Patent Publication Number: US-11049486-B2

Title: Noise reduction apparatus, noise reduction method, and computer-readable recording medium

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of PCT International Application No. PCT/JP2018/015025, filed on Apr. 10, 2018, which designates the United States, incorporated herein by reference, and which claims the benefit of priority from Japanese Patent Application No. 2017-085659, filed on Apr. 24, 2017, incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a noise reduction apparatus, a noise reduction method, and a computer-readable recording medium to reduce noise that occurs in a signal processing unit. 
     2. Related Art 
     There is a conventionally known technology of imaging devices, such as video cameras, to reduce noise due to a noise source near a microphone when stereo recording is conducted (see Japanese Laid-open Patent Publication No. 2006-67355). With this technology, out of first and second microphones, noise is reduced by subtracting a signal of a noise component obtained from the second microphone located near the noise source from a sound signal obtained from the first microphone located far from the noise source. 
     SUMMARY 
     In some embodiments, a noise reduction apparatus includes: an input device to which an electric signal is input from an external device; a signal processing circuit configured to perform predetermined signal processing on the electric signal input to the input device and output a generated signal to an external device; a switch that is provided between the input device and the signal processing circuit, the switch being configured to change over to either one of a connected state in which the input device and the signal processing circuit are electrically connected to each other and a disconnected state in which the input device and the signal processing circuit are electrically disconnected to each other; and a noise processing circuit configured to subtract a noise signal output from the signal processing circuit when the switch is in the disconnected state from the signal output from the signal processing circuit when the switch is in the connected state, and output a subtraction result. 
     In some embodiments, provided is a noise reduction method implemented by a noise reduction apparatus including: an input device to which an electric signal is input from an external device; a signal processing unit configured to perform predetermined signal processing on the electric signal input to the input device and output a generated signal to an external device; and a switch that is provided between the input device and the signal processing circuit, the switch being configured to change over to either one of a connected state in which the input device and the signal processing circuit are electrically connected to each other and a disconnected state in which the input device and the signal processing circuit are electrically disconnected to each other. The noise reduction method includes subtracting a noise signal output from the signal processing circuit when the switch is in the disconnected state from the signal output from the signal processing circuit when the switch is in the connected state, and outputting a subtraction result. 
     In some embodiments, provided is a non-transitory computer-readable recording medium with an executable program for a noise reduction apparatus stored thereon, the noise reduction apparatus including: an input device to which an electric signal is input from an external device; a signal processing unit configured to perform predetermined signal processing on the electric signal input to the input device and output a generated signal to an external device; and a switch that is provided between the input device and the signal processing circuit, the switch being configured to change over to either one of a connected state in which the input device and the signal processing circuit are electrically connected to each other and a disconnected state in which the input device and the signal processing circuit are electrically disconnected to each other. The program causes the noise reduction apparatus to execute: subtracting a noise signal output from the signal processing circuit when the switch is in the disconnected state from the signal output from the signal processing circuit when the switch is in the connected state, and outputting a subtraction result. 
     The above and other features, advantages and technical and industrial significance of this disclosure will be better understood by reading the following detailed description of presently preferred embodiments of the disclosure, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram that illustrates a functional configuration of a noise reduction apparatus according to a first embodiment of the disclosure; 
         FIG. 2  is a flowchart that illustrates the outline of a process performed by the noise reduction apparatus according to the first embodiment of the disclosure; 
         FIG. 3  is a block diagram that illustrates a functional configuration of a sound recording device according to a second embodiment of the disclosure; 
         FIG. 4  is a cross-sectional view that schematically illustrates a configuration of an external input terminal included in the sound recording device according to the second embodiment of the disclosure; 
         FIG. 5  is a diagram that schematically illustrates an example of noise information recorded in a noise-information recording unit included in the sound recording device according to the second embodiment of the disclosure; 
         FIG. 6  is a flowchart that illustrates the outline of a process performed by the sound recording device according to the second embodiment of the disclosure; 
         FIG. 7  is a timing chart of a process performed by the sound recording device according to the second embodiment of the disclosure; 
         FIG. 8  is a diagram that schematically illustrates an example of silent sound data; 
         FIG. 9  is a diagram that schematically illustrates an example of the noise distribution of silent sound data when a converter included in the sound recording device according to the second embodiment of the disclosure performs a DFT process; 
         FIG. 10  is a diagram that schematically illustrates an example of the noise distribution of a calculation result by a calculating unit included in the sound recording device according to the second embodiment of the disclosure; 
         FIG. 11  is a diagram that schematically illustrates a sound signal having undergone an IDFT process by a decoding unit included in the sound recording device according to the second embodiment of the disclosure; 
         FIG. 12  is a diagram that illustrates data of a 1-kHz signal before noise removal; 
         FIG. 13  is a diagram that illustrates data of a 1-kHz signal after noise removal; 
         FIG. 14  is a diagram that schematically illustrates another example of noise information recorded in the noise-information recording unit included in the sound recording device according to the second embodiment of the disclosure; 
         FIG. 15  is a diagram that schematically illustrates another example of noise information recorded in the noise-information recording unit included in the sound recording device according to the second embodiment of the disclosure; and 
         FIG. 16  is a diagram that schematically illustrates another example of noise information recorded in the noise-information recording unit included in the sound recording device according to the second embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects (hereinafter, referred to as “embodiments”) for carrying out the disclosure are explained below with reference to the drawings. Furthermore, the disclosure is not limited to the following embodiments. Moreover, in the drawings referred to in the following explanation, shapes, sizes, and positional relationships are schematically illustrated merely to understand the details of the disclosure. That is, the disclosure is not exclusively limited to the shapes, the sizes, and the positional relationships illustrated in the drawings. 
     First Embodiment 
     Noise Reduction Apparatus 
       FIG. 1  is a block diagram that illustrates a functional configuration of a noise reduction apparatus according to a first embodiment of the disclosure. A noise reduction apparatus  1  illustrated in  FIG. 1  is used in any of the following: a sound recording and playing back device, such as an IC recorder, which acquires sound with for example a microphone, records it as sound data, and outputs the sound data from a speaker, or the like; a capturing device that is capable of recording image data generated by an imaging element, such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), and displaying the image corresponding to the image data; and a headphone that plays back and outputs the sound data from an external device. The noise reduction apparatus  1  is an apparatus that removes self-noise that occurs in a codec circuit, an image processing circuit, or the like, which performs signal processing on sound data or image data. Here, the self-noise is the noise generated due to a voltage fluctuation when a device is started up or the noise peculiar to an electric circuit provided in the device even though no sound data or image data is input. 
     As illustrated in  FIG. 1 , the noise reduction apparatus  1  includes an input device  10 , a switch  11 , a signal processing unit  12 , a noise processing unit  13 , a memory I/F unit  14 , a recording medium  15 , a recording unit  16 , and a control unit  17 . 
     The input device  10  receives electric signals acquired by an external device. Specifically, the input device  10  receives analog or digital sound signals (electric signals) converted from the sound collected by a microphone, and analog or digital image signals (electric signals) generated by an imaging element such as CCD or CMOS. The input device  10  is configured as appropriate depending on the configuration of the noise reduction apparatus  1 . For example, when a portable recording medium is used to transfer sound signals or image signals with an external device, the input device  10  is configured as a reader device that has the recording medium removably attached thereto and that reads the recorded sound signal or image signal. Furthermore, when a server that records sound signals and image signals acquired by an external device is used, the input device  10  is configured as a communication device, or the like, capable of communicating with the server in both directions and, by performing data communication with the server, acquiring a sound signal or image signal. Furthermore, the input device  10  may be configured as an interface device, or the like, to which a sound signal or image signal is input from an external device via a cable. 
     The switch  11  is provided between the input device  10  and the signal processing unit  12  to change over to either one of the connected state in which the input device  10  and the signal processing unit  12  are electrically connected to each other and the disconnected state in which the input device  10  and the signal processing unit  12  are electrically disconnected to each other. The switch  11  changes over to either one of the connected state and the disconnected state under the control of the control unit  17 . The switch  11  is configured by using any of, for example, a mechanical switch such as a toggle switch or push switch, an analog semiconductor switch formed of an IC such as a MOS, and a mechanical relay switch formed of a MOS. 
     Under the control of the control unit  17 , the signal processing unit  12  executes predetermined signal processing on an electric signal, input via the input device  10  and the switch  11 , to generate an output signal and outputs the output signal to the noise processing unit  13 . Here, the predetermined signal processing is amplification processing, A/D conversion processing, gain adjustment processing, format conversion processing for conversion into a predetermined format, and the like, for electric signals. The signal processing unit  12  is configured by using DSP (Digital Signal Processing), FPGA (Field Programmable Gate Array), or the like. 
     Under the control of the control unit  17 , the noise processing unit  13  subtracts a noise signal output from the signal processing unit  12  when the state of the switch  11  is the disconnected state from a signal (output signal) output from the signal processing unit  12  when the state of the switch  11  is the connected state and then outputs it to the memory I/F unit  14 . The noise processing unit  13  is configured by using DSP, FPGA, or the like. 
     Under the control of the control unit  17 , the recording medium  15  records a signal (output signal) output from the noise processing unit  13  via the memory I/F unit  14 . The recording medium  15  is mounted to the noise reduction apparatus  1  in an attachable and detachable manner via the memory I/F unit  14 . The recording medium  15  is configured by using, for example, a memory card. 
     The recording unit  16  records various programs executed by the noise reduction apparatus  1  and various types of data executed by the noise reduction apparatus  1 . The recording unit  16  is configured by using a Flash memory, SDRAM (Synchronous Dynamic Random Access Memory), or the like. Furthermore, the recording unit  16  includes a program recording unit  161  that records a program executed by the noise reduction apparatus  1 . 
     The control unit  17  controls each unit included in the noise reduction apparatus  1  in an integrated manner. The control unit  17  is configured by using a CPU (Central Processing Unit), or the like. The control unit  17  controls the state of the switch  11 . Furthermore, the control unit  17  controls each of the signal processing unit  12 , the noise processing unit  13 , and the memory I/F unit  14 . Specifically, the control unit  17  changes the state of the switch  11  to any one of the connected state and the disconnected state. Furthermore, when the state of the switch  11  is the disconnected state and when there is no data from the input device  10 , the control unit  17  causes the signal processing unit  12  to output a signal (noise signal) to the noise processing unit  13 . 
     Process of the Noise Reduction Apparatus 
     Next, a process performed by the noise reduction apparatus  1  is explained.  FIG. 2  is a flowchart that illustrates the outline of the process performed by the noise reduction apparatus  1 . 
     As illustrated in  FIG. 2 , the control unit  17  first changes the state of the switch  11  to the disconnected state (Step S 101 ) and causes the signal processing unit  12  to output a noise signal while in the disconnected state where the input device  10  and the signal processing unit  12  are disconnected to each other and in the state where no electric signal is input from the input device  10  to the signal processing unit  12 , whereby the noise processing unit  13  is caused to acquire a noise signal from the signal processing unit  12  (Step S 102 ). 
     Then, the control unit  17  changes the state of the switch  11  to the connected state (Step S 103 ) and, in the connected state where the input device  10  and the signal processing unit  12  are connected to each other, causes the signal processing unit  12  to execute signal processing on an electric signal input from the input device  10  and output an output signal, whereby the noise processing unit  13  is caused to acquire the output signal from the signal processing unit  12  (Step S 104 ). 
     Then, the control unit  17  causes the noise processing unit  13  to subtract the noise signal output from the signal processing unit  12  in the disconnected state where the input device  10  and the signal processing unit  12  are disconnected to each other from the output signal output from the signal processing unit  12  in the connected state where the input device  10  and the signal processing unit  12  are connected to each other (Step S 105 ). This makes it possible to reduce at least the noise signal included in the signal processing unit  12  from the output signal. 
     Then, the control unit  17  records a subtraction result, which is obtained after the noise processing unit  13  has removed the noise signal from the output signal, in the recording medium  15  via the memory I/F unit  14  (Step S 106 ). After Step S 106 , the noise reduction apparatus  1  terminates this process. 
     According to the first embodiment of the disclosure described above, as the noise processing unit  13  subtracts a noise signal output from the signal processing unit  12  when the state of the switch  11  is the disconnected state from an output signal output from the signal processing unit  12  when the state of the switch  11  is the connected state and then outputs it, whereby it is possible to reduce the self-noise occurring in the noise reduction apparatus  1  from the output signal. 
     Furthermore, according to the first embodiment of the disclosure, the noise processing unit  13  may cause the recording unit  16  to record a noise signal output from the signal processing unit  12  when the state of the switch  11  is the disconnected state. 
     Second Embodiment 
     Next, a second embodiment of the disclosure is explained. According to the second embodiment, the noise reduction apparatus is applied to a sound recording device including a microphone. After the configuration of the sound recording device according to the second embodiment is explained below, a process performed by the sound recording device according to the second embodiment is explained. Furthermore, the same component as that in the noise reduction apparatus  1  according to the above-described first embodiment is attached with the same reference numeral, and a detailed explanation is omitted. 
     Configuration of the Sound Recording Device 
       FIG. 3  is a block diagram that illustrates a functional configuration of the sound recording device according to the second embodiment of the disclosure. A sound recording device  1 A illustrated in  FIG. 3  is a device that collects sound, generates a sound signal (electric signal) based on the collected sound, and records it. 
     As illustrated in  FIG. 3 , the sound recording device  1 A includes a microphone  21 , an external input terminal  22 , the switch  11 , a sound processing unit  23 , an operating device  24 , a Flash memory  25 , an SDRAM  26 , the memory I/F unit  14 , the recording medium  15 , a driver  27 , a display unit  28 , a temperature detecting unit  29 , a bus  30 , and a control unit  31 . 
     The microphone  21  receives sound, converts it into an analog sound signal (electric signal), and outputs the sound signal to the sound processing unit  23  via the external input terminal  22  and the switch  11 . In the explanation according to the second embodiment, the microphone  21  is a directional microphone; however, this is not a limitation, and it may be a unidirectional microphone and, furthermore, a microphone with changeable directional characteristic may be used. Further, the microphone  21  may be a stereo microphone capable of collecting right and left sound. Moreover, according to the second embodiment, the microphone  21  functions as a first microphone. 
     The plug of an external microphone is inserted into the external input terminal  22 . The external input terminal  22  receives input of an analog sound signal (electric signal) converted from the sound by the external microphone and outputs the received sound signal to the sound processing unit  23  via the switch  11 . Furthermore, the microphone  21  is electrically connected to the external input terminal  22 . The external input terminal  22  electrically connects the external microphone and the switch  11  when the plug of the external microphone is inserted into the external input terminal  22  and electrically connects the microphone  21  and the switch  11  when the plug of the external microphone is not inserted into the external input terminal  22 . The external input terminal  22  is configured by using a microphone jack, or the like. Furthermore, according to the second embodiment, the external microphone functions as a second microphone. 
       FIG. 4  is a cross-sectional view that schematically illustrates a configuration of the external input terminal  22 . 
     As illustrated in  FIG. 4 , the external input terminal  22  includes an insertion portion  221 , a first contact member  222 , a second contact member  223 , and a third contact member  224 . 
     The plug of the external microphone is inserted into the insertion portion  221 . A first end of the first contact member  222  is grounded (GND). When the plug of the external microphone is inserted into the insertion portion  221 , a second end  222   a  of the first contact member  222  is brought into contact with it to be electrically connected. A first end of the second contact member  223  is electrically connected to the switch  11  via an undepicted circuit. When the plug of the external microphone is inserted into the insertion portion  221 , a second end  223   a  of the second contact member  223  is brought into contact with it to be electrically connected. A first end  224   a  of the third contact member  224  is electrically connected to the microphone  21 , a second end  224   b  is electrically connected to the switch  11 , and when the plug of the external microphone is inserted into the insertion portion  221 , the second end  224   b  is electrically disconnected from the switch  11 . Specifically, when the plug of the external microphone is inserted into the insertion portion  221 , the first end  224   a  of the third contact member  224  is brought into contact with the plug of the external microphone so that the second end  224   b  is separated from a terminal  225 , whereby the microphone  21  and the switch  11  are electrically disconnected. Furthermore, the configuration of the external input terminal  22  may be changed as appropriate to other than the shape illustrated in  FIG. 4 . Furthermore, although the microphone  21  is electrically connected to the switch  11  via the external input terminal  22  according to the second embodiment, this is not a limitation, and the microphone  21  and the switch  11  may have a direct electrical connection by omitting the configuration of the external input terminal  22 . 
     With reference back to  FIG. 3 , the configuration of the sound recording device  1 A is continuously explained. 
     Under the control of the control unit  31 , the sound processing unit  23  performs various types of signal processing on a sound signal (electric signal) input via the switch  11 . Under the control of the control unit  31 , the sound processing unit  23  records the sound signal (output signal), on which signal processing has been performed, in the recording medium  15  via the bus  30  and the memory I/F unit  14 . Specifically, under the control of the control unit  31 , the sound processing unit  23  converts a sound signal into sound data in a predetermined format on a frame-by-frame basis and temporarily records it in the SDRAM  26 . For example, under the control of the control unit  31 , the sound processing unit  23  continuously performs the operations for the above-described conversion into sound data and the recording of the sound data in the SDRAM  26  during recording and sequentially records the sound data, recorded in the SDRAM  26 , in the recording medium  15  in a FIFO (First In First Out) manner. The sound processing unit  23  is configured by using DSP, FPGA, or the like. The sound processing unit  23  includes a signal processing unit  231  and a noise processing unit  232 . Furthermore, according to the second embodiment, the sound processing unit  23  functions as a noise reduction apparatus. 
     Under the control of the control unit  31 , the signal processing unit  231  performs predetermined signal processing on a sound signal (electric signal) and outputs it to the noise processing unit  232 . The signal processing unit  231  includes at least an amplifier unit  231   a , an A/D converter  231   b , a filter unit  231   c , an equalizer  231   d , an ALC (Automatic Level Control) unit  231   e , and an ADC Vol unit  231   f.    
     Under the control of the control unit  31 , the amplifier unit  231   a  amplifies the sound signal, input via the switch  11 , and outputs it to the A/D converter  231   b . The amplifier unit  231   a  is configured by using an amplifier circuit such as an amplifier. Furthermore, according to the second embodiment, the amplifier unit  231   a  functions as an amplifying unit. 
     Under the control of the control unit  31 , the A/D converter  231   b  conducts A/D conversion on the analog sound signal, input from the amplifier unit  231   a , to convert it into a digital sound signal (quantized data) and outputs the digital sound signal to the filter unit  231   c . The A/D converter  231   b  is configured by using an A/D conversion circuit, or the like. 
     The filter unit  231   c  cuts off an unnecessary frequency from the digital sound signal, input from the A/D converter  231   b , and outputs it to the equalizer  231   d . The filter unit  231   c  is configured by using, for example, a low-pass filter circuit. 
     Under the control of the control unit  31 , the equalizer  231   d  adjusts a specific frequency with regard to the digital sound signal input from the filter unit  231   c  and outputs it to the ALC unit  231   e . The equalizer  231   d  is configured by using various filters. 
     Under the control of the control unit  31 , the ALC unit  231   e  automatically controls the gain of the sound signal and outputs it to the ADC Vol unit  231   f . The ALC unit  231   e  is configured by using an ALC circuit, or the like. 
     Under the control of the control unit  31 , the ADC Vol unit  231   f  amplifies the digital sound signal, input from the ALC unit  231   e , and outputs it to the noise processing unit  232 . The ADC Vol unit  231   f  is configured by using an ADC Vol circuit, or the like. 
     Under the control of the control unit  31 , the noise processing unit  232  reduces noise included in the sound signal (output signal) input from the sound processing unit  23 . Under the control of the control unit  31 , the noise processing unit  232  records the sound signal with reduced noise in the SDRAM  26  via the bus  30  or in the recording medium  15  via the bus  30  and the memory I/F unit  14 . The noise processing unit  232  is provided after the signal processing unit  231 . The noise processing unit  232  includes a converter  232   a , a calculating unit  232   b , and a decoding unit  232   c.    
     Under the control of the control unit  31 , the converter  232   a  generates first amplitude information by conducting discrete Fourier transform (hereafter, simply referred to as “DFT process”) on a signal (noise signal) output from the signal processing unit  231  when the switch  11  sets the state between the external input terminal  22  and the signal processing unit  231  to the disconnected state. Specifically, the converter  232   a  generates the first amplitude information by performing the DFT process on a digital signal (noise signal) output from the signal processing unit  231 . Furthermore, under the control of the control unit  31 , the converter  232   a  records the first amplitude information in the Flash memory  25  or the SDRAM  26  via the bus  30  or records it in the recording medium  15  via the bus  30  and the memory I/F unit  14 . Furthermore, under the control of the control unit  31 , the converter  232   a  generates second amplitude information by conducting the DFT process on a digital sound signal output from the signal processing unit  231  when the switch  11  sets the state between the external input terminal  22  and the signal processing unit  231  to the connected state and outputs the second amplitude information to the calculating unit  232   b . Specifically, the converter  232   a  generates second phase information by conducting the DFT process on a digital sound signal output from the signal processing unit  231  and generates the second amplitude information based on the second phase information. 
     Under the control of the control unit  31 , the calculating unit  232   b  calculates the difference between the second amplitude information input from the converter  232   a and the first amplitude information recorded in the Flash memory  25  or the SDRAM  26  and outputs the difference to the decoding unit  232   c . Specifically, under the control of the control unit  31 , the calculating unit  232   b  subtracts the first amplitude information recorded in the SDRAM  26  from the second amplitude information input from the converter  232   a  and outputs the subtraction result to the decoding unit  232   c.    
     Under the control of the control unit  31 , the decoding unit  232   c  conducts inverse Fourier transform (hereafter, simply referred to as “IDFT process”) on the difference calculated by the calculating unit  232   b  to generate a decoded sound signal (decoded signal) with noise reduced. Specifically, the decoding unit  232   c  decodes the signal based on the difference between the second amplitude information and the first amplitude information and the second phase information. Under the control of the control unit  31 , the decoding unit  232   c  records the decoded sound signal in the recording medium  15  via the bus  30  and the memory I/F unit  14 . 
     The operating device  24  receives input of signals for giving commands for various operations related to the sound recording device  1 A. The operating device  24  outputs received command signals to the control unit  31  via the bus  30 . For example, the operating device  24  receives input of a start signal for giving a command to the sound recording device  1 A so as to start recording, a termination signal for giving a command to terminate recording, a switch signal for changing over to any of modes (e.g., multiple recording modes) executable by the sound recording device  1 A, an adjustment signal for adjusting the gain of a sound signal, and the like. The operating device  24  is configured by using a button, arrow key, switch, touch panel, and the like. It is obvious that the operating device  24  may form a graphical user interface (GUI), or the like, by using a touch panel, a display monitor, and the like. 
     The Flash memory  25  includes: a program recording unit  251  that records a program executed by the sound recording device  1 A; and a noise-information recording unit  252  that records the noise information relating multiple sets of first amplitude information generated by the converter  232   a  and the temperature detected by the temperature detecting unit  29  described later. Furthermore, the Flash memory  25  records various parameters, and the like, regarding the sound recording device  1 A. 
       FIG. 5  is a diagram that schematically illustrates an example of the noise information recorded in the noise-information recording unit  252 . In  FIG. 5 , the horizontal axis indicates a temperature, the vertical axis indicates a noise level, and a curved line L 1  indicates the relation between the temperature and the noise level. As illustrated in the curved line L 1  of  FIG. 5 , the noise-information recording unit  252  records the first amplitude information (noise level) for each temperature. Although the first amplitude information is related to every temperature in a continuous manner in  FIG. 5 , this is not a limitation, and the temperature detected by the temperature detecting unit  29  described later and the first amplitude information may be recorded by being related discretely. 
     With reference back to  FIG. 3 , the configuration of the sound recording device  1 A is continuously explained. 
     The SDRAM  26  temporarily records various types of information that is being processed by the sound recording device  1 A. Furthermore, the SDRAM  26  temporarily records the first amplitude information generated by the converter  232   a.    
     Under the control of the control unit  31 , the driver  27  controls the display mode of the display unit  28 . For example, under the control of the control unit  31 , the driver  27  causes the display unit  28  to present the gain with regard to a sound signal, the volume of a sound signal, the recording time of a sound signal, and the like. 
     The display unit  28  is configured by using a display panel such as liquid crystal or organic EL (Electro Luminescence), and it displays information input from the driver  27 . 
     The temperature detecting unit  29  detects the ambient temperature of the sound recording device  1 A. The temperature detecting unit  29  outputs a detection result to the control unit  31  via the bus  30 . The temperature detecting unit  29  is configured by using a temperature sensor, or the like. 
     The bus  30  is the transmission path that connects each component in the sound recording device  1 A, and it transmits various types of data generated inside the sound recording device  1 A to each component in the sound recording device  1 A. 
     The control unit  31  controls overall units included in the sound recording device  1 A. The control unit  31  is configured by using a general-purpose processor such as a CPU or a dedicated processor such as various arithmetic circuits performing a specific function, e.g., ASIC (Application Specific Integrated Circuit) or FPGA. When the control unit  31  is a general-purpose processor, it transmits commands, data, and the like, to each unit included in the sound recording device  1 A by reading various programs stored in the program recording unit  251  and controls the overall operation of the sound recording device  1 A in an integrated manner. Furthermore, when the control unit  31  is a dedicated processor, the processor may independently execute various processes, or the processor and the program recording unit  251  may execute various processes in cooperation or in combination by using various types of data, and the like, recorded in the program recording unit  251 . The control unit  31  includes a switch controller  311 , a determining unit  312 , and a noise controller  313 . 
     The switch controller  311  controls the state of the switch  11 . Specifically, the switch controller  311  changes the state of the switch  11  to the disconnected state before the sound recording device  1 A starts recording or after the sound recording device  1 A terminates recording. Furthermore, if a start signal is input from the operating device  24 , the switch controller  311  changes the state of the switch  11  to the connected state after a certain time period has elapsed. 
     The determining unit  312  determines whether the first amplitude information related to the current temperature detected by the temperature detecting unit  29  is recorded in the noise-information recording unit  252  of the Flash memory  25 . 
     In accordance with a determination result of the determining unit  312 , the noise controller  313  selects the first amplitude information used for a calculation operation by the calculating unit  232   b  from the sets of first amplitude information recorded in the Flash memory  25  and outputs it to the calculating unit  232   b.    
     Process of the sound recording device Next, a process performed by the sound recording device  1 A is explained.  FIG. 6  is a flowchart that illustrates the outline of the process performed by the sound recording device  1 A.  FIG. 7  is a timing chart of the process performed by the sound recording device  1 A. In  FIG. 7 , from the upper end, (a) represents the timing of on/off operation of the operating device  24 , (b) represents the sound collection timing of the microphone  21 , (c) represents the reading timing of a sound signal, (d) represents the timing of the DFT process by the converter  232   a , (e) represents the timing of writing or reading the first amplitude information to or from the Flash memory  25 , (f) represents the timing of a difference calculation process by the calculating unit  232   b , (g) represents the timing of the IDFT process by the decoding unit  232   c , (h) represents the timing of the temperature detection by the temperature detecting unit  29 , (i) represents the determination timing by the determining unit  312 , and (j) represents the state of the switch  11 . Furthermore, in  FIG. 7 , the horizontal axis indicates a time. 
     As illustrated in  FIG. 6 , first, when the power button of the operating device  24  is operated and the sound recording device  1 A is started up, the control unit  31  makes various default settings regarding the sound recording device  1 A (Step S 201 ). Here, the default settings include checking the presence or absence of a sound file recorded in the recording medium  15 , checking the remaining amount of battery, setting the time and date, and the like. In this case, the switch controller  311  changes the state of the switch  11  to the disconnected state. 
     Then, when a start signal is input due to an operation on the operating device  24  so that sound recording is started (Step S 202 : Yes), the control unit  31  causes the signal processing unit  231  to output a noise signal and causes the converter  232   a  to load silent sound data (Step S 203 ), causes the converter  232   a  to perform the DFT process on a noise signal that is silent sound data input from the signal processing unit  231  (Step S 204 ), and causes the temperature detecting unit  29  to detect the temperature (Step S 205 ). Specifically, as illustrated in  FIG. 7 , when the recording button of the operating device  24  is operated and a start signal is input (time t 1 ), the control unit  31  causes the signal processing unit  231  to output a noise signal and causes the converter  232   a  to load silent sound data (time t 2 ), causes the converter  232   a  to perform the DFT process on the noise signal that is silent sound data input from the signal processing unit  231  (time t 3 ), and causes the temperature detecting unit  29  to detect the temperature (the time t 3 ). 
       FIG. 8  is a diagram that schematically illustrates an example of silent sound data.  FIG. 9  is a diagram that schematically illustrates an example of the noise distribution of the silent sound data when the converter  232   a  performs the DFT process. In  FIG. 8 , the horizontal axis indicates time, the vertical axis indicates a noise level, and a wavelength D 1  indicates the noise signal that is silent sound data. In  FIG. 9 , the horizontal axis indicates a time, and the vertical axis indicates a frequency (Hz). 
     As illustrated in  FIG. 8 , when the recording button of the operating device  24  is operated, the control unit  31  causes the signal processing unit  231  to output a noise signal and causes the converter  232   a  to load silent sound data (the wavelength D 1 ) and, as illustrated in  FIG. 9 , causes the converter  232   a  to perform the DFT process on the noise signal that is the silent sound data input from the signal processing unit  231 . 
     With reference back to  FIG. 6 , the process after Step S 206  is continuously explained. 
     At Step S 206 , the determining unit  312  determines whether the first amplitude information identical to the data related to the temperature detected by the temperature detecting unit  29  is recorded in the Flash memory  25 . Specifically, as illustrated in  FIG. 7 , the determining unit  312  determines whether the first amplitude information identical to the data related to the temperature detected by the temperature detecting unit  29  is recorded in the Flash memory  25  (the time t 3 ). When the determining unit  312  determines that the first amplitude information identical to the data related to the temperature detected by the temperature detecting unit  29  is recorded in the Flash memory  25  (Step S 206 : Yes), the sound recording device  1 A proceeds to Step S 208  described later. Conversely, when the determining unit  312  determines that the first amplitude information identical to the data related to the temperature detected by the temperature detecting unit  29  is not recorded in the Flash memory  25  (Step S 206 : No), the sound recording device  1 A proceeds to Step S 207  described later. 
     At Step S 207 , the control unit  31  records the first amplitude information generated by the converter  232   a  and the temperature detected by the temperature detecting unit  29  in a related manner in the Flash memory  25 . Specifically, as illustrated in  FIG. 7 , the control unit  31  records the first amplitude information generated by the converter  232   a  and the temperature detected by the temperature detecting unit  29  in a related manner in the Flash memory  25  (time t 4 ). After Step S 207 , the sound recording device  1 A proceeds to Step S 208  described later. 
     Then, the switch controller  311  changes the state of the switch  11  to the connected state (Step S 208 ). Specifically, as illustrated in  FIG. 7 , the switch controller  311  changes the state of the switch  11  to the connected state (the time t 4 ). 
     Then, when a certain time period has elapsed (Step S 209 : Yes), the sound recording device  1 A proceeds to Step S 210  described later. Conversely, when the certain time period has not elapsed (Step S 209 : No), the sound recording device  1 A stands by until the certain time period has elapsed. Here, the reason for the standby until the certain time period (e.g., 0.5 seconds) has elapsed is to prevent the recording of noise that occurs when an electric current is applied after the sound recording device  1 A is started up. 
     At Step S 210 , the control unit  31  causes the signal processing unit  231  to start to record the sound signal from the microphone  21 . Specifically, as illustrated in  FIG. 7 , the control unit  31  causes the signal processing unit  231  to start to record the sound signal (time t 5 ). 
     Then, the control unit  31  causes the converter  232   a  to sequentially perform the DFT process on sound signals output from the signal processing unit  231  in sequence (Step S 211 ). Specifically, as illustrated in  FIG. 7 , the control unit  31  causes the converter  232   a  to perform the DFT process on a sound signal output from the signal processing unit  231  (the time t 5 ). 
     Then, the control unit  31  causes the calculating unit  232   b  to sequentially calculate the difference between the second amplitude information input from the converter  232   a  in sequence and the first amplitude information recorded in the Flash memory  25  (Step S 212 ). Specifically, as illustrated in  FIG. 7 , the control unit  31  causes the calculating unit  232   b  to sequentially calculate the difference between the second amplitude information input from the converter  232   a  in sequence and the first amplitude information recorded in the Flash memory  25  (time t 6 ). 
       FIG. 10  is a diagram that schematically illustrates an example of the noise distribution of a calculation result by the calculating unit  232   b . In  FIG. 10 , the horizontal axis indicates a time, and the vertical axis indicates a frequency (Hz). As illustrated in  FIG. 10 , the calculating unit  232   b  subtracts the first amplitude information from the second amplitude information input from the converter  232   a , thereby reducing a noise signal included in the second amplitude information. 
     After Step S 212 , the control unit  31  causes the decoding unit  232   c  to sequentially perform the IDFT process on the difference input from the calculating unit  232   b  (Step S 213 ). Specifically, as illustrated in  FIG. 7 , the control unit  31  causes the decoding unit  232   c  to sequentially perform the IDFT process on the difference input from the calculating unit  232   b  (time t 7 ). 
       FIG. 11  is a diagram that schematically illustrates a sound signal having undergone the IDFT process by the decoding unit  232   c .  FIG. 12  illustrates data of a 1-kHz signal before noise removal.  FIG. 13  illustrates data of a 1-kHz signal after noise removal. In  FIG. 11 , the horizontal axis indicates a time, the vertical axis indicates a noise level, and a wavelength D 2  indicates a sound signal. In  FIG. 12  and  FIG. 13 , the horizontal axis indicates a frequency [Hz], and the vertical axis indicates a signal level [dBFS]. In  FIG. 12 , a wavelength L 10  indicates the 1-kHz signal before noise removal. Furthermore, in  FIG. 13 , a wavelength L 11  indicates the 1-kHz signal after noise removal. 
     As illustrated in the wavelength D 2  of  FIG. 11 , the noise has been reduced in the sound signal having undergone the IDFT process by the decoding unit  232   c . Specifically, as illustrated in  FIG. 12  and  FIG. 13 , it is understood that noise has been removed with little effect on the level of the recorded sound (1 KHz). This allows a reduction in the self-noise that occurs in the sound recording device  1 A. 
     With reference back to  FIG. 6 , the process after Step S 214  is continuously explained. 
     At Step S 214 , the control unit  31  records the sound signal decoded by the decoding unit  232   c  in the recording medium  15  via the memory I/F unit  14 . 
     Then, when the operating device  24  is operated and the recording of the sound recording device  1 A is stopped (Step S 215 : Yes), the sound recording device  1 A proceeds to Step S 220  described later. Specifically, as illustrated in  FIG. 7 , when the operating device  24  is operated, the recording of the sound recording device  1 A is stopped (time t 11 ). Conversely, when the operating device  24  is not operated and the recording of the sound recording device  1 A is not stopped (Step S 215 : No), the sound recording device  1 A proceeds to Step S 216  described later. 
     At Step S 216 , when a certain time period has elapsed after the temperature detecting unit  29  detects the temperature (Step S 216 : Yes), the control unit  31  causes the temperature detecting unit  29  to detect the temperature (Step S 217 ). Specifically, as illustrated in  FIG. 7 , the control unit  31  causes the temperature detecting unit  29  to detect the temperature (time t 8 ). 
     Then, the determining unit  312  determines whether the first amplitude information identical to the data related to the temperature detected by the temperature detecting unit  29  is recorded in the Flash memory  25  (Step S 218 ). Specifically, as illustrated in  FIG. 7 , the determining unit  312  determines whether the first amplitude information identical to the data related to the temperature detected by the temperature detecting unit  29  is recorded in the Flash memory  25  (time t 9 ). When the determining unit  312  determines that the first amplitude information identical to the data related to the temperature detected by the temperature detecting unit  29  is recorded in the Flash memory  25  (Step S 218 : Yes), the sound recording device  1 A proceeds to Step S 219  described later. Conversely, when the determining unit  312  determines that the first amplitude information identical to the data related to the temperature detected by the temperature detecting unit  29  is not recorded in the Flash memory  25  (Step S 218 : No), the sound recording device  1 A proceeds to the above-described Step S 211 . 
     At Step S 219 , the noise controller  313  updates the first amplitude information used by the calculating unit  232   b  to the first amplitude information related to the temperature detected by the temperature detecting unit  29 . Specifically, as illustrated in  FIG. 7 , the noise controller  313  updates the first amplitude information used by the calculating unit  232   b  to the first amplitude information (R 2 ) related to the temperature detected by the temperature detecting unit  29 . This allows the calculating unit  232   b  to subtract the first amplitude information (R 2 ) related to the current temperature from the second amplitude information generated by the converter  232   a . After Step S 219 , the sound recording device  1 A returns to the above-described Step S 211  and, until the recording is stopped, repeatedly performs the above-described Step S 211  to Step S 219 . In this case, as illustrated in  FIG. 7 , in the sound recording device  1 A, each time a certain time period has elapsed, the temperature detecting unit  29  detects the temperature (time t 10 ), the determining unit  312  determines whether the first amplitude information identical to the data related to the temperature detected by the temperature detecting unit  29  is recorded in the Flash memory  25  (the time t 10 ) and, when the first amplitude information identical to the data related to the temperature detected by the temperature detecting unit  29  is recorded in the Flash memory  25 , the first amplitude information is applied to the calculating unit  232   b . Furthermore, when the above-described Step S 211  to Step S 219  are repeatedly performed, the sound processing unit  23  converts a sound signal into sound data in a predetermined format on a frame-by-frame basis, shifts frames such that the frames are overlapped with each other, and connects sets of sound data (e.g., sound data  1 , sound data  2 ) in a smooth manner while executing the Overlap-add method for addition by applying a weight with a window function, thereby loading a sound signal with noise reduced. 
     At Step S 216 , the sound recording device  1 A returns to the above-described Step S 211  when a certain time period has not elapsed after the temperature detecting unit  29  detects the temperature (Step S 216 : No). 
     At Step S 220 , the switch controller  311  changes the state of the switch  11  to the disconnected state. Specifically, as illustrated in  FIG. 7 , the switch controller  311  changes the state of the switch  11  to the disconnected state (time t 12 ). 
     Then, when a certain time period has elapsed (Step S 221 : Yes), the control unit  31  causes the temperature detecting unit  29  to detect the temperature (Step S 222 ). Specifically, as illustrated in  FIG. 7 , the control unit  31  causes the temperature detecting unit  29  to detect the temperature (time t 13 ). After Step S 222 , the sound recording device  1 A proceeds to Step S 223  described later. Conversely, when the certain time period has not elapsed (Step S 221 : No), the sound recording device  1 A stands by until the certain time period has elapsed. 
     Then, the determining unit  312  determines whether the first amplitude information identical to the data related to the temperature detected by the temperature detecting unit  29  is recorded in the Flash memory  25  (Step S 223 ). Specifically, as illustrated in  FIG. 7 , the determining unit  312  determines whether the first amplitude information identical to the data related to the temperature detected by the temperature detecting unit  29  is recorded in the Flash memory  25  (time t 13 ). When the determining unit  312  determines that the first amplitude information identical to the data related to the temperature detected by the temperature detecting unit  29  is recorded in the Flash memory  25  (Step S 223 : Yes), the sound recording device  1 A terminates this process. Conversely, when the determining unit  312  determines that the first amplitude information identical to the data related to the temperature detected by the temperature detecting unit  29  is not recorded in the Flash memory  25  (Step S 223 : No), the sound recording device  1 A proceeds to Step S 224  described later. 
     At Step S 224 , the control unit  31  causes the signal processing unit  231  to output a noise signal and causes the converter  232   a  to load silent sound data. Specifically, as illustrated in  FIG. 7 , the control unit  31  causes the signal processing unit  231  to output a noise signal and causes the converter  232   a  to load silent sound data (time t 14 ). 
     Then, the control unit  31  causes the converter  232   a  to perform the DFT process on the noise signal that is the silent sound data input from the signal processing unit  231  (Step S 225 ). Specifically, as illustrated in  FIG. 7 , the control unit  31  causes the converter  232   a  to perform the DFT process on the noise signal that is the silent sound data input from the signal processing unit  231  (the time t 14 ). 
     Then, the control unit  31  records the first amplitude information generated by the converter  232   a  and the temperature detected by the temperature detecting unit  29  in a related manner in the Flash memory  25  (Step S 226 ). After Step S 226 , the sound recording device  1 A terminates this process. 
     At Step S 202 , when the recording of sound is not started without inputting a start signal due to an operation on the operating device  24  (Step S 202 : No), the sound recording device  1 A proceeds to Step S 227  described later. 
     Then, when a certain time period has elapsed (Step S 227 : Yes), the sound recording device  1 A terminates this process. Conversely, when the certain time period has not elapsed (Step S 227 : No), the sound recording device  1 A returns to Step S 202 . 
     According to the above-described second embodiment of the disclosure, the noise processing unit  232  subtracts the noise signal output from the signal processing unit  231  when the state of the switch  11  is the disconnected state from the output signal output from the signal processing unit  231  when the state of the switch  11  is the connected state and outputs it, whereby it is possible to reduce self-noise occurring in the sound recording device  1 A from a sound signal. 
     Furthermore, according to the second embodiment of the disclosure, as the switch controller  311  sets the state of the switch  11  to the disconnected state, the noise processing unit  232  may acquire self-noise occurring in the sound recording device  1 A in a silent sound state; thus, the first amplitude information, which is noise, may be acquired with a simple configuration. 
     Furthermore, according to the second embodiment of the disclosure, as each of the sound recording devices  1 A is capable of acquiring the first amplitude information that is noise, self-noise may be reduced with high accuracy without preparing various types of data. 
     Furthermore, according to the second embodiment of the disclosure, the calculating unit  232   b  calculates the difference between the second amplitude information input from the converter  232   a  and the first amplitude information, whereby self-noise occurring in the sound recording device  1 A may be reduced from a sound signal. 
     Furthermore, according to the second embodiment of the disclosure, as the temperature detected by the temperature detecting unit  29  and the first amplitude information generated by the converter  232   a  are recorded in a related manner in the Flash memory  25 , the first amplitude information may be acquired for each temperature. 
     Furthermore, according to the second embodiment of the disclosure, the calculating unit  232   b  acquires the first amplitude information related to the current temperature detected by the temperature detecting unit  29  from the Flash memory  25  and uses the acquired first amplitude information to calculate the difference between the second amplitude information and the first amplitude information, whereby it is possible to reduce noise in accordance with the usage environment of the sound recording device  1 A. 
     Furthermore, according to the second embodiment of the disclosure, when the determining unit  312  determines that the first amplitude information related to the current temperature detected by the temperature detecting unit  29  is not recorded in the Flash memory  25 , the Flash memory  25  records the current temperature detected by the temperature detecting unit  29  and the first amplitude information in a related manner, whereby the first amplitude information may be sequentially updated in accordance with the usage environment of the sound recording device  1 A. 
     Furthermore, according to the second embodiment of the disclosure, when the determining unit  312  determines that the first amplitude information related to the current temperature detected by the temperature detecting unit  29  is not recorded in the Flash memory  25 , the calculating unit  232   b  may calculate the difference between the second amplitude information and the first amplitude information by using the first amplitude information related to the temperature closest to the current temperature recorded in the Flash memory  25 . This makes it possible to reduce self-noise occurring in the sound recording device  1 A from a sound signal. 
     Furthermore, according to the second embodiment of the disclosure, the first amplitude information generated by the converter  232   a  and the temperature detected by the temperature detecting unit  29  are recorded in a related manner in the Flash memory  25 ; however, this is not a limitation and, for example, each of the recording modes executable by the sound recording device  1 A may be related to first amplitude information. For example, as illustrated in  FIG. 14 , the control unit  31  may relate first amplitude information (noise level M 1 , M 2 ) generated by the converter  232   a  to each of recording modes (e.g., mode A, mode B) of the sound recording device  1 A in accordance with a designation signal whose input has been received by the operating device  24  and record them in the Flash memory  25 . Thus, even when a different load is applied to the sound processing unit  23  depending on a different mode and a power fluctuation is different, noise may be reduced in an optimal way for each mode. 
     Furthermore, according to the second embodiment of the disclosure, the amplification factor of the amplifier unit  231   a  and the first amplitude information generated by the converter  232   a  may be recorded in a related manner in the Flash memory  25 . Specifically, as illustrated by a curved line L 2  in  FIG. 15 , the control unit  31  may record the amplification factor of the amplifier unit  231   a  and the first amplitude information generated by the converter  232   a  in a related manner in the Flash memory  25 . This allows a reduction in noise in an optimal way for each gain. 
     Furthermore, according to the second embodiment of the disclosure, the first amplitude information generated by the converter  232   a , the temperature detected by the temperature detecting unit  29 , the amplification factor of the amplifier unit  231   a , and multiple recording modes executable by the sound recording device  1 A may be recorded in a related manner in the Flash memory  25 . Specifically, as illustrated in  FIG. 16 , the control unit  31  records the first amplitude information generated by the converter  232   a , the temperature detected by the temperature detecting unit  29 , the amplification factor of the amplifier unit  231   a , and multiple modes executable by the sound recording device  1 A in a related manner in the Flash memory  25 . This allows a reduction in noise in accordance with various conditions. 
     Other Embodiments 
     Furthermore, the noise reduction apparatus according to the disclosure is applicable to digital still cameras, digital video cameras, mobile phones having a capturing function, tablet-type electronic devices having a capturing function, medical systems that generate image data for medical and industrial fields, captured by a headphone, endoscope, or microscope, and the like, as well as sound recording devices. 
     Furthermore, a program executed by the noise reduction apparatus according to the disclosure is provided by being recorded, in the form of file data that is installable and executable, in a recording medium readable by a computer, such as a CD-ROM, a flexible disk (FD), a CD-R, a DVD (Digital Versatile Disk), a USB medium, or a flash memory. 
     Furthermore, for explanations of the flowchart and the timing chart in this description, a sequential order of operations at the respective steps is indicated by using terms such as “first”, “next”, and “then”; however, the sequential order of an operation necessary to implement the disclosure is not uniquely defined by using those terms. That is, the order of operations in the flowchart and the timing chart described in this description may be changed to such a degree that there is no contradiction. 
     Furthermore, the disclosure is not limited to the above-described embodiment as it is, and at the embodiment phase, components may be modified and embodied without departing from the scope of the disclosure. Further, the components disclosed in the above-described embodiment may be combined as appropriate to form various disclosures. For example, some components may be deleted from the entire components described in the above-described embodiment. Furthermore, the components described in each embodiment may be combined as appropriate. 
     Furthermore, in the description and drawings, if a term is described together with a different term having a broader meaning or the same meaning at least once, it may be replaced with the different term in any part of the description or drawings. Thus, various modifications and applications are possible without departing from the scope of the disclosure. 
     Thus, the disclosure may include various embodiments not described here, and various design changes, and the like, may be made within the range of technical ideas specified in claims. 
     According to the disclosure, there is an advantage such that it is possible to reduce self-noise that occurs in a device. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the disclosure in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.