Patent Publication Number: US-10763804-B2

Title: Audio processing device and method for controlling audio processing device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a Continuation Application of PCT Application No. PCT/JP2017/018877, filed May 19, 2017, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Field of the Invention 
     The present invention relates to a technique for processing audio signals. 
     Description of Related Art 
     There has been proposed a class-D amplifier including a Pulse Width Modulation (PWM) circuit for modulating an audio signal to a binary signal, a switch circuit that is switched by the modulated binary signal, and a low-pass filter for reducing a high-frequency component in the output signal from the switch circuit. In a half-bridge type (single-end type) class-D amplifier, the low-pass filter has an inductor, and a smoothing capacitor of the power supply voltage may be overcharged by a regenerative current from the inductor constituting the low-pass filter. As a result, a phenomenon in which the power supply voltage fluctuates (hereinafter referred to as “power supply pumping phenomenon”) may occur. 
     Japanese Patent No. 5233309 (hereinafter, Patent Document 1) discloses a class-D power amplifier that reduces a fluctuation in power supply voltage due to a power supply pumping phenomenon, by controlling a cutoff frequency of a high-pass filter for reducing a low-frequency component of an input audio signal. In the technique of Patent Document 1, the fluctuation in the power supply voltage is monitored to determine whether or not there is an on-going power supply pumping phenomenon. When a power supply pumping phenomenon occurs, the cutoff frequency of the high-pass filter is increased stepwise, thereby reducing the fluctuation in the power supply voltage due to the power supply pumping phenomenon. 
     However, in the technique of Patent Document 1, the presence or absence of a power supply pumping phenomenon is determined by monitoring fluctuation in the power supply voltage. Therefore, there is a problem in that the control for reducing the fluctuation in the power supply voltage due to the power supply pumping phenomenon is not started until the power supply pumping phenomenon is actually generated. 
     SUMMARY 
     In view of the above-described circumstances, an object of the present invention is to reduce fluctuation (ideally, prevent fluctuation) in a power supply voltage due to a power supply pumping phenomenon. 
     In order to solve the above problem, an audio processing device according to an aspect of the present invention includes: a signal processing circuit configured to select between a first state for outputting a first signal obtained by reducing components that fall below a first frequency in an audio signal and a second state for outputting a second signal obtained by reducing components that fall below a second frequency in the audio signal, and output one of the selected first or second signal as an output signal, where the second frequency is higher than the first frequency; a class-D amplifier that amplifies the output signal; a processor configured to: determine whether or not an intensity of a low-frequency component in the audio signal exceeds a threshold value; and control the signal processing circuit to select: the first state in a case where a determination result is negative, where the intensity of the low-frequency component in the audio signal is determined to not exceed the threshold value; and the second state in a case where the determination result is affirmative, where the intensity of the low-frequency component in the audio signal is determined to exceed the threshold value. 
     An aspect of the present invention provides a method of controlling an audio processing device including: a signal processing circuit configured to select between a first state for outputting a first signal obtained by reducing components that fall below a first frequency in an audio signal and a second state for outputting a second signal obtained by reducing components that fall below a second frequency in the audio signal, and output one of the selected first or second signal as an output signal, where the second frequency is higher than the first frequency; and a class-D amplifier that amplifies the output signal, wherein the method comprises: determining whether or not the intensity of a low-frequency component in the audio signal exceeds a threshold value, and controlling the signal processing circuit to select the first state in a case where a determination result is negative, where the intensity of the low-frequency component in the audio signal is determined to not exceed the threshold value; and controlling the signal processing circuit to select the second state in a case where the determination result is affirmative, where the intensity of the low-frequency component in the audio signal is determined to exceed the threshold value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an audio system according to a first embodiment of the present invention. 
         FIG. 2  is a block diagram showing a power amplification device. 
         FIG. 3  is a block diagram of an element for reducing a change in a power supply voltage due to a power supply pumping phenomenon. 
         FIG. 4  is a frequency response of a first high-pass filter and a second high-pass filter. 
         FIG. 5  is an explanatory drawing showing operations of a state determiner and an output controller. 
         FIG. 6  is a flowchart of an operation performed by a control device. 
         FIG. 7  is a block diagram of an audio processing device according to a second embodiment. 
         FIG. 8  is a block diagram of an audio processing device according to a third embodiment. 
         FIG. 9  is an explanatory drawing of a power supply voltage and a voltage index value. 
         FIG. 10  is a block diagram of an audio processing device according to a fourth embodiment. 
         FIG. 11  is a block diagram of an audio processing device according to a modification. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
       FIG. 1  is a block diagram of an audio system  100  according to a first embodiment of the present invention. As shown in  FIG. 1 , the audio system  100  according to the first embodiment is a computer system for playing various sounds such as musical sounds, vocal sounds, or the like. The audio system  100  includes a signal supply device  11 , an audio processing device  12 , and a sound output device  13 . Any two or more elements of the audio system  100  may be formed unitarily. For example, the signal supply device  11  may be mounted on the audio processing device  12 . 
     The signal supply device  11  is a signal source that supplies a digital audio signal X representing various sounds, such as vocal sounds or musical sounds, to the audio processing device  12 . Examples of the signal supply device  11  include a player that reads an audio signal X from a portable or built-in recording medium. Alternatively, there may be used, as the signal supply device  11 , a sound collecting device that collects sound in the periphery to generate an audio signal X or a communication device that receives an audio signal X from another device via a communication network. 
     The audio processing device  12  processes the audio signal X supplied from the signal supply device  11  to generate an audio signal Z. The sound output device  13  is, for example, a speaker or a headphone. The sound output device  13  plays sound representative of the audio signal Z generated by the audio processing device  12 . 
     As shown in  FIG. 1 , the audio processing device  12  includes a control unit  20 , a signal processing circuit  30 , a D/A converter  40 , and a power amplification device  50 . The audio signal X output from the signal supply device  11  is supplied to the signal processing circuit  30 . In a case where an output of the signal supply device  11  is an analog audio signal X, there may be provided an A/D converter (not shown) that converts the audio signal X from analog to digital. In this case, the signal processing circuit  30  receives a supply of the digital audio signal X converted by the A/D converter. 
     The control unit  20  is a controller configured to control each element of the audio processing device  12 . The control unit  20  includes a control device (an example of “a processor”)  21  and a storage device  22 . The control device  21  is an arithmetic processing circuit such as a Central Processor Unit (CPU), for example. The control device  21  controls the signal processing circuit  30  and the power amplification device  50  by executing a program stored in the storage device  22 . The storage device  22  stores therein a program to be executed by the control device  21  and various data used by the control device  21 . For example, there may be used, as the storage device  22 , a known recording medium, such as a semiconductor recording medium or a magnetic recording medium, or a combination of a plurality of types of recording media. 
     The signal processing circuit  30  is configured by, for example, a Digital Signal Processor (DSP) for audio signal processing. The signal processing circuit  30  performs signal processing on the audio signal X supplied from the signal supply device  11 , to an audio signal Y 0 . Examples of the signal processing performed by the signal processing circuit  30  include crossover processing for dividing the audio signal X into bands, delay processing for delaying the audio signal X, equalizer processing for adjusting the frequency characteristics of the audio signal X, limiter processing for limiting a voltage range of an audio signal X, or howling suppression processing for suppressing howling. A part or all of the functions of the signal processing circuit  30  may be realized by the control device  21 . 
     The D/A converter  40  in  FIG. 1  converts the digital audio signal Y 0  generated by the signal processing circuit  30  into an analog audio signal Y 1 . The power amplification device  50  generates an audio signal Z by amplifying the audio signal Y 1 . The sound output device  13  receives a supply of the audio signal Z amplified by the power amplification device  50 . 
       FIG. 2  is a block diagram showing the power amplification device  50 . As shown in  FIG. 2 , the power amplification device  50  according to the first embodiment includes a class-D amplifier  51  and a power source  52 . The class-D amplifier  51  amplifies the audio signal Y 1  supplied from the D/A converter  40  to generate an audio signal Z. The power source  52  supplies power to the class-D amplifier  51 . Specifically, the class-D amplifier  51  receives supplies of a positive power supply voltage Vp and a negative power supply voltage Vm from the power source  52 . 
     As shown in  FIG. 2 , the power source  52  includes a positive power supply  521 , a negative power supply  522 , a smoothing capacitor  523 , and a smoothing capacitor  524 . The positive power supply  521  generates a positive power supply voltage Vp, and the negative power supply  522  generates a negative power supply voltage Vm. The smoothing capacitor  523  has a capacitance for smoothing the positive power supply voltage Vp. The smoothing capacitor  524  has a capacitance for smoothing the negative power supply voltage Vm. For example, an electrolytic capacitor may be used as the smoothing capacitor  523  and the smoothing capacitor  524 . 
     As shown in  FIG. 2 , the class-D amplifier  51  according to the first embodiment is a half-bridge type (single-end type) digital amplifier. The class-D amplifier  51  includes a modulation circuit  511 , a switching circuit  512 , and a low-pass filter  513 . The modulation circuit  511  generates a Pulse Width Modulation (PWM) signal Y 2  by pulse width modulation with respect to the audio signal Y 1 . The PWM signal Y 2  is a binary signal which fluctuates with a duty ratio corresponding to the level of the audio signal Y 1 . Specifically, there may be used, as the modulation circuit  511 , the PWM circuit of the triangular wave comparison type (other excited oscillation type) which generates the PWM signal Y 2  by comparing the audio signal Y 1  with the triangular wave, or a self-excited oscillation PWM circuit which generates the PWM signal Y 2  by the self-excited oscillation by the negative feedback. 
     The switching circuit  512  amplifies the PWM signal Y 2  generated by the modulation circuit  511  by a switching operation, thereby generating an amplified signal Y 3 . The switching circuit  512  according to the first embodiment includes a drive circuit  550 , a first switch  551 , and a second switch  552 . Each of the first switch  551  and the second switch  552  is a switching element such as a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), for example. The first switch  551  is interposed between the positive power supply  521  and an output point N. The second switch  552  is interposed between the negative power supply  522  and the output point N. The drive circuit  550  controls either of the first switch  551  and the second switch  552  to be in the ON state in accordance with the PWM signal Y 2  generated by the modulation circuit  511 . Specifically, the drive circuit  550  controls the first switch  551  to be in the ON state when the PWM signal Y 2  is at a high level, and controls the second switch  552  to be in the ON state when the PWM signal Y 2  is at a low level. When the first switch  551  is controlled to be in the ON state, the positive power supply voltage Vp is applied to the output point N. When the second switch  552  is controlled to be in the ON state, the negative power supply voltage Vm is applied to the output point N. That is, the amplified signal Y 3  generated at the output point N is a rectangular wave that changes from one of the positive power supply voltage Vp and the negative power supply voltage Vm to the other at a duty ratio similar to that of the PWM signal Y 2 . 
     The low-pass filter  513  generates an audio signal Z obtained by reducing a high-frequency component of the amplified signal Y 3  that is generated by the class-D amplifier  51 . It is of note that the high-frequency components of the amplified signal Y 3  are, for example, the band component including the oscillation frequency of the modulation circuit  511 . That is, the low-pass filter  513  extracts the low-frequency component including the audible band from the amplified signal Y 3 , as the audio signal Z. As shown in  FIG. 2 , the low-pass filter  513  includes a capacitor Cf and an inductor Lf. 
     In the class-D amplifier  51  having the structure shown above, a power supply pumping phenomenon may occur. The power supply pumping phenomenon will be described below. The power supply pumping phenomenon is a phenomenon in which the positive power supply voltage Vp and the negative power supply voltage Vm fluctuate. This phenomenon is caused by overcharging the smoothing capacitor  523  and the smoothing capacitor  524  by a regenerative current from the inductor Lf of the low-pass filter  513 . Assuming that a sine wave is supplied to the load (e.g., sound output device  13 ) from the class-D amplifier  51 , the fluctuation amount ΔV of the negative power supply voltage Vm due to the power supply pumping phenomenon is expressed by the following Equation (1). Although the variation amount of the negative power supply voltage Vm is shown in Equation (1), similar fluctuations may also occur in the positive power supply voltage Vp. 
     
       
         
           
             
               
                 
                   
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                             4 
                             ⁢ 
                             Vdd 
                           
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                             π 
                             ⁢ 
                             
                                 
                             
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                             Vs 
                           
                         
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                       8 
                       ⁢ 
                       π 
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                         fCR 
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     In Equation (1), the symbol Vs is the peak voltage [V] of the sine wave, and the symbol Vdd is the absolute value of the positive power supply voltage Vp or the negative power supply voltage Vm. The symbol f is the frequency [Hz] of the sine wave. Further, the symbol C is a capacitance value [F] of the smoothing capacitor  524 , and the symbol RL is a load resistance [Ω]. As will be understood from Equation (1), the lower the frequency of the audio signal Z output from the class-D amplifier  51  is, the more the fluctuation amount ΔV (that is, an impact of the power supply pumping phenomenon) of the positive power supply voltage Vp or the negative power supply voltage Vm tends to be increased. Therefore, by reducing the low-frequency component in the audio signal X before amplification by the class-D amplifier  51 , it is possible to reduce the fluctuation in the power supply voltage due to the power supply pumping phenomenon. 
       FIG. 3  is a block diagram of an element in the audio processing device  12  for reducing the fluctuation in the power supply voltage due to the power supply pumping phenomenon. As shown in  FIG. 3 , the signal processing circuit  30  according to the first embodiment includes a low-frequency reducer  31  for reducing the low-frequency component of the audio signal X. The audio signal X processed by the low-frequency reducer  31  is the same one generated by the signal processing circuit  30 . 
     As shown in  FIG. 3 , the low-frequency reducer  31  according to the first embodiment includes a signal processor  32  and an output controller  33 . The signal processor  32  includes a first High-Pass Filter (HPF)  321  and a second High-Pass Filter (HPF)  322 . As shown by a broken line in  FIG. 4 , the first high-pass filter  321  generates a first signal X 1  by reducing components that fall below a first frequency F 1  in the audio signal X. As shown by a solid line in  FIG. 4 , the second high-pass filter  322  generates a second signal X 2  by reducing components that fall below a second frequency F 2  in the audio signal X. 
     As shown in  FIG. 5 , the second frequency F 2  is higher than the first frequency F 1  (F 2 &gt;F 1 ). For example, the first frequency F 1  is set to a predetermined value within a range from 3 Hz to 20 Hz, inclusive. For example, the second frequency F 2  is set to a predetermined value within a range from 20 Hz to 100 Hz, inclusive. Each of the first high-pass filter  321  and the second high-pass filter  322  is composed of, for example, a fourth-order filter. Therefore, the gradient of the attenuation band in the frequency response of the first high-pass filter  321  is 24 dB/Oct. The gradient of the attenuation band in the frequency response of the second high-pass filter  322  is also 24 dB/Oct. As will be understood from the above description, the signal processor  32  according to the first embodiment generates a first signal X 1  obtained by reducing components that fall below the first frequency F 1  in the audio signal X and a second signal X 2  obtained by reducing components that fall below the second frequency F 2  in the audio signal X. The second frequency F 2  is higher than the first frequency F 1 . 
     The output controller  33  shown in  FIG. 3  outputs the weighted sum of the first signal X 1  generated by the first high-pass filter  321  and the second signal X 2  generated by the second high-pass filter  322  as an audio signal Y 0 . Specifically, the output controller  33  includes a multiplier  331 , a multiplier  332 , and an adder  333 . The multiplier  331  multiplies the first signal X 1  by a weighted value (1−α). The multiplier  332  multiplies the second signal X 2  by a weighted value α. The adder  333  adds an output signal from the multiplier  331  and an output signal from the multiplier  332 , to generate an audio signal Y 0 . That is, the audio signal Y 0  is expressed by the following Equation (2).
 
 Y 0=(1−α) X 1+α· X 2  (2)
 
     The weighted value α of the second signal X 2  takes a numerical value within a range from 0 to 1. The weighted value (1−α) of the first signal X 1  may also fluctuate within a range from 0 to 1. As one of the weighted value α and the weighted value (1−α) increases, the other one decreases. When the weighted value α is set to 0, the output controller  33  outputs the first signal X 1  as an audio signal Y 0 . This state will be called the “first state”. On the other hand, when the weighted value α is set to 1, the output controller  33  outputs the second signal X 2  as an audio signal Y 0 . This state will be called the “second state”. By controlling the weighted value α, the output controller  33  changes from one state to another between the first state and the second state. In the second state, the output controller  33  outputs a second signal X 2  having a reduced component that falls below the second frequency F 2 . Therefore, as compared with the first state, the low-frequency component of the audio signal Z, which is supplied from the class-D amplifier  51  to the sound output device  13 , is reduced. Consequently, this allows for reduction of the fluctuation in the power supply voltage due to the power supply pumping phenomenon. 
     As described above with reference to the above-described Equation (1), as the frequency of the audio signal Z output from the class-D amplifier  51  is lower, the power supply pumping phenomenon is more likely to occur. Therefore, the higher the intensity (for example, volume or power) of the low-frequency component of the audio signal X supplied from the signal supply device  11 , the more likely the power supply pumping phenomenon is to occur. In consideration of the above-described tendency, in the first embodiment, an audio processing device determines that there is a possibility that a power supply pumping phenomenon occurs when the intensity of the low-frequency component in the audio signal X is greater than the threshold value. 
     As shown in  FIG. 3 , the audio processing device  12  according to the first embodiment includes a Low-Pass Filter (LPF)  72 . The low-pass filter  72  extracts low-frequency components that fall below a predetermined frequency in the audio signal X, to output a low-frequency signal S. That is, the low-frequency signal S represents a low-frequency component included in the audio signal X. The low-pass filter  72  generates a low-frequency signal S representing, for example, a low-frequency component that falls below the first frequency F 1  or the second frequency F 2  (for example, a component that falls below a suitable frequency within a range from 3 Hz to 100 Hz, inclusive). The low-pass filter  72  is incorporated in, for example, the signal processing circuit  30 . 
     As shown in  FIG. 3 , the control device  21  according to the first embodiment executes a program stored in the storage device  22 , whereby functions of a state determiner  61  and an operation controller  62  are realized. A part or all of the functions of the control device  21  may be realized by the signal processing circuit  30 . The function of the low-pass filter  72  may also be realized by executing the program by the control device  21 . 
       FIG. 5  is an explanatory drawing of the operation of the control device  21  (the state determiner  61  and the operation controller  62 ). The state determiner  61  determines whether or not a power supply pumping phenomenon occurs in the power source  52 . Specifically, the state determiner  61  compares an intensity (hereinafter referred to as “low-frequency intensity”) L of the low-frequency signal S generated by the low-pass filter  72  with a predetermined threshold value. Then, the state determiner  61  determines whether or not there is a possibility that a power supply pumping phenomenon occurs in accordance with the comparison result. The low-frequency intensity L takes a numerical value on an envelope generated by a known envelope filter, for example, from the low-frequency signal S. As described above, the greater the low-frequency intensity L, the more likely the power supply pumping phenomenon will occur. In consideration of the above tendency, the state determiner  61  according to the first embodiment determines that there is a possibility that a power supply pumping phenomenon occurs when the low-frequency intensity L is higher than the predetermined threshold value Tb (that is, when the low-frequency intensity L rises across the threshold value Tb). 
     The operation controller  62  controls the weighted value α to be applied to the generation of the audio signal Y 0  by the output controller  33  in accordance with the determination result from the state determiner  61 . As shown in  FIG. 5 , when the low-frequency intensity L rises across the threshold value Tb (time point t 1 ), the operation controller  62  increases the weighted value α with time from the start point to the end point of the transition period V 1  of the predetermined length. Specifically, in the transition period V 1 , the weighted value (1−α) of the first signal X linearly decreases from 1 to 0, and the weighted value α of the second signal X 2  linearly increases from 0 to 1. That is, a cross-fade, in which the fade-out of the first signal X 1  and the fade-in of the second signal X 2  are parallel to each other, is realized. As shown above, the output controller  33  gradually changes in the transition period V 1  from the first state (α=0), in which the first signal X 1  is output, until the second state (α=1), in which the second signal X 2  is output. 
     On the other hand, when the low-frequency intensity L falls across the threshold value Tb (time point t 2 ), the operation controller  62  decreases the weighted value α with time from the start point to the end point of the transition period V 2  of the predetermined length. Specifically, in the transition period V 2 , the weighted value (1−α) of the first signal X 1  linearly increases from 0 to 1, and the weighted value α of the second signal X 2  linearly decreases from 1 to 0. That is, a cross-fade, in which the fade-in of the first signal X 1  and the fade-out of the second signal X 2  are parallel to each other, is realized. Therefore, the output controller  33  gradually changes in the transition period V 2  from the second state (α=1), in which the second signal X 2  is output, until the first state (α=0), in which the first signal X 1  is to output. The time lengths of the transition period V 1  and the transition period V 2  are freely selectable. For example, the transition period V 2  may be set to a length that is longer than the transition period V 1 . Specifically, the transition period V 1  is set to a time length of, for example, 10 milliseconds or less. The transition period V 2  is set to a time length of, for example, 100 milliseconds or less. 
       FIG. 6  is a flowchart of operations performed by the control device  21  (the state determiner  61  and the operation controller  62 ). For example, the processing shown in  FIG. 6  starts when it is triggered by an interrupt occurring at a predetermined cycle. When the processing shown in  FIG. 6  is started, the state determiner  61  receives the low-frequency signal S output from the low-pass filter  72  (Sb 1 ), and calculates the low-frequency intensity L (Sb 2 ). 
     The state determiner  61  determines whether or not the low-frequency intensity L exceeds the threshold value Tb (Sb 3 ). When the low-frequency intensity L exceeds the threshold value Tb (Sb 3 : YES), that is, when there is a possibility that a power supply pumping phenomenon occurs, the operation controller  62  executes processing for setting the weighted value α to 1 (Sb 4 ). That is, the processing for controlling the output controller  33  to be in the second state is executed. Specifically, when the output controller  33  is already in the second state (α=1), the operation controller  62  maintains the weighted value α at 1. When the output controller  33  is in the first state (α=0), as shown in  FIG. 5 , the operation controller  62  changes the weighted value α from 0 to 1 over time from the start point to the end point of the transition period V 1  of the predetermined length. That is, the output controller  33  changes from the first state to the second state. 
     On the other hand, when it is estimated that the low-frequency  5   s  intensity L is below the threshold value Tb (Sb 3 : NO), that is, when the power supply pumping phenomenon does not occur, the operation controller  62  executes processing for setting the weighted value α to 0 (Sb 5 ). That is, the processing for controlling the output controller  33  to the first state is executed. Specifically, when the output controller  33  is already in the first state (α=0), the operation controller  62  maintains the weighted value α at 0. On the other hand, when the output controller  33  is in the second state (α=1), as shown in  FIG. 5 , the operation controller  62  reduces the weighted value α from 1 to 0 over time from the start point to the end point of the transition period V 2  of the predetermined length. That is, the output controller  33  changes from the second state to the first state. 
     As described above, the output controller  33  is controlled to be in the first state when the determination result from the state determiner  61  is negative (when there is no possibility that the power supply pumping phenomenon occurs). On the other hand, the output controller  33  is controlled to be in the second state when the determination result from the state determiner  61  is affirmative (when there is a possibility that the power supply pumping phenomenon occurs). In the second state, since the second signal X 2 , which is obtained by reducing components that fall below the second frequency F 2 , which is higher than the first frequency F 1 , is output from the output controller  33 , there is reduced fluctuation in the power supply voltage due to the power supply pumping phenomenon as compared with the first state. 
     In the first embodiment, when the intensity (low-frequency intensity L) of the low-frequency component in the audio signal X exceeds the threshold value Tb, it is determined that there is a possibility that a power supply pumping phenomenon occurs. Therefore, even when the power supply pumping phenomenon is not actually generated, it is possible to execute processing for reducing fluctuations in the power supply voltage due to the power supply pumping phenomenon (processing for controlling the output controller  33  to the second state). That is, according to the first embodiment, it is possible to prevent fluctuations in the power supply voltage due to a power supply pumping phenomenon. 
     Further, in the first embodiment, since the output controller  33  changes from one state to another between the first state and the second state, it is not necessary to change the cutoff frequency of the high-pass filter for processing the audio signal X according to the presence or absence of the power supply pumping phenomenon. Specifically, in the first embodiment, there is reduction in fluctuation of the power supply voltage due to the power supply pumping phenomenon by the very simple control for changing one weighted value α. Therefore, there is an advantage in that there is reduction of load that is applied to the audio processing device  12  and the like to reduce fluctuation in the power supply voltage due to the power supply pumping phenomenon as compared with the technique of Patent Document 1 in which the cutoff frequency of the high-pass filter is changed. 
     Due to the difference in frequency response between the first high-pass filter  321  and the second high-pass filter  322  (in particular, phase characteristic), there is a possibility that phases of the first signal X 1  and the second signal X 2  may be different from each other. Therefore, in the configuration in which the first signal X 1  and the second signal X 2  are selectively switched by the output controller  33 , the level of the audio signal Y 0  output from the output controller  33  may fluctuate discontinuously at the time of switching, which may cause noise (for example, a noise “puff”). In the first embodiment, the weighted value α of the second signal X 2  decreases over time in parallel to the increase in the weighted value (1−α) of the first signal X 1  over time. On the other hand, the weighted value α of the second signal X 2  increases over time in parallel to the decrease in the weighted value (1−α) of the first signal X 1  over time. That is, the first signal X 1  and the second signal X 2  are cross-faded. Therefore, the first embodiment provides another advantage that there is reduced noise caused by a difference in phase between the first signal X 1  and the second signal X 2 . 
     Second Embodiment 
     A second embodiment according to the present invention will be described. In the embodiments shown in the following, elements having the same actions and functions as in the first embodiment are denoted by the same respective reference numerals as used for like elements in the description of the first embodiment, and detailed description thereof is omitted where appropriate. 
       FIG. 7  is a block diagram of elements for reducing fluctuation in power supply voltage due to the power supply pumping phenomenon in the audio processing device  12  according to the second embodiment. Similarly to the first embodiment, the second high-pass filter  322  of the signal processor  32  in the second embodiment outputs the second signal X 2  by reducing the components that fall below the second frequency F 2  in the audio signal X. As shown in  FIG. 7 , the second high-pass filter  322  is a fourth-order filter consisting of a second-order filter  351  and a second-order filter  352 . The second signal X 2  output from the downstream filter  352  is supplied to the output controller  33 . 
     As shown in  FIG. 7 , the audio processing device  12  according to the first embodiment includes a subtractor circuit  71 . The subtractor circuit  71  subtracts the high-frequency signal Xh, which is output from the filter  351  of the second high-pass filter  322 , from the audio signal X before processing by the second high-pass filter  322 , to generate a low-frequency signal S. The high-frequency signal Xh output from the filter  351  corresponds to a high-frequency component that is higher than the second frequency F 2  of the audio signal X. Therefore, the low-frequency signal S generated by subtracting the high-frequency signal Xh from the audio signal X corresponds to a low-frequency component that falls below the second frequency F 2  of the audio signal X. It is also possible to supply the second signal X 2  output from the second high-pass filter  322  (filter  352 ) to the subtractor circuit  71  as the high-frequency signal Xh. 
     The following operations (a) and (b) are the same as those in the first embodiment. (a) The operation in which the state determiner  61  determines whether or not there is a possibility that a power supply pumping phenomenon occurs according to intensity (low-frequency intensity L) of a low-frequency signal S. (b) The operation in which the operation controller  62  controls the weighted value α in accordance with the determination result from the state determiner  61 . Therefore, the second embodiment achieves the same effects as those of the first embodiment. In the second embodiment, the second high-pass filter  322  (filter  351 ) for generating the second signal X 2  from the audio signal X is used to generate the low-frequency signal S used for determining the power supply pumping phenomenon. Therefore, there is an advantage in that the configuration of the audio processing device  12  is simplified, as compared with the configuration of the audio processing device according to the first embodiment that generates the low-frequency signal S using the low-pass filter  72  that is separate from the second high-pass filter  322 . 
     Third Embodiment 
       FIG. 8  is a block diagram of elements in the audio processing device  12  according to the third embodiment for reducing fluctuation in power supply voltage due to a power supply pumping phenomenon. As shown in  FIG. 8 , the audio processing device  12  according to the third embodiment has a configuration in which the voltage detector  53  is added to the same elements as those in the first embodiment. The voltage detector  53  detects the positive power supply voltage Vp and the negative power supply voltage Vm supplied to the class-D amplifier  51  by a power source  52  of the power amplification device  50 . Specifically, the voltage detector  53  is configured to include an A/D converter (not shown) configured to generate digital data representing, for example, each of the positive power supply voltage Vp and the negative power supply voltage Vm. 
     The state determiner  61  according to the third embodiment determines whether or not there is a possibility that a power supply pumping phenomenon occurs in accordance with the low-frequency intensity L of the audio signal X (hereinafter referred to as the “first determination”). Furthermore, the state determiner  61  determines whether or not there is an on-going power supply pumping phenomenon (hereinafter referred to as the “second determination”) in accordance with the positive power supply voltage Vp and the negative power supply voltage Vm detected by the voltage detector  53 . Specifically, in the second determination, the state determiner  61  compares the voltage index value Q, which corresponds to the positive power supply voltage Vp and the negative power supply voltage Vm, with a threshold value to determine whether or not there is a power supply pumping phenomenon. 
       FIG. 9  shows time changes of the positive power supply voltage Vp, the negative power supply voltage Vm, and the voltage index value Q. As shown in  FIG. 9 , the voltage index value Q takes a value obtained by smoothing the larger one of the positive power supply voltage Vp and the absolute value |Vm| of the negative power supply voltage Vm (maximum value max {Vp, |Vm|}) on a time axis. For example, the state determiner  61  calculates the value on the envelope generated by a known envelope filter from the time series of the maximum value max {Vp, |Vm|}, as the voltage index value Q. 
     As will be understood from the above description, as the positive power supply voltage Vp or the negative power supply voltage Vm fluctuates from a predetermined value by a power supply pumping phenomenon, the voltage index value Q takes a larger numerical value. That is, it is possible to determine that the larger the voltage index value Q, the higher the probability of the power supply pumping phenomenon. The state determiner  61  according to the third embodiment determines that the power supply pumping phenomenon has occurred when a voltage index value Q is higher than a threshold value Ta, and determines that the power supply pumping phenomenon has been eliminated when the voltage index value Q is lower than the threshold value Ta. The threshold value Ta is set to a predetermined value exceeding the voltage index value Q observed in a state in which the power supply pumping phenomenon does not occur. 
     The operation controller  62  according to the third embodiment controls the output controller  33  in accordance with the result of the first determination using the low-frequency intensity L and the result of the second determination using the voltage index value Q. Specifically, the operation controller  62  controls the output controller  33  to be in the second state when at least one of the results of the first determination and the second determination is affirmative, and controls the output controller  33  to be in the first state when both the results of the first determination and the second determination are negative. 
     Also the third embodiment achieves the same effects as those of the first embodiment. Furthermore, in the third embodiment, in addition to the above-mentioned first determination, the second determination is executed. In the first determination, the state determiner  61  determines whether or not there is a possibility that a power supply pumping phenomenon occurs in accordance with the low-frequency intensity L. In a second determination, the state determiner  61  determines whether or not there is an on-going power supply pumping phenomenon in accordance with the voltage index value Q. Therefore, there is an advantage in that there is effective reduction in fluctuation in the power supply voltage due to the power supply pumping phenomenon as compared with the first embodiment in which only the first determination is performed. In the second embodiment, the second high-pass filter  322  (filter  351 ) and the subtractor circuit  71  are used to generate the low-frequency signal S. This configuration of the second embodiment is also applicable to the third embodiment as well. 
     Fourth Embodiment 
       FIG. 10  is a block diagram of elements for reducing fluctuation in power supply voltage due to a power supply pumping phenomenon in the audio processing device  12  according to the fourth embodiment. As shown in  FIG. 10 , in the audio processing device  12  according to the fourth embodiment, the low-frequency reducer  31  (the signal processor  32  and the output controller  33 ) according to the first embodiment is replaced by the low-frequency reducer  31   a  shown in  FIG. 10 . The low-frequency reducer  31   a  is a high-pass filter for reducing components that fall below the cutoff frequency Fc in the audio signal X. 
     The cutoff frequency Fc of the low-frequency reducer  31   a  is variable. When the cutoff frequency Fc is set to the first frequency F 1 , the low-frequency reducer  31   a  turns to be in the first state in which an audio signal Y 0  obtained by reducing components that fall below the first frequency F 1  in the audio signal X is output. On the other hand, when the cutoff frequency Fc is set to the second frequency F 2 , the low-frequency reducer  31   a  turns to be in the second state in which the audio signal Y 0  obtained by reducing components that fall below the second frequency F 2  in the audio signal X is output. The low-frequency reducer  31   a  changes from one state to another between the first state and the second state under the control of the operation controller  62 . In the second state, the low-frequency reducer  31   a  outputs the audio signal Y 0  obtained by reducing components that fall below the second frequency F 2 , which is higher than the first frequency F 1 . Accordingly, there is reduced fluctuation in the power supply voltage due to the power supply pumping phenomenon as compared with the first state. 
     Similarly to the first embodiment, the state determiner  61  determines whether or not the low-frequency intensity L of the audio signal X exceeds the threshold value Tb (that is, whether or not there is a possibility that a power supply pumping phenomenon occurs). The operation controller  62  according to the fourth embodiment controls the cutoff frequency Fc of the low-frequency reducer  31   a  in accordance with the determination result from the state determiner  61 . 
     Specifically, when the low-frequency intensity L is higher than the threshold value Tb (when there is a possibility that a power supply pumping phenomenon occurs), the operation controller  62  raises the cutoff frequency Fc from the first frequency F 1  to the second frequency F 2 . For example, the operation controller  62  increases the cutoff frequency Fc stepwise from the first frequency F 1  to the second frequency F 2  by a predetermined value. On the other hand, when the low-frequency intensity L is lower than the threshold value Tb (when it is estimated that the power supply pumping phenomenon does not occur), the operation controller  62  lowers the cutoff frequency Fc from the second frequency F 2  to the first frequency F 1 . For example, the operation controller  62  increases the cutoff frequency Fc stepwise from the second frequency F 2  to the first frequency F 1  by a predetermined value. As will be understood from the above description, similarly to the operation controller  62  according to the first embodiment, the operation controller  62  of the fourth embodiment controls the low-frequency reducer  31   a  to the first state when the determination result from the state determiner  61  is negative, and controls the low-frequency reducer  31   a  to be in the second state when the determination result from the state determiner  61  is affirmative. 
     As shown above, in the fourth embodiment, when the low-frequency intensity L in the audio signal X exceeds the threshold value Tb, the state determiner  61  determines that there is a possibility that a power supply pumping phenomenon occurs. Therefore, even when the power supply pumping phenomenon is not actually generated, there can be executed processing for reducing fluctuations in the power supply voltage due to the power supply pumping phenomenon (processing for increasing the cutoff frequency Fc to the second frequency F 2 ). That is, the fourth embodiment allows for reduction of (and ideally prevention of) fluctuation in the power supply voltage due to the power supply pumping phenomenon in advance in the same manner as in the first embodiment. 
     In the second embodiment, the second high-pass filter  322  and the subtractor circuit  71  are used to generate the low-frequency signal S. The configuration of the second embodiment may be also applicable to the fourth embodiment. Furthermore, in the third embodiment, the low-frequency reducer  31  is controlled in accordance with the result of the first determination using the low-frequency intensity L and the result of the second determination using the voltage index value Q. This configuration of the third embodiment may also be applicable to the fourth embodiment. 
     Modifications 
     The embodiments described above may be modified in various ways. Examples of specific modifications will now be described. Two or more modes selected freely among the following may also be combined. 
     (1) In the first embodiment to the third embodiment, the first signal X 1  and the second signal X 2  are cross-faded. However, in a case in which noise due to the phase difference between the first signal X 1  and the second signal X 2  does not cause a particular problem, the output controller  33  may selectively output the first signal X 1  and the second signal X 2  as the audio signal Y 0 . That is, it is possible to omit the calculation of the weighted sum of the first signal X 1  and the second signal X 2  and the cross fade between the first signal X and the second signal X 2 . 
     For example, there is used, as the output controller  33 , a switch for selecting either of the first signal X and the second signal X 2 . The operation controller  62  controls the output controller  33  so that when the determination result from the state determiner  61  is affirmative, the output controller  33  is in the first state in which the first signal X 1  is output, and when the determination result is negative, the output controller  33  is in the second state in which the second signal X 2  is output. Also, in the above configuration, as compared with the technique of Patent Document 1 in which the cutoff frequency of the high-pass filter is changed, there is reduction of load that is applied to the audio processing device and the like to reduce fluctuation in the power supply voltage due to the power supply pumping phenomenon. As will be understood from the above description, the operation controller  62  according to the aspect of the present invention is expressed comprehensively as an element configured to control the output controller  33  to be in the first state when the determination result from the state determiner  61  is negative, and control the output controller  33  to be in the second state when the determination result is affirmative. There is basically preferred an audio processing device that is configured to gradually change from one state to another between the first state and the second state. However, the audio processing device may instantaneously change from one state to another between the first state and the second state. 
     (2) In first embodiment to the third embodiment, the signal processor  32  is provided with the first high-pass filter  321  and the second high-pass filter  322 , but the first high-pass filter  321  may be omitted. For example, in an audio processing device configured to reduce components that fall below the first frequency F in the signal processing before the processing by the low-frequency reducer  31 , the first high-pass filter  321  is omitted from the low-frequency reducer  31 , as shown in  FIG. 11 . That is, the output controller  33  receives a supply of the audio signal X, as an audio signal X 1 , having reduced components that fall below the first frequency F 1 . 
     (3) In the second embodiment, a part (filter  351 ) of the second high-pass filter  322  is used both for generating the second signal X 2  and for generating the low-frequency signal S. However the configuration in which the signal processor  32  is used for generating the low-frequency signal S is not limited to the above example. The subtractor circuit  71  may receive a supply of the first signal X 1  generated by the first high-pass filter  321  as the high-frequency signal Xh. Alternatively, the subtractor circuit  71  may receive a supply of a signal generated by some of filters constituting first-order high-pass filters  321  (for example, a filter other than the last stage) as the high-frequency signal Xh. In the above configuration, the low-frequency signal S, which represents a low-frequency component that falls below the first frequency F 1 , is generated by the subtractor circuit  71 . 
     (4) in the first embodiment to the third embodiment, the weighted value (1−α) of the first signal X and the weighted value α of the second signal X 2  linear vary; however, the aspects of the variation of the weighted value α and the weighted value (1−α) is not limited to the above example. Specifically, each of the weighted value α and the weighted value (1−α) may also be changed in a curved manner. In each of the above-described embodiments, the weighted value α is changed from 0 to 1; however, the minimum value of the weighted value α (an example of the second value) and the maximum value (an example of the first value) are not limited to the above examples. For example, the minimum value of the weighted value α may be set to a positive number close to 0 (for example, 0.1). Alternatively, for example, the maximum value of the weighted value α may be set to a value close to 1 (for example, 0.9). 
     (5) For example, the following aspect is understood from the foregoing embodiments. 
     Aspect 1 
     An audio processing device according to an aspect (Aspect 1) of the present invention includes: a signal processing circuit configured to select between a first state for outputting a first signal obtained by reducing components that fall below a first frequency in an audio signal and a second state for outputting a second signal obtained by reducing components that fall below a second frequency in the audio signal, and output one of the selected first or second signal as an output signal, where the second frequency is higher than the first frequency; a class-D amplifier that amplifies the output signal; a processor configured to: determine whether or not an intensity of a low-frequency component in the audio signal exceeds a threshold value; and control the signal processing circuit to select: the first state in a case where a determination result is negative, where the intensity of the low-frequency component in the audio signal is determined to not exceed the threshold value; and the second state in a case where the determination result is affirmative, where the intensity of the low-frequency component in the audio signal is determined to exceed the threshold value. 
     In the aspect described above, it is determined whether or not the intensity of the low-frequency component in the audio signal exceeds a threshold value (that is, whether or not there is a possibility that a power supply pumping phenomenon occurs). Accordingly, there is executed the processing for reducing the fluctuation in the power supply voltage due to the power supply pumping phenomenon (processing for controlling the output controller to enter the second state) even when the power supply pumping phenomenon does not actually occur. Therefore, there is an advantage in that there is reduced (ideally, avoided) fluctuation in the power supply voltage due to the power supply pumping phenomenon. 
     Aspect 2 
     In the example of Aspect 1 (Aspect 2), the signal processing circuit includes: a high-pass filter configured to reduce components that fall below the second frequency in the audio signal; and an output controller configured to select between the first state and the second state, in which, in the second state, the output controller outputs the output signal from the high-pass filter, and in which, the audio processing device further comprises a subtractor circuit configured to subtract, from the audio signal, the output signal from the high-pass filter to generate a signal having the low-frequency component. 
     Therefore, there is an advantage in that the configuration of the audio processing device is simplified as compared with a configuration of generating a signal of a low-frequency component for determining a power supply pumping phenomenon by an element separate from the high-pass filter. 
     Aspect 3 
     An aspect (Aspect 3) of the present invention provides a method of controlling an audio processing device including: a signal processing circuit configured to select between a first state for outputting a first signal obtained by reducing components that fall below a first frequency in an audio signal and a second state for outputting a second signal obtained by reducing components that fall below a second frequency in the audio signal, and output one of the selected first or second signal as an output signal, where the second frequency is higher than the first frequency; and a class-D amplifier that amplifies the output signal, wherein the method comprises: determining whether or not the intensity of a low-frequency component in the audio signal exceeds a threshold value, and controlling the signal processing circuit to select the first state in a case where a determination result is negative, where the intensity of the low-frequency component in the audio signal is determined to not exceed the threshold value; and controlling the signal processing circuit to select the second state in a case where the determination result is affirmative, where the intensity of the low-frequency component in the audio signal is determined to exceed the threshold value. 
     In the aspect described above, since it is determined whether or not the intensity of the low-frequency component in the audio signal exceeds a threshold value (that is, whether or not there is a possibility that a power supply pumping phenomenon occurs). Accordingly, there is executed the processing for reducing fluctuation in the power supply voltage due to the power supply pumping phenomenon (processing for controlling the output controller to enter the second state) even when the power supply pumping phenomenon does not actually occur. Therefore, there is an advantage in that there is reduced (ideally, avoided) fluctuation in the power supply voltage due to the power supply pumping phenomenon. 
     In the example of Aspect 3 (Aspect 4), the signal processing circuit includes: a high-pass filter configured to reduce components that fall below the second frequency in the audio signal; and an output controller configured to select between the first state and the second state, in which, in the second state the output controller outputs the output signal from the high-pass filter, and in which, the audio processing device further comprises a subtractor circuit configured to subtract, from the audio signal, the output signal from the high-pass filter to generate a signal having the low-frequency component. 
     DESCRIPTION OF REFERENCE SIGNS 
     
         
           100  audio system; 
           11  signal supply device; 
           12  audio processing device; 
           13  sound output device; 
           20  control unit; 
           21  control device; 
           22  storage device; 
           51  class-D amplifier, 
           511  modulation circuit; 
           512  switching circuit; 
           513  low-pass filter; 
           52  power source; 
           521 ,  522  constant-voltage power supply; 
           523 ,  524  smoothing capacitor; 
           31 ,  31   a  low-frequency reducer; 
           32  signal processor; 
           321  first high-pass filter; 
           322  second high-pass filter, 
           33  output controller; 
           331 ,  332  multiplier, 
           333  adder; 
           61  state determiner; 
           62  operation controller, 
           71  subtractor circuit; 
           72  low-pass filter