Patent Publication Number: US-2022217467-A1

Title: Control method, control system, and storage medium

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of International Application PCT/JP2020/013576 filed on Mar. 26, 2020, the content of this application is incorporated herein by reference in their entirety. 
    
    
     1. FIELD OF THE INVENTION 
     The present invention relates to a control method, a control system, and a storage medium. 
     2. DESCRIPTION OF THE RELATED ART 
     There is known a technology for controlling a frequency response of a filter to which an audio signal is input. JP 2681349 B2 includes description of a filter in which a target amplitude characteristic and a target phase characteristic are set independently of each other. This filter has a latency changed in accordance with the set target amplitude characteristic and the set target phase characteristic. 
     SUMMARY OF THE INVENTION 
     However, in the technology of JP 2681349 B2, no consideration is given to a case in which a latency has a predetermined value. The filter of Patent Literature 1 has a finite length, and hence a frequency response defined by a target amplitude characteristic and a target phase characteristic cannot be accommodated in the filter unless some compromise is made between the target amplitude characteristic and the target phase characteristic. When an attempt is made to accommodate the frequency response in the filter, there occurs a large difference between an amplitude characteristic of the frequency response and the target amplitude characteristic. 
     The present invention has been made in view of the above-mentioned problems, and an object thereof is to reduce a difference between an amplitude characteristic of a frequency response and a target amplitude characteristic. 
     Solution to Problem 
     In order to solve the above-mentioned problems, according to one embodiment of the present invention, there is provided a control method of controlling a frequency response of a filter for processing an audio signal, the frequency response being defined by an amplitude characteristic and a phase characteristic, the control method including: acquiring a target amplitude characteristic, a target phase characteristic, and a latency value; obtaining a first frequency response in accordance with the target amplitude characteristic and the target phase characteristic; obtaining a second frequency response by modifying the first frequency response so that a latency of the second frequency response satisfies the latency value; and obtaining a third frequency response to be set for the filter by correcting the second frequency response, the second frequency response being corrected so as to reduce a difference between (i) an amplitude characteristic of the second frequency response and (ii) the target amplitude characteristic. 
     According to one embodiment of the present invention, there is provided a control system for controlling a frequency response of a filter for processing an audio signal, the control system including: one or more processors; and one or more memories, wherein the frequency response is defined by an amplitude characteristic and a phase characteristic, and wherein the one or more processors are configured to execute a program stored in the one or more memories to cause the control system to: acquire a target amplitude characteristic, a target phase characteristic, and a latency value; obtain a first frequency response in accordance with the target amplitude characteristic and the target phase characteristic; obtain a second frequency response by modifying the first frequency response so that a latency of the second frequency response satisfies the latency value; and obtain a third frequency response to be set for the filter by correcting the second frequency response, the second frequency response being corrected by reducing a difference between (i) an amplitude characteristic of the second frequency response and (ii) the target amplitude characteristic. 
     According to one embodiment of the present invention, there is provided a storage medium including one or more storage media having stored thereon a computer-readable program for controlling a frequency response of a filter for processing an audio signal, wherein the frequency response is defined by an amplitude characteristic and a phase characteristic, and wherein the computer-readable program causes one or more processors to perform operations of: acquiring a target amplitude characteristic, a target phase characteristic, and a latency value; obtaining a first frequency response in accordance with the target amplitude characteristic and the target phase characteristic; obtaining a second frequency response by modifying the first frequency response so that a latency of the second frequency response satisfies the latency value; and obtaining a third frequency response to be set for the filter by correcting the second frequency response, the second frequency response being corrected by reducing a difference between (i) an amplitude characteristic of the second frequency response and (ii) the target amplitude characteristic. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram for illustrating an example of a control device according to an embodiment of the present invention. 
         FIG. 2  is a diagram for illustrating an example of a filter of an SPU. 
         FIG. 3  is a block diagram for illustrating an example of functions implemented by the control device. 
         FIG. 4  is a diagram for illustrating an example of frequency responses controlled by the control device. 
         FIG. 5  is a graph for showing how an amplitude characteristic of a third frequency response is changed in accordance with a priority. 
         FIG. 6  is a graph for showing how a phase characteristic of the third frequency response is changed in accordance with the priority. 
         FIG. 7  is a diagram for illustrating an example of a filter control screen. 
         FIG. 8  is a flow chart for illustrating an example of a process to be executed by the control device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     1. Hardware Configuration of Control Device 
     Now, an example of an embodiment according to the present invention is described with reference to the accompanying drawings.  FIG. 1  is a diagram for illustrating an example of a control device according to the embodiment. A control device  10  controls a frequency response of a filter for processing an audio signal. For example, the control device  10  is a digital mixer, a signal processor, an audio amplifier, an electronic musical instrument, a personal computer, a tablet terminal, a smartphone, or a digital assistant. 
     The audio signal is a digital or analog signal representing a sound. The filter is a circuit for processing an input audio signal and outputs the processed audio signal. The filter in this embodiment is a finite impulse response (FIR) filter having a finite length. The frequency response is a filter characteristic on a time axis. The frequency response is defined by an amplitude characteristic and a phase characteristic. The frequency response is used for setting a coefficient for the filter. The controlling of the frequency response refers to obtaining of the frequency response. 
     In this embodiment, “obtaining” means obtaining as a result of processing. For example, the frequency response is obtained as a result of processing, for example, inverse Fourier transform, and hence the control device  10  “obtains” the frequency response. The “obtaining” can also be rephrased as creating, defining, or generating. Meanwhile, “acquiring” means receiving. For example, information set by a user operation is received from the outside, and hence the control device  10  acquires the set information. The “acquiring” can also be rephrased as receiving. In this embodiment, the “obtaining” and the “acquiring” are thus used properly. 
     As illustrated in  FIG. 1 , the control device  10  includes a CPU  11 , a nonvolatile memory  12 , a RAM  13 , an operating unit  14 , a display unit  15 , an input unit  16 , a signal processing unit (SPU)  17 , and a digital analog converter (DAC)  18 . The control device  10  is connected to a powered speaker  20 . The speaker  20  may be integrally provided to the control device  10 . 
     The CPU  11  includes at least one processor. The CPU  11  executes predetermined process based on a program and data that are stored in the nonvolatile memory  12 . The nonvolatile memory  12  is a memory, for example, a ROM, an EEPROM, a flash memory, or a hard disk drive. The RAM  13  is an example of a volatile memory. The operating unit  14  is an input device, for example, a touch panel, a keyboard, a mouse, a button, or a lever. The display unit  15  is a display, for example, a liquid crystal display or an organic EL display. 
     The input unit  16  acquires an audio signal. In this embodiment, the input unit  16  acquires a digital audio signal. The input unit  16  may acquire an analog audio signal. In this case, the input unit  16  converts an analog audio signal into a digital audio signal through use of an A/D converter. For example, the input unit  16  includes an interface for receiving input of an audio signal from the outside. The input unit  16  may acquire audio data stored in the nonvolatile memory  12 . The input unit  16  inputs the acquired audio signal to the SPU  17 . The SPU  17  includes a filter for processing the input audio signal. 
       FIG. 2  is a diagram for illustrating an example of the filter of the SPU  17 . As illustrated in  FIG. 2 , a filter F includes delay circuits Z 1  to Zn- 1  and multipliers M 1  to Mn. The variable “n” represents the number of taps of the filter F. The variable “n” may be any natural number. Coefficients α 1  to an are set for the multipliers M 1  to Mn, respectively. The coefficients α 1  to αn are values corresponding to a third frequency response described later. The filter F is an FIR filter for convolving the coefficients α 1  to αn into an audio signal. 
     The audio signal input from the input unit  16  is input to the multiplier M 1  and the delay circuit Z 1 . The audio signal input to the delay circuit Z 1  is delayed by a predetermined time period, and input to the multiplier M 2  and the delay circuit Z 2 . After that, in the same manner, the audio signal is delayed by each of the delay circuits Z 3  to Zn- 1 . The delayed audio signal is input to each of the multipliers M 3  to Mn. 
     The multipliers M 1  to Mn each multiply the audio signal input to itself by each of the coefficients α 1  to αn, respectively. Each of the multipliers M 1  to Mn inputs the audio signal multiplied by its own one of the coefficients α 1  to αn to an adder A. The adder A adds up the audio signals output from the multipliers M 1  to Mn. The adder A inputs the added audio signals to the DAC  18 . 
     The DAC  18  is a circuit for converting a digital audio signal into an analog audio signal. The DAC  18  outputs the converted analog audio signal to the powered speaker  20 . The speaker  20  outputs a sound corresponding to the input analog audio signal. 
     The hardware configuration of the control device  10  is not limited to the above-mentioned example. For example, the control device  10  may include a communication interface for wired communication or wireless communication. In addition, for example, the control device  10  may include a reading device (for example, an optical disc drive or a memory card slot) for reading a computer-readable information storage medium. In addition, for example, the control device  10  may include an input/output terminal (for example, a USB port) for inputting/outputting data. The program and data described as being stored in the nonvolatile memory  12  in this embodiment may be supplied to the control device  10  through the communication interface, the reading device, or the input/output terminal. 
     2. Functions to be Implemented by Control Device 
       FIG. 3  is a block diagram for illustrating an example of functional units to be implemented by the control device  10 .  FIG. 4  is a diagram for illustrating an example of the frequency responses controlled by the control device  10 . As illustrated in  FIG. 3 , in the control device  10 , a first acquisition unit  100  (target amplitude characteristic acquisition unit), a second acquisition unit  101  (target phase characteristic acquisition unit), a third acquisition unit  102  (latency value acquisition unit), a fourth acquisition unit  103  (priority acquisition unit), a first processing unit  104  (first frequency response processing unit), a second processing unit  105  (second frequency response processing unit), a third processing unit  106  (third frequency response processing unit), and a display control unit  107  are implemented. Those functions are implemented mainly by the CPU  11 . 
     2-1. First Acquisition Unit (Target Amplitude Characteristic Acquisition Unit) 
     The first acquisition unit  100  acquires a target amplitude characteristic r 1 . The target amplitude characteristic r 1  is a target amplitude characteristic (ideal amplitude characteristic). The amplitude characteristic is a characteristic of an amplitude on a frequency axis. As illustrated in  FIG. 4 , the horizontal axis of the target amplitude characteristic r 1  indicates a frequency. The vertical axis of the target amplitude characteristic r 1  indicates an amplitude change (gain). A curve of the target amplitude characteristic r 1  indicates a magnitude of the amplitude for each frequency. 
     In this embodiment, the target amplitude characteristic r 1  is set in accordance with a user operation. For example, a user sets the target amplitude characteristic r 1  on a filter control screen described later. The first acquisition unit  100  acquires the target amplitude characteristic r 1  set by the user operation, and records the target amplitude characteristic r 1  in the RAM  13 . The user operation is not limited to the operation on the filter control screen, and may be another operation using the operating unit  14 . For example, the target amplitude characteristic r 1  may be set directly from the button, lever, or the like of the operating unit  14  instead of the user operation using a screen. 
     The target amplitude characteristic r 1  is not required to be set by the user. In this case, the target amplitude characteristic r 1  indicates a predetermined curve. For example, this target amplitude characteristic r 1  is stored in advance in the nonvolatile memory  12 . The first acquisition unit  100  acquires the target amplitude characteristic r 1  stored in advance in the nonvolatile memory  12 . A plurality of target amplitude characteristics r 1  may be stored in advance in the nonvolatile memory  12 . In this case, the first acquisition unit  100  acquires any one of the plurality of target amplitude characteristics r 1 . For example, the first acquisition unit  100  acquires the target amplitude characteristic r 1  selected by the user. The target amplitude characteristic r 1  may be automatically set based on a frequency amplitude characteristic of a room, which has been measured by emitting a test sound from the speaker  20 . For example, such a target amplitude characteristic r 1  as to suppress a peak of the frequency amplitude characteristic may be automatically set. 
     2-2. Second Acquisition Unit (Target Phase Characteristic Acquisition Unit) 
     The second acquisition unit  101  acquires a target phase characteristic r 2 . The target phase characteristic r 2  is a target phase characteristic (ideal phase characteristic). The phase characteristic is a characteristic of the phase on the frequency axis. As illustrated in  FIG. 4 , the horizontal axis of the target phase characteristic r 2  indicates a frequency. The vertical axis of the target phase characteristic r 2  indicates a phase change (degree of lead or lag). A curve of the target phase characteristic r 2  indicates the phase change for each frequency. 
     In this embodiment, the target phase characteristic r 2  is set in accordance with a user operation. For example, a user sets the target phase characteristic r 2  on the filter control screen described later. The second acquisition unit  101  acquires the target phase characteristic r 2  set by the user operation, and records the target phase characteristic r 2  in the RAM  13 . The user operation is not limited to the operation on the filter control screen, and may be another operation using the operating unit  14 . For example, the target phase characteristic r 2  may be set directly from the button, lever, or the like of the operating unit  14  instead of the user operation using a screen. 
     The target phase characteristic r 2  is not required to be set by the user. In this case, the target phase characteristic r 2  indicates a predetermined curve. For example, this target phase characteristic is stored in advance in the nonvolatile memory  12 . The second acquisition unit  101  acquires the target phase characteristic r 2  stored in advance in the nonvolatile memory  12 . A plurality of target phase characteristics r 2  may be stored in advance in the nonvolatile memory  12 . In this case, the second acquisition unit  101  acquires any one of the plurality of target phase characteristics r 2 . For example, the second acquisition unit  101  acquires the target phase characteristic r 2  selected by the user. The target phase characteristic r 2  may be automatically set based on a frequency phase characteristic of a room, which has been measured by emitting a test sound from the speaker  20 . For example, such a target phase characteristic r 2  as to smooth the frequency phase characteristic may be automatically set. 
     2-3. Third Acquisition Unit (Latency Value Acquisition Unit) 
     The third acquisition unit  102  acquires a latency value L. The latency value L is a numerical value indicating a latency (time length) of the filter. The latency is a delay of the audio signal that has passed through the filter from the audio signal input to the filter. As the latency value L becomes larger, the sound is further delayed by the filter. The latency value L is represented by a numerical value within a numerical range defined in advance. For example, the latency value L is represented by a numerical value of 0 milliseconds or more and T milliseconds or less. The variable T represents a positive number indicating an upper limit value of the latency value L. 
     In this embodiment, the latency value L is set in accordance with a user operation. For example, the user sets the latency value L on the filter screen described later. The third acquisition unit  102  acquires the latency value L set by the user operation, and records the latency value L in the RAM  13 . The user operation is not limited to the operation on the filter control screen, and may be another operation using the operating unit  14 . For example, the latency value L may be set directly from the button, lever, or the like of the operating unit  14  instead of the user operation using the screen. 
     The latency value L is not required to be set by the user. In this case, the latency value L is a value defined in advance. For example, this latency value L is stored in advance in the nonvolatile memory  12 . The third acquisition unit  102  acquires the latency value L stored in advance in the nonvolatile memory  12 . A plurality of latency values L may be stored in advance in the nonvolatile memory  12 . In this case, the third acquisition unit  102  acquires any one of the plurality of latency values L. For example, the third acquisition unit  102  acquires the latency value L selected by the user. 
     2-4. Fourth Acquisition Unit (Priority Acquisition Unit) 
     The fourth acquisition unit  103  acquires a priority P. The priority P is a numerical value representing a degree of correction of the frequency response. For example, the priority P represents a ratio of correction of the amplitude characteristic. In this case, the frequency response is corrected so that the amplitude characteristic is further prioritized as the priority P becomes higher. The priority P is represented by a numerical value within a numerical range defined in advance. For example, the priority P is represented by a numerical value of 0 or more and 1 or less. 
     In this embodiment, the priority P is set in accordance with a user operation. For example, the user sets the priority P on the filter screen described later. The fourth acquisition unit  103  acquires the priority P set by the user operation, and records the priority P in the RAM  13 . The user operation is not limited to the operation on the filter control screen, and may be another operation using the operating unit  14 . For example, the priority P may be set directly from the button, lever, or the like of the operating unit  14  instead of the user operation using the screen. 
     The priority P is not required to be set by the user. In this case, the priority P is a value defined in advance. For example, this priority P is stored in advance in the nonvolatile memory  12 . The fourth acquisition unit  103  acquires the priority P stored in advance in the nonvolatile memory  12 . A plurality of priorities P may be stored in advance in the nonvolatile memory  12 . In this case, the fourth acquisition unit  103  acquires any one of the plurality of priorities P. For example, the fourth acquisition unit  103  acquires the priority P selected by the user. In another case, the priority P may be automatically determined based on a predetermined rule. For example, the value of the priority P may be automatically determined based on a Euclidean distance between the amplitude characteristic of the filter and the target amplitude characteristic. 
     2-5. First Processing Unit (First Frequency Response Processing Unit) 
     The first processing unit  104  obtains a first impulse response IR 1  in accordance with a first frequency response FR 1  including the target amplitude characteristic r 1  and the target phase characteristic r 2 . The response FR 1  in a frequency domain is equivalent to the response IR in a time domain. 
     As illustrated in  FIG. 4 , the first processing unit  104  performs inverse Fourier transform (iFFT) on the response FR 1  to obtain the response IR 1 . The response FR 1  is transformed from the frequency domain to the time domain by the inverse Fourier transform. Consequently, the horizontal axis of the response IR 1  indicates time. The vertical axis of the response IR 1  indicates a magnitude (power in  FIG. 4 ) of the coefficient of the filter F. The curve of response IR 1  indicates the magnitude of the coefficient for each time (for each tap). In addition, the response IR 1  has a peak near the center (time zero). When a frequency resolution of the response FR 1  is equal to or less than a resolution corresponding to the number of taps of the filter F, the obtained response IR 1  accurately corresponds to the response FR 1 . When the frequency resolution of the response FR 1  is higher than the resolution corresponding to the number of taps of the filter F, the obtained response IR 1  includes an error. 
     2-6. Second Processing Unit (Second Frequency Response Processing Unit) 
     The second processing unit  105  modifies the response IR 1  so that a latency according to the response IR 1  satisfies the latency value L, to thereby obtain a second impulse response IR 2 . The response IR 2  corresponds to a second frequency response FR 2 , which has a predetermined latency, and is close to the response FR 1 . 
     For example, the second processing unit  105  trims (deletes) a portion of the response IR 1  before a negative time, which corresponds to the latency, so as to satisfy the latency value L set by the user, to thereby obtain the second impulse response IR 2 . The trimming is an example of modifying for eliminating a leading part of an impulse response. Through the trimming, the corresponding response FR 1  is substantially modified. Deforming, replacing, or amending the curve of the response FR 1  or response IR 1  corresponds to the modifying. The modifying may be performed on the response FR 1  in the frequency domain. 
     The second processing unit  105  trims the response IR 1  so that a latency according to a curve after the trimming satisfies the latency value L. For example, the second processing unit  105  determines an amount (length) of trimming a head of the curve based on the latency value L. For example, the second processing unit  105  trims the response IR 1  so that a period from the left end of the curve to the peak has a time length corresponding to the latency value L. As the latency value L becomes larger, the trimming amount becomes smaller. Energy of the audio signal that has passed through the filter is reduced due to this trimming, and hence such modifying as to compensate for the reduction may be performed. 
     2-7. Third Processing Unit (Third Frequency Response Processing Unit) 
     The third processing unit  106  corrects the response IR 2  so that a difference between an amplitude characteristic r 3  of the second frequency response FR 2  corresponding to the response IR 2  and the target amplitude characteristic r 1  is reduced, to thereby obtain a third impulse response IR 3  to be set for the filter F. The response IR 3  corresponds to a third frequency response FR 3 , which has a predetermined latency, and has been corrected to become closer to the target amplitude characteristic r 1 . The response FR 2  (amplitude characteristic r 3  and phase characteristic r 4 ) is obtained by Fourier-transforming the response IR 2 , and is equivalent to the response IR 2 . 
     For example, the third processing unit  106  includes an FFT unit  106 A, a division unit  106 B, a multiplication unit  106 C, a correction control unit  106 D, a correction unit  106 E, a time-shift unit  106 F, and a coefficient setting unit  106 G. The FFT unit  106 A performs Fourier transform (FFT) on the response IR 2  to obtain the response FR 2  including the amplitude characteristic r 3  and the phase characteristic r 4 . As illustrated in  FIG. 4 , the trimming performed by the second processing unit  105  causes a difference between the target amplitude characteristic r 1  and the amplitude characteristic r 3 . When this difference is large, the user wishes to bring the amplitude characteristic r 3  closer to the original target amplitude characteristic r 1 . In view of this, in this embodiment, the response FR 2  is corrected so as to reduce this difference. 
     In this respect, as illustrated in  FIG. 4 , the trimming performed by the second processing unit  105  also causes a difference between the target phase characteristic r 2  and the phase characteristic r 4 . In this case, the difference between the target amplitude characteristic r 1  and the amplitude characteristic r 3  and the difference between the target phase characteristic r 2  and the phase characteristic r 4  are in a trade-off relationship. As one of the differences becomes smaller, the other difference becomes larger. The degree of correction of the response FR 2  is determined based on the priority P. In this embodiment, the response FR 2  or the response IR 2  is corrected in accordance with the priority P in the following manner. 
     The division unit  106 B calculates an amplitude characteristic Δr of the difference between the target amplitude characteristic r 1  and the amplitude characteristic r 3 . This difference is calculated by subtracting a decibel value in the frequency domain. The subtraction of the decibel value corresponds to division of a linear value, and is therefore indicated by a division sign in  FIG. 3 . The multiplication unit  106 C multiplies the amplitude characteristic Δr of the difference, which is a calculation result of the division unit  106 B, by the priority P. The correction control unit  106 D obtains a frequency response ΔFR for correction corresponding to the amplitude characteristic being a calculation result of the multiplication unit  106 C. The amplitude characteristic ΔFR for correction is obtained by a minimum phase. For the minimum phase, a known technology can be used. 
     The correction unit  106 E corrects the response IR 2  in accordance with the frequency response ΔFR for correction to obtain the third impulse response IR 3 . Transforming, restoring, or compensating the curve of the response FR 2  or response IR 2  itself so that the amplitude characteristic r 3  of the third frequency response FR 3  corresponding to the response IR 3  becomes closer to the target amplitude characteristic r 1  corresponds to the correcting. This correction is, for example, a process for obtaining the response IR 3  by convolving an impulse response ΔIR corresponding to the response ΔFR into the response IR 2 . As illustrated in  FIG. 4 , a difference between the target amplitude characteristic r 1  and an amplitude characteristic r 5  of the response FR 3  is smaller than the difference between the target amplitude characteristic r 1  and the amplitude characteristic r 3  of the response FR 2 . The response FR 3  is obtained by Fourier-transforming the response IR 3 , and is equivalent to the response IR 3 . 
     The time-shift unit  106 F time-shifts the response IR 3 . Due to the time-shifting, the head (leftmost portion) of the curve indicated by the response IR 3  becomes t=0. The coefficient setting unit  106 G sets a value on the curve at t=0 as the coefficient α 1  for the first tap. The coefficient setting unit  106 G sets values on the curve corresponding to a predetermined sampling interval Δt as the coefficients α 2  to αn for the second tap and the subsequent taps. The setting of the coefficients α 1  to αn refers to rewriting of the values of the coefficients α 1  to αn. 
     As described above, the third processing unit  106  changes the degree of correction of the response IR 2  based on the priority P. The degree of correction refers to a degree to which the amplitude characteristic r 3  of the response FR 3  is brought closer to the target amplitude characteristic r 1 . The degree of correction can also be referred to as a correction amount. For example, a relationship between the priority P and the degree of correction is defined in a program code. This relationship may be defined in a mathematical expression or a table in place of the program code. The third processing unit  106  corrects the response IR 2  based on the degree of correction corresponding to the priority P. 
       FIG. 5  is a graph for showing how the amplitude characteristic r 5  of the response FR 3  is changed in accordance with the priority P. In  FIG. 5 , the curve of the target amplitude characteristic r 1  is indicated by the solid line. The curve of the amplitude characteristic r 5  is indicated by the lines (dotted line, broken line, and one-dot chain line) other than the solid line. As shown in  FIG. 5 , when the priority P is changed so as to prioritize the amplitude characteristic r 5 , the difference between the amplitude characteristic r 5  of the response FR 3  and the target amplitude characteristic r 1  becomes smaller. For example, as the priority P becomes higher, the difference between the amplitude characteristic r 5  of the response FR 3  and the target amplitude characteristic r 1  becomes smaller. In the example of  FIG. 5 , when the priority P is 1, the amplitude characteristic r 5  and the target amplitude characteristic r 1  match each other. 
       FIG. 6  is a graph for showing how a phase characteristic r 6  of the response FR 3  is changed in accordance with the priority P. In  FIG. 6 , the curve of the target phase characteristic r 2  is indicated by the solid line. The curve of the phase characteristic r 6  is indicated by the lines (dotted line, broken line, and one-dot chain line) other than the solid line. As shown in  FIG. 6 , when the priority P is changed so as to prioritize the phase characteristic r 6 , the difference between the phase characteristic r 6  of the response FR 3  and the target phase characteristic r 2  becomes smaller. For example, as the priority P becomes lower, the difference between the phase characteristic r 6  of the response FR 3  and the target phase characteristic r 2  becomes smaller. 
     2-8. Display Control Unit 
     The display control unit  107  causes the display unit  15  to display the filter control screen.  FIG. 7  is a diagram for illustrating an example of the filter control screen. The filter control screen is a user interface for performing work of setting the filter F. As illustrated in  FIG. 7 , an amplitude characteristic image I 1 , a phase characteristic image I 2 , a latency value slider SL 1 , and a priority slider SL 2  are displayed on a filter control screen G. 
     The target amplitude characteristic r 1  and the amplitude characteristic r 5  of the response FR 3  are displayed in the amplitude characteristic image I 1 . In the example of  FIG. 7 , the curve of the target amplitude characteristic r 1  is indicated by the solid line. The curve of the amplitude characteristic r 5  is indicated by the broken line. The user sets the target amplitude characteristic r 1  by changing the curve shape of the solid line through, for example, a dragging operation of the mouse. For example, the user changes the curve of the solid line by moving each of icons i 1  to i 4  on the curve. When the curve of the target amplitude characteristic r 1  is changed, the response FR 3  is updated according to the changed target amplitude characteristic, and the curve of the amplitude characteristic r 5  is changed accordingly. The user can add a new icon to the curve at any position, or delete an unnecessary existing icon. 
     The target phase characteristic r 2  and the phase characteristic r 6  of the response FR 3  are displayed in the phase characteristic image I 2 . In the example of  FIG. 7 , the curve of the target phase characteristic r 2  is indicated by the solid line. The curve of the phase characteristic r 6  is indicated by the broken line. The user sets the target phase characteristic r 2  by changing the curve shape of the solid line through, for example, a dragging operation of the mouse. For example, the user changes the curve of the solid line by moving each of icons i 5  to i 8  on the curve. When the curve of the target phase characteristic r 2  is changed, the response FR 3  is updated according to the changed target phase characteristic r 2 , and the curve of the phase characteristic r 6  is changed accordingly. 
     The latency value slider SL 1  is an image for receiving setting of the latency value L. The user sets the latency value L by moving a knob of the slider SL 1  through, for example, a dragging operation of the mouse. For example, the user sets the latency value L by moving the knob of the slider SL 1  horizontally. In the example of  FIG. 7 , the latency value L increases as the knob of the slider SL 1  is moved further to the rightward direction (along the horizontal axis direction). When the latency value L is changed, a degree of trimming is changed, the response FR 3  is updated according to the changed degree of trimming, and the curves of the amplitude characteristic r 5  and the phase characteristic r 6  are changed accordingly. 
     The priority slider SL 2  is an image for setting the priority P. The user sets the priority P by moving a knob of the slider SL 2  through, for example, a dragging operation of the mouse. For example, the user sets the priority P by moving the knob of the slider SL 2  vertically. In the example of  FIG. 7 , the priority P is changed so that the amplitude characteristic is further prioritized as the knob of the slider SL 2  moves further to the upward direction (toward the direction of the amplitude characteristic image I 1 ). The priority P is changed so that the phase characteristic is further prioritized as the knob of the slider SL 2  moves further to the downward direction (toward the direction of the phase characteristic image I 2 ). When the priority P is changed, that means the degree of correction is changed, the response FR 3  is updated according to the changed priority P, and the curves of the amplitude characteristic r 5  and the phase characteristic r 6  are changed accordingly. 
     As described above, the display control unit  107  may cause the display unit  15  to display both a pair of the amplitude characteristic r 5  of the response FR 3  and the target amplitude characteristic r 1  and a pair of the phase characteristic r 6  and the target phase characteristic r 2  in a comparable manner. The display control unit  107  may cause only one of the pairs to be displayed in a comparable manner. That is, any one of the amplitude characteristic image I 1  and the phase characteristic image I 2  may be displayed. 
     The displaying in a comparable manner refers to displaying of two characteristics in comparison. The two characteristics may be distinguished in any manner, and are not limited to being distinguished by the solid line and the broken line as illustrated in  FIG. 7 . For example, the two characteristics may be distinguished by lines having mutually different colors or lines having mutually different values of thickness. In addition, for example, instead of overlaying the two curves in the same area in a comparable manner, the curves may be arranged vertically or horizontally in a comparable manner. 
     3. Process to be Executed by Control Device 
       FIG. 8  is a flow chart for illustrating an example of a process to be executed by the control device  10 . The process illustrated in  FIG. 8  is executed by the CPU  11  operating in accordance with the program stored in the nonvolatile memory  12 . This process is an example of processes to be executed by the functional units illustrated in  FIG. 3 . 
     As illustrated in  FIG. 8 , the CPU  11  acquires initial values of the target amplitude characteristic r 1 , the target phase characteristic r 2 , the latency value L, and the priority P (Step S 1 ). It is assumed that those initial values are stored in advance in the nonvolatile memory  12 . When previous settings are present in the nonvolatile memory  12 , the values in the previous settings are acquired as the initial values. When the previous setting is not present in the nonvolatile memory  12 , initial values preset by a program creator are acquired. The initial values acquired in Step S 1  are temporarily recorded in the RAM  13 . 
     The CPU  11  displays the filter control screen G on the display unit  15  based on the initial values acquired in Step S 1  (Step S 2 ). The CPU  11  receives a user operation from the operating unit  14  (Step S 3 ). The user performs any one of an amplitude setting operation of changing the curve shape of the target amplitude characteristic r 1 , a phase setting operation of changing the curve shape of the target phase characteristic r 2 , a latency value setting operation of moving the latency value slider SL 1 , a priority setting operation of moving the priority slider SL 2 , and a predetermined end operation. 
     When the amplitude setting operation is received (Step S 3 : amplitude setting), the CPU  11  changes the target amplitude characteristic r 1  recorded in the RAM  13  (Step S 4 ). The target amplitude characteristic r 1  is changed so as to show a curve that has been changed. The curve of the solid line in the amplitude characteristic image I 1  is updated in accordance with the amplitude setting operation. 
     When the phase setting operation is received (Step S 3 : phase setting), the CPU  11  changes the target phase characteristic r 2  recorded in the RAM  13  (Step S 5 ). The target phase characteristic r 2  is changed so as to show a curve that has been changed. The curve of the solid line in the phase characteristic image I 2  is updated in accordance with the phase setting operation. 
     When the latency value setting operation is received (Step S 3 : latency value setting), the CPU  11  changes the latency value L recorded in the RAM  13  (Step S 6 ). The latency value L becomes a value corresponding to a position of the latency value slider SL 1 . 
     The CPU  11  obtains the response FR 1  in accordance with the target amplitude characteristic r 1  and the target phase characteristic r 2  (Step S 7 ). In Step S 7 , the CPU  11  inverse-Fourier-transforms the response FR 1  including the target amplitude characteristic r 1  and the target phase characteristic r 2  to obtain the response IR 1 . 
     The CPU  11  trims the response IR 1  so that the latency according to the response IR 1  satisfies the latency value L, to thereby obtain the response IR 2  (Step S 8 ). In Step S 8 , the CPU  11  trims a head portion of the response IR 1  in accordance with the latency value L. The CPU  11  Fourier-transforms the response IR 2  to obtain the response FR 2 . 
     The CPU  11  corrects, based on the priority P, the response IR 2  so that the difference between the amplitude characteristic r 3  of the response FR 2  and the target amplitude characteristic r 1  is reduced, to thereby obtain the response IR 3  (Step S 9 ). In Step S 9 , the CPU  11  first obtains the amplitude characteristic for correction by multiplying the difference between the target amplitude characteristic r 1  and the amplitude characteristic r 3  of the response FR 2  by the priority P, transforms the amplitude characteristic for correction into the response ΔIR for correction under a condition for the minimum phase, and convolves the response ΔIR into the response IR 2 , to thereby obtain the response IR 3 . 
     The CPU  11  time-shifts the response IR 3 , and sets the coefficients α 1  to αn of the filter F (Step S 10 ). Each of the coefficients α 1  to αn becomes a value on the curve of the time-shifted response IR 3 . 
     The CPU  11  updates the display of the filter control screen G (Step S 11 ). In Step S 11 , the CPU  11  Fourier-transforms the response IR 3  to obtain the response FR 3  including the amplitude characteristic r 5  and the phase characteristic r 6 . The curve of the broken line in the amplitude characteristic image I 1  is updated so as to indicate the amplitude characteristic r 5 . The curve of the broken line in the phase characteristic image I 2  is updated so as to indicate the phase characteristic r 6 . 
     When the priority setting operation is received in Step S 3  (Step S 3 : priority setting), the CPU  11  changes the priority P recorded in the RAM  13  (Step S 12 ). After that, Steps S 9  to S 11  are executed. The priority P does not affect the curve of the response IR 1  and the curve of the response IR 2 . For that reason, after Step S 12 , Step S 7  and Step S 8  are not executed. The omission of Step S 7  and Step S 8  enables the response IR 3  to be quickly obtained. Undesired process is not executed, and hence it is possible to reduce a processing load on the control device  10 . 
     When Step S 11  is executed, the procedure returns to Step S 3 . The process described above is repeated until the desired amplitude characteristic r 5  and phase characteristic r 6  are obtained. When the desired amplitude characteristic r 5  and phase characteristic r 6  are obtained, the user performs the predetermined end operation. When the end operation is received in Step S 3  (Step S 3 : end operation), this process is brought to an end. The audio signal filtering process using the coefficients α 1  to αn in the SPU  17  is continued even after the end of this process. When the end operation is received, the latest target amplitude characteristic r 1 , target phase characteristic r 2 , latency value L, and priority P that are stored in the RAM  13  are recorded in the nonvolatile memory  12 . Those recorded values are acquired as initial values in Step S 1  when the process of  FIG. 8  is executed again. 
     The control device  10  in this embodiment trims the response IR 1  so as to satisfy the latency value L, to thereby obtain the response IR 2 . The control device  10  corrects the response IR 2  so as to reduce the difference between the amplitude characteristic r 3  of the response FR 2  and the target amplitude characteristic r 1 , to thereby obtain the response IR 3 . Thus, it is possible to reduce the difference between the amplitude characteristic r 5  to be set for the filter F and the target amplitude characteristic r 1 . The sound modified by the filter F having the corrected response IR 3  is output from the speaker  20 . 
     In addition, the control device  10  sets the target amplitude characteristic r 1  and the target phase characteristic r 2  in accordance with the user operation. This enables the user to freely set each of the target amplitude characteristic r 1  and the target phase characteristic r 2 . A degree of freedom for the user to set the filter F is improved. For example, the filter F can be set in accordance with an environment in which the speaker  20  is installed. 
     Further, the control device  10  changes the degree of correction of the response IR 2  based on the priority P. Thus, it is possible to obtain the response IR 3  that reflects the target amplitude characteristic r 1  and the target phase characteristic r 2  in a freely-set balance. 
     Further, the control device  10  sets the priority P in accordance with the user operation. This enables the user to freely set the priority P. The degree of freedom for the user to set the filter F is improved, and the work of setting the filter F can be supported. For example, it is possible to set the amplitude characteristic r 5  and the phase characteristic r 6  corresponding to the environment in which the speaker  20  is installed. 
     Further, when the priority P is changed so as to prioritize the amplitude characteristic, the difference between the amplitude characteristic r 5  of the response FR 3  and the target amplitude characteristic r 1  becomes smaller. When the priority P is changed so as to prioritize the phase characteristic, the difference between the phase characteristic r 6  of the response FR 3  and the target phase characteristic r 2  becomes smaller. The user can intuitively grasp the change of the response FR 3  corresponding to the priority P. Through adjustment of the priority P, the user can set, for the filter F, the response FR 3  in which the target amplitude characteristic r 1  and the target phase characteristic r 2  are reflected in a desired balance. 
     Further, the control device  10  sets the latency value L by the user operation. The user may freely set the latency value L. The degree of freedom for the user to set the filter F is thus improved. For example, it is possible to set the latency value L corresponding to a purpose of use of the speaker  20 . For example, for SR purposes, it is preferred to use a shorter latency value (on the order of several tens of milliseconds). For music production purposes, a longer latency value may be set without any problem. 
     Further, the control device  10  displays the amplitude characteristic r 5  of the response FR 3  and the target amplitude characteristic r 1  in a comparable manner. Further, the control device  10  displays the phase characteristic r 6  of the response FR 3  and the target phase characteristic r 2  in a comparable manner. This facilitates the user&#39;s understanding of the difference between the target characteristic and the current characteristic, and can support the user in the work of setting the filter F. 
     4. Modifications 
     The present invention is not limited to the embodiment described above, and can be modified suitably without departing from the spirit of the present invention. 
     For example, the priority P may be set independently for each frequency of the audio signal. In this modification example, the priority P is stored for each frequency band. There are as many priorities P as there are frequency bands. The frequency band and the priority P have a one-to-one relationship. In this modification, there are provided three frequency bands, namely, high, medium, and low frequency bands. The number of frequency bands may be any number. For example, there may be provided two frequency bands or four or more frequency bands. 
     For example, three priorities P corresponding to the three frequency bands, respectively, are stored. The priorities P for the frequency bands may be different from each other or may be the same. For example, the priorities P for the frequency bands are set independently of each other in accordance with the user operation. The user may be able to set only the priorities P for some frequency bands. 
     The third processing unit  106  corrects the response IR 2  in accordance with a plurality of priorities P for a plurality of frequency bands to obtain the response IR 3 . When the priority P differs between the two frequency bands, the degree of correction may be crossfaded at a boundary portion between the two frequency bands. The third processing unit  106  corrects the response IR 2  to the degrees of correction corresponding to the priorities P for the frequency bands. In this modification, the multiplier  106 C multiplies the calculation result of the divider  106 B by the degrees of correction based on the plurality of priorities P of the plurality of frequency bands. The process after the correction control unit  106 D is as described in the embodiment. 
     According to the above-mentioned modification, the priority P is set independently for each frequency. Thus, the response IR 3  corrected to mutually different degrees of correction for the frequencies is obtained. Through the correction corresponding to the frequencies, the user can more freely control the sound to be output from the speaker  20 . 
     In the embodiment, the case in which trimming is performed after the response IR 1  is obtained has been described. The order of steps to be executed by the control device  10  is not limited thereto. For example, the control device  10  may change at least one of the target amplitude characteristic r 1  or the target phase characteristic r 2  in accordance with the latency value L, and then obtain the response IR 1  corresponding to the target amplitude characteristic r 1  and target phase characteristic r 2  that have been changed. 
     In the embodiment, the case in which both the target amplitude characteristic r 1  and the target phase characteristic r 2  are set in accordance with the user operation has been described. At least one of the target amplitude characteristic r 1  or the target phase characteristic r 2  may be set in accordance with the user operation. For example, only any one of those may be set in accordance with the user operation. Further, for example, both of those may be fixed values without being set in accordance with the user operation. 
     In the embodiment, the case in which the degree of correction of the response IR 2  is changed based on the priority P has been described. The control device  10  may correct the response IR 2  without using the priority P in particular. For example, the control device  10  may correct the response IR 2  through use of the entire difference between the target amplitude characteristic r 1  and the amplitude characteristic r 3  of the response FR 2  (priority=1). Further, for example, the control device  10  may correct the response IR 2  through use of the characteristic obtained by multiplying the difference between the target amplitude characteristic r 1  and the amplitude characteristic r 3  of the response FR 2  by the ratio of the fixed value. 
     The amplitude characteristic r 5  of the response FR 3  and the target amplitude characteristic r 1  are not required to be displayed in a comparable manner. The phase characteristic r 6  of the response FR 3  and the target phase characteristic r 2  are not required to be displayed in a comparable manner. The filter control screen G is not required to be displayed in particular. The control system is not limited to one control device  10 . The control system may include a plurality of devices connected by a network or a serial bus.