the present embodiment. According to the arrangement illustrated in  FIG. 11 , the noise cancelling system  100  performs the noise reduction processing of the AFF method and the noise reduction processing of the FB method using a common microphone and a common speaker. Hereinafter, the arrangement of a microphone and a speaker will be described in detail. 
     The signal processing section  150  uses, as the AFF microphone  110 A and the AFF speaker  120 A, a microphone and a speaker, respectively, that have a fixed relative positional relationship. In the example illustrated in  FIG. 11 , the AFF microphone  110 A and the AFF speaker  120 A are installed on a headrest. For example, when reclining or the like of a seat is performed, absolute positions change while a relative positional relationship of the FB microphone  110 B and the FB speaker  120 B remaining fixed. On the other hand, the signal processing section  150  uses, as the FB microphone  110 B and the FB speaker  120 B, a microphone and a speaker, respectively, that have a fixed relative positional relationship. In the example illustrated in  FIG. 11 , the FB microphone  110 B and the FB speaker  120 B are installed on the headrest. For example, when reclining or the like of the seat is performed, absolute positions change while a relative positional relationship of the FB microphone  110 B and the FB speaker  120 B remaining fixed. 
     Here, the signal processing section  150  uses, as the AFF microphone  110 A and the FB microphone  110 B, the same microphone having a variable absolute position. In the example illustrated in  FIG. 11 , the signal processing section  150  uses one microphone installed on the headrest, as the AFF microphone  110 A and also as the FB microphone  110 B. A signal output from the microphone used as the AFF microphone  110 A and the FB microphone  110 B is input to the adaptive algorithm  151  and used for the generation of the first noise reduction signal, and input to the FB filter  153  and used for the generation of the second noise reduction signal. In addition, the signal processing section  150  uses, as the AFF speaker  120 A and the FB speaker  120 B, the same speaker having a variable absolute position. In the example illustrated in  FIG. 11 , the signal processing section  150  uses one speaker installed on the headrest, as the AFF speaker  120 A and also as the FB speaker  120 B. The speaker used as the AFF speaker  120 A and the FB speaker  120 B outputs a signal indicating an addition result of the first noise reduction signal and the second noise reduction signal that is obtained by the adder  154 . 
     In this arrangement, because there is one microphone and one speaker, and a relative positional relationship of the microphone and the speaker is fixed, the effect caused by the second arrangement and the effect caused by the third arrangement are both caused. 
     &lt;3.5. Fifth Arrangement&gt; 
       FIG. 12  is a diagram for describing a fifth arrangement of the noise cancelling system  100  according to the present embodiment. According to the arrangement illustrated in  FIG. 12 , the noise cancelling system  100  performs the noise reduction processing of the AFF method and the noise reduction processing of the FB method using a common microphone and a common speaker. Hereinafter, the arrangement of a microphone and a speaker will be described in detail. 
     The signal processing section  150  uses, as the AFF microphone  110 A and the AFF speaker  120 A, a microphone and a speaker, respectively, that have a fixed relative positional relationship. In the example illustrated in  FIG. 11 , the AFF microphone  110 A and the AFF speaker  120 A are installed on a ceiling inner side. For example, even if reclining or the like of a seat is performed, absolute positions and a relative positional relationship of the AFF microphone  110 A and the AFF speaker  120 A remain fixed. On the other hand, the signal processing section  150  uses, as the FB microphone  110 B and the FB speaker  120 B, a microphone and a speaker, respectively, that have a fixed relative positional relationship. In the example illustrated in  FIG. 11 , the FB microphone  110 B and the FB speaker  120 B are installed on a ceiling inner side. For example, even if reclining or the like of the seat is performed, absolute positions and a relative positional relationship of the FB microphone  110 B and the FB speaker  120 B remain fixed. 
     Here, the signal processing section  150  uses, as the AFF microphone  110 A and the FB microphone  110 B, the same microphone having a fixed absolute position. In the example illustrated in  FIG. 12 , the signal processing section  150  uses one microphone installed on the ceiling inner side, as the AFF microphone  110 A and also as the FB microphone  110 B. A signal output from the microphone used as the AFF microphone  110 A and the FB microphone  110 B is input to the adaptive algorithm  151  and used for the generation of the first noise reduction signal, and input to the FB filter  153  and used for the generation of the second noise reduction signal. In addition, the signal processing section  150  uses, as the AFF speaker  120 A and the FB speaker  120 B, the same speaker having a fixed absolute position. In the example illustrated in  FIG. 12 , the signal processing section  150  uses one speaker installed on the ceiling inner side, as the AFF speaker  120 A and also as the FB speaker  120 B. The speaker used as the AFF speaker  120 A and the FB speaker  120 B outputs a signal indicating an addition result of the first noise reduction signal and the second noise reduction signal that is obtained by the adder  154 . 
     In this arrangement, because there is one microphone and one speaker, and a relative positional relationship of the microphone and the speaker is fixed, the effect caused by the second arrangement and the effect caused by the third arrangement are both caused. 
     &lt;3.6. Sixth Arrangement&gt; 
       FIG. 13  is a diagram for describing a sixth arrangement of the noise cancelling system  100  according to the present embodiment. According to the arrangement illustrated in  FIG. 13 , the noise cancelling system  100  performs the noise reduction processing of the AFF method using a plurality of microphones and a plurality of speakers. Hereinafter, the arrangement of microphones and speakers will be described in detail. 
     The signal processing section  150  uses, as the AFF microphone  110 A and the AFF speaker  120 A, a plurality of microphones and a plurality of speakers, respectively. In the example illustrated in  FIG. 13 , an AFF microphone  110 A- 1  is installed on a ceiling inner side and an AFF microphone  110 A- 2  is installed on a headrest. In addition, an AFF speaker  120 A- 1  is installed on a door inner side and an AFF speaker  120 A- 2  is installed on the headrest. 
     The adaptive algorithm  151  sets a plurality of filter coefficients of the FF filter  152 . More specifically, the adaptive algorithm  151  calculates the filter coefficients using an adaptive algorithm on the basis of a reference signal output from the sensor section  130 , and error microphone signals output from the AFF microphones  110 A- 1  and  100 A- 2 . Here, the adaptive algorithm  151  calculates two filter coefficients including a filter coefficient for the AFF speaker  120 A- 1  and a filter coefficient for the AFF speaker  120 A- 2 . Then, the adaptive algorithm  151  sequentially rewrites two filter coefficients to be set for the FF filter  152 , to the two calculated filter coefficients. 
     The FF filter  152  generates a plurality of first noise reduction signals for the plurality of AFF speakers  120 A. More specifically, first of all, the FF filter  152  calculates a reference signal (i.e. noise) on the basis of an rpm signal output from the sensor section  130 . Subsequently, the FF filter  152  generates the first noise reduction signal by applying the filter coefficient for the AFF speaker  120 A- 1  that has been set by the adaptive algorithm  151 , to the reference signal, and causes the AFF speaker  120 A- 1  to output the generated first noise reduction signal. In addition, the FF filter  152  generates the first noise reduction signal by applying (i.e. convolving) the filter coefficient for the AFF speaker  120 A- 2  that has been set by the adaptive algorithm  151 , to the reference signal, and causes the AFF speaker  120 A- 2  to output the generated first noise reduction signal. 
     Note that a microphone used as the AFF microphone  110 A may be the same as a microphone used as the FB microphone  110 B, or may be different from the microphone used as the FB microphone  110 B. In the example illustrated in  FIG. 13 , the signal processing section  150  uses a microphone installed on the ceiling inner side, as the AFF microphone  110 A- 1 , and uses one microphone installed on the headrest, as the AFF microphone  110 A- 2  and also as the FB microphone  110 B. A signal output from the microphone used as the AFF microphone  110 A- 2  and the FB microphone  110 B is input to the adaptive algorithm  151  and used for the generation of the first noise reduction signal, and input to the FB filter  153  and used for the generation of the second noise reduction signal. 
     In a similar manner, a speaker used as the AFF speaker  120 A may be the same as a speaker used as the FB speaker  120 B, or may be different from the speaker used as the FB speaker  120 B. In the example illustrated in  FIG. 13 , the signal processing section  150  uses a speaker installed on the door inner side, as the AFF speaker  120 A- 1 , and uses one speaker installed on the headrest, as the AFF speaker  120 A- 2  and also as the FB speaker  120 B. The speaker used as the AFF speaker  120 A- 2  and the FB speaker  120 B outputs a signal indicating an addition result of the first noise reduction signal and the second noise reduction signal that is obtained by the adder  154 . 
     In addition, a relative positional relationship and absolute positions of a microphone and a speaker that are respectively used as the AFF microphone  110 A and the AFF speaker  120 A may be fixed or may be variable. For example, while the AFF microphone  110 A- 1  has a fixed relative positional relationship with the AFF speaker  120 A- 1 , the AFF microphone  110 A- 1  has a variable relative positional relationship with the AFF speaker  120 A- 2 . In addition, an absolute position of the AFF microphone  110 A- 1  is fixed, whereas an absolute position of the AFF microphone  110 A- 2  is variable. 
     The FB microphone  110 B and the FB speaker  120 B are as described in the first arrangement. Thus, the detailed description here will be omitted. 
     According to this arrangement, there is a plurality of routes from the AFF microphone  110 A to the AFF speaker  120 A. Because an effect of noise reduction processing varies depending on the route, by providing a plurality of routes, it becomes possible to enhance an effect of noise reduction processing. 
     &lt;3.7. Switching of Arrangement&gt; 
     Each of the arrangements listed in the table described above has been described above in detail. The noise cancelling system  100  may switch these arrangements in accordance with a situation. 
     More specifically, the setting section  160  sets an operation mode of the signal processing section  150 . Then, the signal processing section  150  switches at least any of a microphone used as the AFF microphone  110 A, a microphone used as the FB microphone  110 B, a speaker used as the AFF speaker  120 A, or a speaker used as the FB speaker  120 B, in accordance with the operation mode set by the setting section  160 . In this manner, the noise cancelling system  100  can adopt an appropriate arrangement in accordance with a situation. 
     For example, the setting section  160  may set an operation mode in accordance with the presence or absence of an output of a signal other than a noise reduction signal (i.e. first noise reduction signal or second noise reduction signal) from a speaker that can be used as the AFF speaker  120 A. Specifically, the setting section  160  sets an operation mode in accordance with the presence or absence of an output of a signal for music reproduction from a speaker for music reproduction that is installed on a door inner side. For example, in a case where a signal for music reproduction is output from a speaker for music reproduction that is installed on a door inner side, the setting section  160  sets an operation mode of adopting the above-described third arrangement. The noise cancelling system  100  can thereby separate the speaker for music reproduction from noise reduction processing, prevent load on music reproduction processing, and maintain sound quality of music, in a case where music is reproduced. On the other hand, in a case where a signal for music reproduction is not output from a speaker for music reproduction that is installed on a door inner side, the setting section  160  sets an operation mode of adopting the above-described first arrangement. The noise cancelling system  100  can thereby reduce a noise more effectively especially in a low bandwidth, by using a speaker for music reproduction that has a large caliber, for noise reduction processing, in a case where music is not reproduced. 
     Additionally, the setting section  160  may automatically set an operation mode on the basis of any trigger. In this case, the noise cancelling system  100  may notify the user of the set operation mode using any output apparatus such as a navigation apparatus. In addition, the setting section  160  may set an operation mode in accordance with a manual input from the user. 
     4. HARDWARE CONFIGURATION EXAMPLE 
     The technology according to the present disclosure can be applied to various products. For example, the noise cancelling system  100  may be implemented as an apparatus to be mounted on a vehicle of a certain type such as an automobile, an electric bicycle, a hybrid electric vehicle, and a motorcycle. In addition, at least part of the structural elements of the noise cancelling system  100  may be implemented in a module for an apparatus to be mounted on a vehicle (e.g. integrated circuit module formed by one die). 
       FIG. 14  is a block diagram illustrating an example of the schematic configuration of the vehicle control system  900  to which the technology according to the present disclosure can be applied. The vehicle control system  900  includes an electronic control unit  902 , a storage apparatus  904 , an input apparatus  906 , a vehicle-outside sensor  908 , a vehicle condition sensor  910 , an occupant sensor  912 , a communication IF  914 , an output apparatus  916 , a power generation apparatus  918 , a braking apparatus  920 , a steering  922 , and a lamp activation apparatus  924 . 
     The electronic control unit  902  functions as an operation processing apparatus and a control apparatus, and controls the overall operation of the vehicle control system  900  in accordance with a variety of programs. The electronic control unit  902  can be configured as an electronic control unit (ECU) along with the storage apparatus  904  described below. A plurality of ECUs (i.e. electronic control unit  902  and storage apparatus  904 ) may be included in the vehicle control system  900 . For example, ECUs for controlling each of various sensors or various drive systems may be provided thereto, and an ECU for controlling the plurality of those ECUs in cooperative manner may be further provided. The plurality of these ECUs are connected via an in-vehicle communication network such as a controller area network (CAN), a local interconnect network (LIN), a local area network (LAN), or Flexray that is compliant with any standard. The electronic control unit  902  forms one or more structural elements (i.e. the signal processing section  150  and/or the setting section  160 ) included in the control section  170  illustrated in  FIG. 3 , for example. In the present embodiment, the electronic control unit  902  generates the first noise reduction signal and the second noise reduction signal. 
     The storage apparatus  904  is an apparatus for data storage which is configured as an example of a storage section of the vehicle control system  900 . The storage apparatus  904  is implemented, for example, by a magnetic storage section device such as a HDD, a semiconductor storage device, an optical storage device, a magneto-optical storage device, or the like. The storage apparatus  904  may include a storage medium, a recording apparatus that records data in the storage medium, a readout apparatus that reads out data from the storage medium, a deletion apparatus that deletes data recoded in the storage medium, and the like. This storage apparatus  904  stores a program to be executed by the electronic control unit  902 , various kinds of data, various kinds of data acquired from the outside, and the like. The storage apparatus  904  can form the storage section  140  illustrated in  FIG. 3 , for example. 
     The input apparatus  906  is implemented, for example, by an apparatus such as a mouse, a keyboard, a touch panel, a button, a microphone, a switch, and a lever into which an occupant (driver or passenger) inputs information. In addition, the input apparatus  906  may be, for example, a remote control apparatus using infrared light or other radio waves, or may be an external connection device such as a mobile phone or a PDA supporting the operation of the vehicle control system  900 . In addition, the input apparatus  906  may be, for example, a camera. In that case, an occupant can input information according to a gesture. Further, the input apparatus  906  may include an input control circuit or the like that generates an input signal, for example, on the basis of information input by a user using the above-described input means, and outputs the generated input signal to the electronic control unit  902 . The occupant is able to input various kinds of data to the vehicle control system  900  or instruct the vehicle control system  900  about a processing operation by operating this input apparatus  906 . 
     The vehicle-outside sensor  908  is implemented by a sensor that detects information of the outside of the vehicle. For example, the vehicle-outside sensor  908  may include a sonar apparatus, a radar apparatus, a light detection and ranging or laser imaging detection and ranging (LIDAR) apparatus, a camera, a stereo-camera, a time of flight (ToF) camera, an infrared sensor, an environment sensor, a microphone, or the like. 
     The vehicle condition sensor  910  is implemented by a sensor that detects information regarding a vehicle condition. For example, the vehicle condition sensor  910  may include a sensor that detects an operation performed by a driver such as an accelerator opening degree, brake stepping force, or a steering wheel angle. In addition, the vehicle condition sensor  910  may include a sensor that detects a condition of a power source such as the number of revolutions or torque of an internal combustion engine or a motor. In addition, the vehicle condition sensor  910  may include a sensor such as a gyro sensor or an acceleration sensor for detecting information regarding the movement of the vehicle. In addition, the vehicle condition sensor  910  may include a global navigation satellite system (GNSS) module that receives GNSS signals (e.g. global positioning system (GPS) signals from a GPS satellite) from a GNSS satellite, and measures position information including the latitude, longitude, and altitude of the apparatus. Note that, with respect to the position information, the vehicle condition sensor  910  may sense the position through Wi-Fi (registered trademark), transmission and reception to and from a mobile phone/PHS/smartphone or the like, near field communication, or the like. The vehicle condition sensor  910  can form the sensor section  130  illustrated in  FIG. 3 , for example. 
     The occupant sensor  912  is implemented by a sensor that detects information regarding an occupant. For example, the occupant sensor  912  may include a camera, a microphone, and an environment sensor provided to a vehicle compartment. In addition, the occupant sensor  912  may include a biometric sensor that detects biometric information of an occupant. The biometric sensor is attached, for example, to a seating face, the steering wheel, or the like, and is capable of detecting biometric information of the occupant sitting on the seat or the driver gripping the steering. The occupant sensor  912  can form the microphone  110  illustrated in  FIG. 3 , for example. In the present embodiment, the microphone  110  is formed on a door inner side, a ceiling inner side, or a headrest, and detects a sound in the vehicle. 
     Note that various sensors such as the vehicle-outside sensor  908 , the vehicle condition sensor  910 , and the occupant sensor  912  each output information showing a detection result to the electronic control unit  902 . These various sensors may set the sensing area, accuracy, or the like on the basis of the control of the electronic control unit  902 . In addition, these various sensors may include a recognition module that performs recognition processing based on raw data such as processing of recognizing the driving position of an own vehicle on a road, for example, on the basis of the position of a lane line included in a taken captured image. 
     The communication IF  914  is a communication interface that mediates communication performed by the vehicle control system  900  with another apparatus. The communication IF  914  can include, for example, a V2X communication module. Note that V2X communication is a concept including vehicle-to-vehicle communication and vehicle-to-infrastructure communication. Additionally, the communication IF  914  may also include a communication module for a wireless local area network (LAN), Wi-Fi (registered trademark), 3G, long term evolution (LTE), Bluetooth (registered trademark), near field communication (NFC) or wireless USB (WUSB). This communication IF  914  is capable of transmitting and receiving signals or the like, for example, to and from the Internet or other communication devices outside the vehicle in compliance with a predetermined protocol, for example, TCP/IP or the like. 
     The output apparatus  916  is implemented by an apparatus capable of visually or aurally notifying an occupant of acquired information. Such an apparatus includes a display apparatus such as an instrument panel, a head-up display, a projector or a lamp, and a sound output apparatus such as a speaker or headphones. Specifically, the display apparatus visually displays results obtained from various kinds of processing performed by the vehicle control system  900  in a variety of forms such as text, an image, a table, and a graph. At that time, a virtual object such as an augmented reality (AR) object may be displayed. Meanwhile, the sound output apparatus converts audio signals including reproduced sound data, acoustic data, or the like into analog signals, and aurally outputs the analog signals. The above-described sound output apparatus can form the speaker  120  illustrated in  FIG. 3 , for example. In the present embodiment, the speaker  120  is formed on a door inner side, a ceiling inner side, or a headrest, and the first noise reduction signal and the second noise reduction signal are output. 
     The power generation apparatus  918  is an apparatus for generating driving force for the vehicle. The power generation apparatus  918  may be implemented, for example, by an internal combustion engine. In that case, the power generation apparatus  918  performs start control, stop control, throttle valve opening degree control, fuel injection control, exhaust gas recirculation (EGR) control, or the like on the basis of a control command from an electronic control unit  902 . In addition, the power generation apparatus  918  may be implemented, for example, by a motor, an inverter, and a battery. In that case, the power generation apparatus  918  can supply electric power from the battery to the motor via the inverter on the basis of a control command from the electronic control unit  902 , and perform a motor operation (so-called powering) to output positive torque and a regenerative operation to cause the motor to absorb torque to generate electric power, and charge the battery via the inverter. 
     The braking apparatus  920  is an apparatus for providing braking force to the vehicle, or causing the vehicle to decelerate or stop. The braking apparatus  920  can include, for example, a brake installed at each wheel, and a brake pipe for transmitting the force of stepping on the brake pedal to the brake, an electronic circuit or the like. In addition, the braking apparatus  920  may include a control apparatus such as an antilock brake system (ABS) or an electronic stability control (ESC) for activating a mechanism of preventing a slide or a skid caused by brake control. 
     The steering  922  is an apparatus for controlling the advancing direction (steering angle) of the vehicle. The steering  922  can include, for example, a steering wheel, a steering shaft, a steering gear, a tie rod, and the like. In addition, the steering  922  can include a power steering for assisting a driver in steering. Further, the steering  922  can include a power source such as a motor for allowing for automatic steering. 
     The lamp activation apparatus  924  is an apparatus that activates various lamps such as a head light, a turn signal, a position lamp, a fog light, or a stop lamp. The lamp activation apparatus  924  controls, for example, the blinking of the lamps, the amount of light, the light-emitting direction, or the like. 
     Note that the power generation apparatus  918 , the braking apparatus  920 , the steering  922 , and the lamp activation apparatus  924  may come into operation on the basis of a manual operation performed by a driver or on the basis of an automatic operation performed by the electronic control unit  902 . 
     An example of a hardware configuration that can implement the functions of the noise cancelling system  100  according to the present embodiment has been described above. Each of the above-described structural elements may be implemented using a general-purpose member, or may be implemented by hardware specialized in the function of each structural element. Accordingly, it is possible to appropriately change a hardware configuration to be used, in accordance with a technical level at the time when the present embodiment is implemented. 
     Note that a computer program for implementing each of the functions of the noise cancelling system  100  according to the present embodiment as described above can be created, and may be mounted on an ECU or the like. In addition, a computer-readable recording medium storing such a computer program can be provided. The recording medium is a magnetic disc, an optical disc, a magneto-optical disc, a flash memory, or the like, for example. In addition, the above-described computer program may be delivered via a network, for example, without using the recording medium. 
     5. CONCLUSION 
     An embodiment of the present disclosure has been described above in detail with reference to  FIGS. 1 to 14 . As described above, the noise cancelling system  100  according to the present embodiment generates a first noise reduction signal using an adaptive filter on the basis of a signal output from a first input apparatus, causes the generated first noise reduction signal to be output by a first output apparatus, generates a second noise reduction signal using a fixed filter on the basis of a signal output from a second input apparatus, and causes the generated second noise reduction signal to be output by a second output apparatus. In the present embodiment, because two types of noise reduction processing with different methods are used in combination, it becomes possible to reduce both noises targeted by the respective methods. 
     For example, a case where the noise cancelling system  100  is mounted on a vehicle will be described. The noise cancelling system  100  uses, in combination, an adaptive filter excelling at reduction of a variable peaky noise such as an engine noise, and a fixed filter excelling at reduction of a broad noise such as a road noise and a wind noise. Thus, the noise cancelling system  100  according to the present embodiment can reduce more noises in a wide bandwidth at the time of actual running of the vehicle. 
     The preferred embodiment(s) of the present disclosure has/have been described above with reference to the accompanying drawings, whilst the present disclosure is not limited to the above examples. A person skilled in the art may find various alterations and modifications within the scope of the appended claims, and it should be understood that they will naturally come under the technical scope of the present disclosure. 
     For example, the above-described arrangements may be appropriately used in combination. 
     In addition, the noise cancelling system  100  described in this specification may be implemented as an independent apparatus, or a part or all thereof may be implemented as a separate apparatus. For example, among the functional configuration examples of the noise cancelling system  100  that are illustrated in  FIG. 3 , the storage section  140 , the signal processing section  150 , and the setting section  160  may be included in an apparatus such as a server that is connected via a network or the like with the microphone  110 , the speaker  120 , and the sensor section  130 . 
     In addition, the signal processing described in this specification with reference to  FIG. 4  or the like is executed in an order following a flow of data that is indicated by an arrow. Nevertheless, an additional processing step may be added. In addition, the noise reduction processing of the AFF method and the noise reduction processing of the FB method can be concurrently executed. 
     Further, the effects described in this specification are merely illustrative or exemplified effects, and are not limitative. That is, with or in the place of the above effects, the technology according to the present disclosure may achieve other effects that are clear to those skilled in the art from the description of this specification. 
     Additionally, the present technology may also be configured as below. 
     (1) 
     A signal processing apparatus including: 
     a signal processing section configured to 
     generate a first noise reduction signal using an adaptive filter on the basis of a signal output from a first input apparatus, and cause the generated first noise reduction signal to be output by a first output apparatus, and 
     generate a second noise reduction signal using a fixed filter on the basis of a signal output from a second input apparatus, and cause the generated second noise reduction signal to be output by a second output apparatus. 
     (2) 
     The signal processing apparatus according to (1), 
     in which the first noise reduction signal targets a noise with a narrow frequency bandwidth, and 
     the second noise reduction signal targets a noise with a wide frequency bandwidth. 
     (3) 
     The signal processing apparatus according to (2), 
     in which the first noise reduction signal targets a noise corresponding to the number of revolutions of a power source that performs a revolving motion, and 
     the second noise reduction signal targets a noise corresponding to a movement performed by a movable object using the power source. 
     (4) 
     The signal processing apparatus according to (3), in which the power source is an internal combustion engine or a motor, and the movable object is a vehicle. 
     (5) 
     The signal processing apparatus according to any one of (1) to (4), in which the signal processing section uses, as the first input apparatus and the first output apparatus, an input apparatus and an output apparatus that have a fixed relative positional relationship, respectively, and uses, as the second input apparatus and the second output apparatus, an input apparatus and an output apparatus that have a fixed relative positional relationship, respectively. 
     (6) 
     The signal processing apparatus according to (5), in which the signal processing section 
     uses, as the first input apparatus and the first output apparatus, an input apparatus and an output apparatus that have fixed absolute positions, respectively, and 
     uses, as the second input apparatus and the second output apparatus, an input apparatus and an output apparatus that have variable absolute positions, respectively. 
     (7) 
     The signal processing apparatus according to (5), in which the signal processing section uses, as the first input apparatus and the second input apparatus, a same input apparatus having a variable absolute position, and uses, as the first output apparatus and the second output apparatus, a same output apparatus having a variable absolute position. 
     (8) 
     The signal processing apparatus according to (5), in which the signal processing section uses, as the first input apparatus and the second input apparatus, a same input apparatus having a fixed absolute position, and uses, as the first output apparatus and the second output apparatus, a same output apparatus having a fixed absolute position. 
     (9) 
     The signal processing apparatus according to any one of (1) to (4), in which the signal processing section uses, as the first input apparatus and the first output apparatus, an input apparatus and an output apparatus that have a variable relative positional relationship, respectively, and uses, as the second input apparatus and the second output apparatus, an input apparatus and an output apparatus that have a fixed relative positional relationship, respectively. 
     (10) 
     The signal processing apparatus according to (9), in which the signal processing section uses, as the first input apparatus and the second input apparatus, a same input apparatus having a variable absolute position. 
     (11) 
     The signal processing apparatus according to (9), in which the signal processing section  150  uses, as the first output apparatus and the second output apparatus, a same output apparatus having a variable absolute position. 
     (12) 
     The signal processing apparatus according to any one of (1) to (11), in which the signal processing section uses, as the first input apparatus and the first output apparatus, a plurality of input apparatuses and a plurality of output apparatuses, respectively. 
     (13) 
     The signal processing apparatus according to any one of (1) to (12), in which the signal processing section uses an output apparatus for music reproduction, as the first output apparatus. 
     (14) 
     The signal processing apparatus according to any one of (1) to (13), in which the signal processing section uses, as the second input apparatus and the second output apparatus, an input apparatus and an output apparatus, respectively, that are provided on one component among one or more components forming a seat. 
     (15) 
     The signal processing apparatus according to any one of (1) to (14), further including 
     a setting section configured to set an operation mode of the signal processing section, 
     in which the signal processing section switches at least any of an input apparatus used as the first input apparatus, an input apparatus used as the second input apparatus, an output apparatus used as the first output apparatus, or an output apparatus used as the second output apparatus, in accordance with the operation mode set by the setting section. 
     (16) 
     The signal processing apparatus according to (15), in which the setting section sets the operation mode in accordance with presence or absence of an output of a signal other than the first noise reduction signal or the second noise reduction signal from an output apparatus that can be used as the first output apparatus. 
     (17) 
     A method executed by a processor, the method including: 
     generating a first noise reduction signal using an adaptive filter on the basis of a signal output from a first input apparatus, and causing the generated first noise reduction signal to be output by a first output apparatus; and 
     generating a second noise reduction signal using a fixed filter on the basis of a signal output from a second input apparatus, and causing the generated second noise reduction signal to be output by a second output apparatus. 
     (18) 
     A recording medium having a program recorded thereon, the program causing a computer to function as: 
     a signal processing section configured to 
     generate a first noise reduction signal using an adaptive filter on the basis of a signal output from a first input apparatus, and cause the generated first noise reduction signal to be output by a first output apparatus, and 
     generate a second noise reduction signal using a fixed filter on the basis of a signal output from a second input apparatus, and cause the generated second noise reduction signal to be output by a second output apparatus. 
     REFERENCE SIGNS LIST 
     
         
           100  noise cancelling system 
           110  microphone 
           110 A AFF microphone 
           110 B FB microphone 
           120  speaker 
           120 A AFF speaker 
           120 B FB speaker 
           130  sensor section 
           140  storage section 
           150  signal processing section 
           151  adaptive algorithm 
           152  FF filter 
           153  FB filter 
           154  adder 
           160  setting section 
           170  control section