Patent Publication Number: US-2018042499-A1

Title: Biological signal detecting device, biological signal processing device, and blood pressure measuring system

Description:
TECHNICAL FIELD 
     The present invention relates to a biological signal detecting device adapted to detect a biological signal having periodicity, a biological signal processing device adapted to remove noise from the biological signal, and a blood pressure measuring system that uses the aforesaid devices. 
     BACKGROUND ART 
     A heart sound, which results from the movement of a heart, includes high-frequency components caused by blood pressure. It is known that the heart sound has strong correlation with the blood pressure, and it is possible to estimate the blood pressure by suitably processing heart sound data using a computer. 
     Patent document 1 discloses the technical content of a central blood pressure measuring device that measures the central blood pressure based on the heart sound. 
     Patent document 2 discloses the content of a technique for removing noise components when detecting the heartbeat. 
     CITATION LIST 
     Patent Literature 
     Patent document 1: Japanese Unexamined Patent Application Publication No. 2012-16450 
     Patent document 2: WO2014/084162 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the central blood pressure measuring device disclosed in Patent document 1, an acceleration sensor is used to collect the heart sound. Since the acceleration sensor is expensive, it is preferred that a general microphone (such as an electret condenser microphone, a dynamic microphone, a ceramic microphone or the like) is used, if possible, to reduce the cost of the device. However, if the aforesaid general microphone is used, noise caused by the clothes, the skin, the muscles, the bones and the like of a subject will also be collected at the same time. 
     In recent years, a wearable sensor possible to be constantly worn by a subject attracts a lot of attention in the market; however, in order to make the blood pressure measuring device as a wearable sensor, low price, small size, and low power consumption are preconditions. Thus, in order to make the blood pressure measuring device as a wearable sensor, it is essential to employ a technique in which a low-priced microphone is used, yet the noise can be effectively removed. 
     Further, if it a radio wave can be used instead of a heartbeat sensor or an electrocardiogram detector, it will be possible to continuously detect the heartbeat in a non-contact manner. 
     The present invention is made to solve the aforesaid problems, and it is an object of the present invention to provide a biological signal detecting device that uses a radio wave to detect a biological signal, a biological signal processing device that removes noise from the biological signal, and a blood pressure measuring system that uses the aforesaid devices to continuously detect the blood pressure of a subject. 
     Solution to Problem 
     To solve the aforesaid problems, a signal noise removing device according to an aspect of the present invention includes: an oscillation source that generates a high-frequency signal; a first antenna that sends a radio wave based on the high-frequency signal; a second antenna that receives the radio wave sent from the first antenna; a third antenna that receives the radio wave sent from the first antenna; and a synchronous detection circuit that uses the high-frequency signal to demodulate a modulated signal based on the radio wave received from the second antenna and the radio wave received from the third antenna. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a biological signal detecting device that uses a radio wave to detect a biological signal, a biological signal processing device that removes noise from the biological signal, and a blood pressure measuring system that uses the aforesaid devices to continuously detect the blood pressure of a subject. 
     Other problems, configurations and effects than those described above will be made clear by the following description of each embodiment. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing an overall configuration of a blood pressure measuring system; 
         FIG. 2A  and  FIG. 2B  are a block diagram showing a hardware configuration of a sensor driving device and a block diagram showing a hardware configuration of an information processing device; 
         FIG. 3A  and  FIG. 3B  are block diagrams showing software functions of the blood pressure measuring system in both cases: one is the case where arithmetic processing for measuring blood pressure is executed by the information processing device, and the other is the case where arithmetic processing for measuring blood pressure is executed by the sensor driving device; 
         FIG. 4  is a block diagram showing software functions of a signal processing section; 
         FIG. 5A  and  FIG. 5B  are block diagrams of software functions showing an example of a noise removal processing section; 
         FIG. 6  shows a waveform diagram of heartbeat data, a waveform diagram of heart sound data, and a waveform diagram of heart sound data delayed by a delay; 
         FIG. 7  shows a waveform diagram of heartbeat data and a waveform diagram of heart sound data; 
         FIG. 8  shows a waveform diagram of a heartbeat waveform before being subjected to reposition processing and a waveform diagram of a heartbeat waveform after being subjected to the reposition processing; 
         FIG. 9  is a schematic view showing an overall configuration of an in-vehicle blood pressure measuring system; 
         FIG. 10  is a functional block diagram of a biological signal detecting device; and 
         FIG. 11  is a functional block diagram of a heart sound detecting device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following embodiment, a signal noise removing device, which constitutes a blood pressure measuring device, will be described first, and a biological signal detecting device will be described thereafter. 
     The signal noise removing device is an improved modification based on the art disclosed in Patent document 2. 
     In the signal noise removing device of the present embodiment, an orthogonal transformation and an orthogonal inverse transformation having strong noise filtering function are used in order to remove the noise mixed into a heart sound. 
     As widely known, an orthogonal transformation includes multiplication processing of a periodic signal. By performing the multiplication processing, frequency components other than a specific frequency are remarkably attenuated. Such feature is extremely useful as a noise filter. 
     A heart sound includes a blood flow sound. The blood flow sound is caused by the friction between blood and the inner walls of blood vessels when the blood flows in the blood vessels. Accordingly, the heart sound has strong correlation with the heartbeat cycle (i.e., the heart sound is a frequency component whose frequency is an integral multiple of the frequency of the heartbeat cycle). 
     Thus, if it is possible to only extract frequency components having strong correlation with the heartbeat cycle, it will be possible to remove the noise not associated with the heartbeat cycle. 
     However, the heartbeat cycle is subtly not constant, but includes fluctuations peculiar to living bodies. 
     Thus, the heartbeat is separately detected by a pulse sensor, the peak of the heartbeat is detected to calculate the average value of the heartbeat cycle, and the signal waveform of the heart sound synchronous with the heartbeat is forcibly fitted to the average cycle of the heartbeat (i.e., resample). The noise components can be removed from the heart sound by performing an orthogonal transformation and then an orthogonal inverse transformation after the signal waveform of the heart sound has been forcibly fitted to the average cycle of the heartbeat. 
     After having performed the orthogonal inverse transformation, the waveform extracted by the orthogonal inverse transformation is restored to the original heartbeat cycle based on addresses in the buffer stored when detecting the peak of the heartbeat. By performing the above processing, the noise can be removed from the heart sound. 
     Further, Patent document 1 discloses a technique in which the blood pressure is estimated by using a correlation between the amplitude of the heart sound and the blood pressure. 
     If the signal of the noise-removed heart sound outputted by the aforesaid signal noise removing device is inputted as it is to a blood pressure measuring device disclosed in Patent document 1, it will be possible to achieve a blood pressure measuring device using a microphone. 
     Further, it is also possible to directly estimate the blood pressure from the orthogonal transformation of the signal noise removing device by using a correlation between the frequency components of the heart sound and the blood pressure. 
     In the biological signal detecting device, a radio wave, instead of a heartbeat sensor or an electrocardiogram detector, is used to detect a biological signal in a non-contact manner. The impedance of the human body varies in response to his (or her) heartbeat. Thus, variation in impedance of a human body is detected when a radio wave is passed through the human body. However, while the radio wave is being passed through the human body, the propagation state of the radio wave will largely fluctuate if the human body moves even slightly. To solve such a problem, two receiving modules are provided to cancel out the fluctuation component by using synchronous detection and differential amplification. 
     First Embodiment: Overall Configuration of a Blood Pressure Measuring System 
       FIG. 1  is a schematic view showing an overall configuration of a blood pressure measuring system  101 . 
     A heartbeat sensor  104  attached to an outer ear  103   a  of a subject  103  and a heart sound microphone  105  attached to the chest of the subject  103  are connected to a sensor driving device  102 . The sensor driving device  102  detects a heartbeat signal from the heartbeat sensor  104 , detects a heart sound signal from the heart sound microphone  105 , and performs data communication with an information processing device  106  by using a short-range wireless communication means, such as Bluetooth (registered trademark) or the like; wherein examples of the information processing device  106  include a mobile wireless terminal  106   a  (such as a smartphone), a personal computer  106   b , and the like. Further, the sensor driving device  102  calculates the blood pressure from the heart sound, and displays the calculated blood pressure on a display of the information processing device  106 . The details about the data communication will be described later with reference to  FIG. 3A  and  FIG. 3B . 
     First Embodiment: Hardware Configurations of Sensor Driving Device  102  and Information Processing Device  106   
       FIG. 2A  is a block diagram showing a hardware configuration of the sensor driving device  102 . 
     In the sensor driving device  102  (which includes a microcomputer), a CPU  201 , a ROM  202 , a RAM  203 , a first A/D converter  204 , a second A/D converter  205 , a first buffer  206  and a second buffer  207  are connected to a bus  208 , wherein the first buffer  206  and the second buffer  207  constitute a serial port. 
     The heartbeat sensor  104  is configured by a combination of a green LED  209  and a photodiode  210 , for example. A power source voltage is applied to the anode of the LED  209  through a resistor  211 . The power source voltage is also applied to the cathode of the photodiode  210  through a resistor  212 . 
     The cathode of the LED  209  is connected to the first buffer  206 . The first buffer  206  (which is a widely known CMOS inverter) functions as a switch that is ON/OFF controlled through the bus  208  to thereby control the connection between the cathode of the LED  209  and a ground node. The first A/D converter  204  is connected to the anode of the photodiode  210  through a first operational amplifier  213  to convert the variation of the blood flow in the outer ear  103   a  of the subject into digital data, wherein the first operational amplifier  213  is adapted to perform current-voltage conversion and voltage amplification. 
     The second A/D converter  205  is connected to the heart sound microphone  105  through a second operational amplifier  214  to convert the heart sound detected from the chest of the subject into digital data, wherein the second operational amplifier  214  is adapted to perform voltage amplification. 
     A short-range wireless communication section  215  is connected to the second buffer  207  to transmit the data outputted by the sensor driving device  102  to the information processing device  106  (see  FIG. 1 ). 
       FIG. 2B  is a block diagram showing a hardware configuration of the information processing device  106 . 
     The information processing device  106  includes a CPU  221 , a ROM  222 , a RAM  223 , a nonvolatile storage  224 , a display  225 , an operating section  226 , and a short-range wireless communication section  227 ; and all these components are connected to a bus  228 . 
     Here, in the case where the information processing device  106  is a mobile wireless terminal  106   a  (such as a smartphone or the like), the display  225  will be a liquid crystal display, and the operating section  226  will be a capacitance type position detecting device. The display  225  and the operating section  226  are superimposed on each other to form a touch panel display  229 . 
     First Embodiment: Software Functions of Sensor Driving Device  102  and Information Processing Device  106   
       FIG. 3A  and  FIG. 3B  are block diagrams each showing software functions of the blood pressure measuring system  101 ; wherein  FIG. 3A  shows an example in which arithmetic processing for measuring blood pressure is executed by the information processing device  106 , and  FIG. 3B  shows an example in which arithmetic processing for measuring blood pressure is executed by the sensor driving device  102 . 
     In the example shown in  FIG. 3A , the LED  209  is driven to intermittently emit light by a light emission controller  301  of the sensor driving device  102 . The light emitted by the LED  209  is transmitted though the outer ear  103   a  of the subject  103 , the transmitted light is detected by the photodiode  210 , and detected light is converted into heartbeat data by the first A/D converter  204 . 
     The heart sound of the subject is detected from his (or her) chest by the heart sound microphone  105 , and converted into heart sound data by the second A/D converter  205 . 
     The heartbeat data and the heart sound data are transmitted to the information processing device  106  by the short-range wireless communication section  215  of the sensor driving device  102 . 
     The information processing device  106  receives the heartbeat data and the heart sound data from the sensor driving device  102  through the short-range wireless communication section  227 . The heartbeat pulse and the heart sound data with the noise removed (hereinafter referred to as “noise-removed heart sound data”) are outputted by a signal processing section  302  of the information processing device  106 . A blood pressure calculator  303  analyzes the noise-removed heart sound data obtained from the signal processing section  302 , and outputs blood pressure data. The heartbeat pulse and the blood pressure data are inputted to an input/output controller  304 , and displayed on the display  225  as a heartbeat value and a blood pressure value. 
     In the example shown in  FIG. 3B , the LED  209  is also driven to intermittently emit light by the light emission controller  301  of the sensor driving device  102 . The light emitted by the LED  209  is transmitted though the outer ear  103   a  of the subject  103 , the transmitted light is detected by the photodiode  210 , and the detected light is converted into heartbeat data by the first A/D converter  204 . 
     The heart sound of the subject is detected from his (or her) chest by the heart sound microphone  105 , and converted into heart sound data by the second A/D converter  205 . 
     The signal processing section  302  receives the heartbeat data and the heart sound data, and outputs the heartbeat pulse and the noise-removed heart sound data. The blood pressure calculator  303  analyzes the noise-removed heart sound data, and outputs the blood pressure data. The heartbeat pulse and the blood pressure data are transmitted to the information processing device  106  by the short-range wireless communication section  227  of the sensor driving device  102 . 
     The information processing device  106  receives the heartbeat pulse and the blood pressure data from the sensor driving device  102  through the short-range wireless communication section  227 . The heartbeat pulse and the blood pressure data are inputted to the input/output controller  304 , and displayed on the display  225  as the heartbeat value and the blood pressure value. 
     In other words, the function of the signal processing section  302  and the blood pressure calculator  303  may be provided both on the side of the information processing device  106  and on the side of the sensor driving device  102 . 
     The signal processing section  302  may also be called as a “biological signal processing device” that removes the noise from the heart sound data based on the heartbeat data and the heart sound data. 
     First Embodiment: Software Functions of Signal Processing Section  302   
       FIG. 4  is a block diagrams showing software functions of the signal processing section  302 . 
       FIG. 5A  and  FIG. 5B  are block diagrams of software functions showing an example of a noise removal processing section  411 . 
     The heartbeat signal outputted by the photodiode  210  (which constitutes the heartbeat sensor  104 ) is converted into the heartbeat data by the first A/D converter  204 , and stored in the heartbeat buffer  401 . On the other hand, the heart sound signal outputted by the heart sound microphone  105  is converted into the heart sound data by the second A/D converter  205 , and stored in a heart sound buffer  403  through a delay  402 . The first A/D converter  204  and the second A/D converter  205  are configured so that the sample frequency of the first A/D converter  204  is equal to the sample frequency of the second A/D converter  205 , and the heartbeat buffer  401  and the heart sound buffer  403  are configured so that the number of stored samples of the heartbeat buffer  401  is equal to the number of stored samples of the heart sound buffer  403 . 
     The functions of the heartbeat buffer  401 , the heart sound buffer  403  and the delay  402  will be described below with reference to  FIG. 6 . 
       FIG. 6  shows a waveform diagram of heartbeat data, a waveform diagram of heart sound data, and a waveform diagram of heart sound data delayed by a delay. 
     In the three waveforms shown in  FIG. 6 , the top one represents a waveform diagram of the heart sound data at detection point P 404  of  FIG. 4 , the middle one represents a waveform diagram of the heartbeat data at detection point P 405  of  FIG. 4 , and the bottom one represents a waveform diagram of the heart sound data delayed by the delay  402  at detection point P 406  of  FIG. 4 . 
     There is a delay D 601  on the time axis existing between the heart sound generated near the heart of the human body and the heartbeat detected from the outer ear  103   a , the delay D 601  being based on the distance between the heart and the outer ear  103   a . The delay  402  cancels out the delay D 601 , so that the phase of the heartbeat data stored in the heartbeat buffer  401  and the phase of the heart sound data stored in the heart sound buffer  403  coincide with each other on the time axis. 
     Now back to the description of the signal processing section  302  with reference to  FIG. 4 . 
     Address information of the peak of the heartbeat and address information of the range of the heartbeat are detected from the heartbeat data within the heartbeat buffer  401  by a heartbeat detector  407 . 
     Here, the address information to be detected by the heartbeat detector  407  will be explained below by comparing the heartbeat data with the heart sound data with reference to  FIG. 7 . 
       FIG. 7  shows a waveform diagram of the heartbeat data stored in the heartbeat buffer  401  and a waveform diagram of the heart sound data stored in the heart sound buffer  403 . The two waveform diagrams shown in  FIG. 7  are equivalent to enlarged middle waveform diagram and enlarged bottom waveform diagram shown in  FIG. 6 . 
     Based on the heartbeat data stored in the heartbeat buffer  401 , the heartbeat detector  407  outputs R-wave address information P 701  and cut-out address information A 702 . 
     Generally, the heartbeat waveform of a healthy person includes a P-wave, a Q-wave, an R-wave, an S-wave, and a T-wave, and appearance order of these waves never changes. 
     First, in order to remove a DC offset component from the heartbeat data in the heartbeat buffer  401 , the heartbeat detector  407  calculates a virtual zero potential by performing arithmetic processing. Further, the heartbeat detector  407  detects the address of the peak of the R-wave in the heartbeat buffer  401 . The detected address of the peak of the R-wave is the R-wave address information P 701 . 
     Next, based on the virtual zero potential, the heartbeat detector  407  detects the address of the start of the P-wave and the address of the end of the T-wave, the both addresses existing before and after the R-wave address information P 701 . The address of the start of the P-wave and the address of the end of the T-wave represent the cut-out address information A 702 . 
     When the R-wave address information P 701  in the heartbeat buffer  401  is acquired, the interval between heartbeats (i.e., RR interval) can be obtained. 
     Incidentally, in addition to acquiring the R-wave address information P 701 , the heartbeat detector  407  also outputs the heartbeat pulse. The heartbeat pulse is supplied to the input/output controller  304  (see  FIG. 3A  and  FIG. 3B ), and the input/output controller  304  measures the heartbeat value from the interval of the heartbeat pulse. 
     Now back to the description of the signal processing section  302  again with reference to  FIG. 4 . 
     Based on the R-wave address information P 701  and the cut-out R-wave address information A 702  obtained from the heartbeat detector  407 , a reposition processing section  408  performs reposition processing on the heart sound data in the heart sound buffer  403 . The data having been subjected to the reposition processing is outputted to and stored in a reposition buffer  409  as repositioned heart sound data. 
     Here, the reposition processing of the reposition processing section  408  will be explained below using the heartbeat data with reference to  FIG. 8 . 
       FIG. 8  shows a waveform diagram of a heartbeat waveform before being subjected to the reposition processing and a waveform diagram of a heartbeat waveform after being subjected to the reposition processing. The data actually to be subjected to the reposition processing is the heart sound data; however, in order to facilitate understanding, the reposition processing is explained below using an example in which the processing of the reposition processing section  408  is performed on a heartbeat waveform. 
     It is known that the RR interval of a healthy person periodically increases and decreases repeatedly. In other words, the RR interval continuously varies, instead of being constantly unchanged. If an orthogonal transformation (such as a discrete Fourier transformation) is performed on the heart sound data as it is, it will be difficult to remove the noise components due to the periodic variation. To solve such problem, before inputting the heart sound data into the noise removal processing section  411  (which removes noises by performing an orthogonal transformation) provided in the subsequent stage, the waveform of the heart sound data is forcibly arranged at an equal interval. As described above, the heartbeat detector  407  has detected the R-wave address information P 701 , and therefore the RR interval has been obtained. The reposition processing section  408  first extracts all RR intervals (i.e., the distances between the R-wave address information P 701 ) detected from the heartbeat buffer  401 , and calculates the average value of the extracted RR intervals. The average value is referred to as “RR average”. Next, based on the RR average, the reposition processing section  408  rearranges the R-wave address information P 701  detected by the heartbeat detector  407  at an interval equivalent to the RR average, and the result obtained is referred to as repositioned R-wave address information P 801 . Further, the reposition processing section  408  cuts out the heart sound data in the heart sound buffer  403  based on the cut-out R-wave address information A 702 , moves the cut out heart sound data by an address moving amount G 802 , which represents a difference between the R-wave address information P 701  and the repositioned R-wave address information P 801 , and stores the result in a reposition buffer  409 . If simply cutting out the heart sound data, since noise occurs in edge portions of the cut out heart sound data, interpolation processing will be performed on the repositioned heart sound data in the reposition buffer  409  by an interpolation processing section  410  provided within the reposition processing section  408 . Such interpolation processing can be achieved by using a known interpolation, such as a linear interpolation, a Lagrange interpolation, a spline interpolation or the like; particularly, a good arithmetic result can be obtained by the Lagrange interpolation. 
     Thus, the reposition processing section  408  outputs the repositioned heart sound data and the repositioned R-wave address information P 801 . The repositioned heart sound data is stored in the reposition buffer  409 . The repositioned R-wave address information P 801  is supplied to a position restoration processing section  413  (which will be described later), in which the repositioned R-wave address information P 801 , together with the R-wave address information P 701  and the cut-out address information A 702  outputted by the heartbeat detector  407 , is used in processing for restoring the position of the heart sound waveform of the noise-removed repositioned heart sound data to its original position on the time axis. 
     The noise of the repositioned heart sound data stored in the reposition buffer  409  is removed by the noise removal processing section  411 . The noise removal processing section  411  removes the noise by using an orthogonal transformation and an orthogonal inverse transformation. There are various known orthogonal transformation methods; among these methods, two typical orthogonal transformation methods will be used to describe the processing of removing the noise with reference to  FIG. 5A  and  FIG. 5B . 
       FIG. 5A  shows an example of a block diagram of software functions of a noise removal processing section  411  using a discrete cosine transformation. 
     The repositioned heart sound data (which is a discrete data row) stored in the reposition buffer  409  is converted into a coefficient data row by a DCT transformation processing section  501 , wherein the coefficient data row has the same sample number as that of the discrete data row. A high-order coefficient data thinning processing is performed on the coefficient data row by a coefficient filter  502 . The coefficient data row having been subjected to the high-order coefficient data thinning processing is converted into a decoded discrete data row by a DCT inverse transformation processing section  503 , and stored in a processed buffer  412 , wherein the decoded discrete data row has the same sample number as that of the discrete data row. 
     The decoded discrete data row becomes the noise-removed repositioned heart sound data. 
       FIG. 5B  shows an example of a block diagram of software functions of a noise removal processing section  411  using a discrete Fourier transformation. 
     The repositioned heart sound data (which is a discrete data row) stored in the reposition buffer  409  is converted into a complex data row by a FFT transformation processing section  511 , wherein the complex data row has the same sample number as that of the discrete data row. A real part data row and an imaginary part data row of the complex data row are converted into an amplitude data row and a frequency/phase data row by a vector operation processing section  512 , wherein the amplitude data row and the frequency/phase data row each have the same sample number as that of the discrete data row. The amplitude data row has its high-frequency components eliminated by a high-pass filter (hereinafter referred to as “LPF”)  513 . The amplitude data row having its high-frequency components eliminated and the frequency/phase data row are converted into a complex data row by an inverse vector operation processing section  514 . Further, the complex data row is converted into a decoded discrete data row by a FFT inverse transformation processing section  515 , and the decoded discrete data row is stored in the processed buffer  412 . 
     The decoded discrete data row becomes the noise-removed repositioned heart sound data. 
     By the position restoration processing section  413 , the position of the heart sound waveform of the noise-removed repositioned heart sound data in the processed buffer  412  on the time axis is restored to its original position before being moved by the reposition processing section  408 . In other words, a heart sound waveform is cut out from the noise-removed repositioned heart sound data in the processed buffer based on address information obtained by shifting the cut-out address information A 702  based on the difference between the R-wave address information P 701  and the repositioned R-wave address information P 801 . Further, position restoration is performed based on the difference between the R-wave address information P 701  and the repositioned R-wave address information P 801 , and the result is outputted as the noise-removed heart sound data. 
     A heart sound includes a blood flow sound. The blood flow sound is caused by the friction between blood and the inner walls of blood vessels when the blood flows in the blood vessels. Accordingly, the heart sound has strong correlation with the heartbeat cycle. 
     Thus, if it is possible to only extract frequency components having strong correlation with the heartbeat cycle, it will be possible to remove the noise not associated with the heartbeat cycle. 
     However, the heartbeat cycle is subtly not constant, but includes fluctuations peculiar to living bodies. 
     Thus, the heartbeat detector  407  is used to extract the R-wave address information P 701  and the cut-out address information A 702  from the heartbeat data. Next, the reposition processing section  408  calculates the average value of the RR interval based on the R-wave address information P 701  to derive the repositioned R-wave address information P 801 . Further, the reposition processing section  408  forcibly fits the heart sound data synchronous with the heartbeat data to the average cycle of the heartbeat. The noise component not associated with the heartbeat cycle can be removed from the heart sound by fitting the cut out heart sound waveform at an equal interval, and then performing an orthogonal transformation and an orthogonal inverse transformation. 
     After having performed the orthogonal inverse transformation, the waveform cut out by the orthogonal inverse transformation is restored to the original heartbeat cycle based on the R-wave address information P 701 , the repositioned R-wave address information P 801 , and the cut-out address information A 702 . By performing the above processing, the noise can be removed from the heart sound. 
     As shown in  FIG. 3A  and  FIG. 3B , the noise-removed heart sound data is inputted to the blood pressure calculator  303 . The blood pressure calculator  303  refers to a table in the blood pressure calculator  303  to calculate the blood pressure from the amplitude of the noise-removed heart sound data. 
     Second Embodiment: Overall Configuration of Biological Signal Detecting Device 
     The blood pressure measuring system  101  has been disclosed in the first embodiment. By providing a heart sound signal and a heartbeat signal to the blood pressure measuring system  101  of the first embodiment, it is possible to remove the noise from the heart sound signal, and calculate the blood pressure. In the blood pressure measuring system  101 , the heart sound signal is detected by an acoustic microphone, and the heartbeat signal is detected by a photoelectric sensor formed by the LED  209  and the photodiode  210 . These sensors need to be brought into contact with the human body. If these sensors can be uses in a non-contact manner, it will be possible to achieve an in-vehicle blood pressure measuring system  901  to be incorporated into a vehicle or the like. 
       FIG. 9  is a schematic view showing an overall configuration of the in-vehicle blood pressure measuring system  901 . 
     The in-vehicle blood pressure measuring system  901  includes a biological signal detecting device  902 , a heart sound detecting device  903 , and a blood pressure measuring device  904 . Among these components, the blood pressure measuring device  904  is equivalent to the signal processing section  302  and blood pressure calculator  303  of the blood pressure measuring system  101 . 
     The heart sound detecting device  903  irradiates a weak radio wave with a frequency of about 60 MHz emitted from a helical antenna  905  to a driver  906 , and detects the heart sound signal from the reflected wave reflected by the driver  906 . 
     The biological signal detecting device  902  has three antennas connected thereto, which are a first antenna  907 , a second antenna  909 , and a third antenna  910 . 
     The first antenna  907  is a square-shaped metal plate whose side length is about 5 to 10 cm, and is embedded in a backrest  908   a  of a driver seat  908 . 
     Similar to the first antenna  907 , the second antenna  909  is also a square-shaped metal plate whose side length is about 5 to 20 cm, and is embedded in a seat  908   b  of the driver seat  908 . 
     The third antenna  910  is a metal wire embedded in a steering  911  of the vehicle. Alternatively, the third antenna  910  may also be embedded in a dashboard provided near the steering  911 , instead of being embedded in the steering  911 . At this time, the third antenna  910  is formed in the same shape as that of the first antenna  907  and the second antenna  909 , i.e., formed as a square-shaped metal plate whose side length is about 5 to 10 cm. 
     An AC resistance detector  912  of the biological signal detecting device  902  emits a weak unmodulated radio wave from the first antenna  907 , wherein the unmodulated radio wave has a HF band ranging from several MHz to several tens of MHz. The radio wave is received from the second antenna  909  and the third antenna  910 . Since the AC resistance detector  912  outputs a signal containing the biological signals, a band-pass filter (hereinafter referred to as “BPF”)  913  removes a DC offset component and a high-frequency noise component, and outputs a biological signal substantially equivalent to an electrocardiographic signal. 
     Thus, the biological signal obtained from the biological signal detecting device  902  and the heart sound signal obtained from the heart sound detecting device  903  are inputted to the blood pressure measuring device  904 , and the blood pressure of the subject  103  is measured. 
       FIG. 10  is a functional block diagram of the biological signal detecting device  902 . 
     An oscillation source  1001  generates an unmodulated high-frequency signal having a HF band ranging from several MHz to several tens of MHz. The unmodulated high-frequency signal is transmitted from the first antenna  907  as an unmodulated radio wave. 
     The second antenna  909  has a tuned circuit connected thereto, wherein the tuned circuit includes a coil L 1002  and a capacitor C 1003 . The third antenna  910  also has a tuned circuit connected thereto, wherein the tuned circuit includes a coil L 1004  and a capacitor C 1005 . 
     The output signal of the tuned circuit connected to the second antenna  909  is applied to a first mixer  1006 . The unmodulated high-frequency signal of the oscillation source  1001  is also inputted to the first mixer  1006 . 
     The output signal of the tuned circuit connected to the third antenna  910  is applied to a second mixer  1007 . The unmodulated high-frequency signal of the oscillation source  1001  is inputted to the second mixer  1007  through a first π/2 phase shifting circuit  1008 . In other words, a signal obtained by delaying the unmodulated high-frequency signal of the oscillation source  1001  by π/2 phase is inputted to the second mixer  1007 . Incidentally, a dual gate FET, for example, can be used as the first mixer  1006  and the second mixer  1007 . 
     The output signal of the tuned circuit connected to the second antenna  909  and the unmodulated high-frequency signal of the oscillation source  1001  are supplied to the first mixer  1006 , a sum signal and a difference signal of these frequency components are supplied to a first LPF  1009  as the output signal of the first mixer  1006 . Of the inputted sum signal and difference signal of the frequency components, the first LPF  1009  only outputs the difference signal. Further, the output signal of the first LPF  1009  is delayed by a second π/2 phase shifting circuit  1010  by π/2 phase. 
     The output signal of the tuned circuit connected to the third antenna  910  and the signal obtained by delaying the unmodulated high-frequency signal of the oscillation source  1001  by π/2 phase are supplied to the second mixer  1007 , and a sum signal and a difference signal of these frequency components are supplied to a second LPF  1011  as the output signal of the second mixer  1007 . Of the inputted sum signal and difference signal of the frequency components, the second LPF  1011  only outputs the difference signal. 
     The output signal of the second π/2 phase shifting circuit  1010  and the output signal of the second LPF  1011  are inputted to a differential amplifier  1012 . The differential amplifier  1012  cancels out the same in-phase components of the output signal of the second π/2 phase shifting circuit  1010  and the output signal of the second LPF  1011 , and only outputs the reverse-phase components of the both output signals. Further, the output signal of the differential amplifier  1012  is inputted to the BPF  913 . 
     Except for the two tuned circuits, the first mixer  1006 , the second mixer  1007 , the first π/2 phase shifting circuit  1008 , the first LPF  1009 , the second π/2 phase shifting circuit  1010 , and the second LPF  1011  are identical to those of a known synchronous detection circuit. 
     The heartbeat (i.e., change in pressure of blood flow) results in a phenomenon of so-called “variation in impedance of the human body”. A biological signal substantially equal to the heartbeat signal is detected by connecting electrodes to a plurality of points of the human body, supplying a weak AC signal to the electrodes, and detecting variation in impedance of the human body. The biological signal detecting device  902  according to the present embodiment uses a radio wave to detect the variation in impedance of the human body in a non-contact manner. 
     However, when propagating a radio wave from a transmission side to a reception side with an object interposed therebetween, the gain of the detection signal on the reception side will largely fluctuate depending on the object interposed between the transmission side and the reception side. In the case shown in  FIG. 9 , the signal level on the reception side will largely fluctuate if the driver  906  (i.e., the subject) moves even slightly. 
     To solve such problem, a synchronous detection circuit is used in the biological signal detecting device  902  of the present embodiment. By providing two antennas and two tuned circuits on the reception side, the fluctuation of the signal level based on the movement of the human body is cancelled out as the in-phase components by the differential amplifier  1012 , and the fluctuation of the signal level based on the variation in impedance of the human body is detected as the reverse-phase components by the differential amplifier  1012 . In other words, the variation in impedance of the human body is equivalent to generating an amplitude modulation with respect to the unmodulated high-frequency signal. 
       FIG. 11  is a functional block diagram of the heart sound detecting device  903 . The technical content of the heart sound detecting device  903  is disclosed in a patent application for a “pulse sensor” (JP2013-217093) which is filed by the inventor of the present application. 
     The heart sound detecting device  903  can be divided into the following two elements. 
     The first element is transmitting a radio wave (which is a traveling wave) to an object, and receiving and extracting a reflected wave reflected from the object. The first element includes a pulse wave generator  1102 , a BPF  1103 , a first RF amplifier  1104 , a directional coupler  1105 , and the aforesaid helical antenna  905  (as a fourth antenna). 
     The second element is generating a frequency difference signal from the traveling wave and the reflected wave, and extracting a signal of the pulse. The second element includes a second RF amplifier  1108 , a third RF amplifier  1109 , a third mixer  1110 , a fourth mixer  1112 , a third LPF  1114 , a fourth LPF  1115 , a differential amplifier  1116 , and a fifth LPF  1117 . 
     The pulse wave generator  1102  (which may also be referred to as a “signal generator”) generates a pulse having relatively low frequency. The frequency of the pulse generated by the pulse wave generator  1102  is, for example, 1 MHz. 
     The BPF  1103  extracts harmonic components from the pulse generated by the pulse wave generator  1102 . The central frequency and bandwidth of the BPF  1103  is, for example, 60 MHz±3 MHz. For example, a circuit structure obtained by connecting multiple stages of LC resonance circuits can be used as the BPF  1103 . 
     The first RF amplifier  1104  amplifies the signal of the harmonic components passed through the BPF  1103 . 
     The signal of the harmonic components having been amplified by the first RF amplifier  1104  is inputted to an input terminal of the directional coupler  1105  (the terminal indicated as “IN” in  FIG. 11 ). Further, the signal of the harmonic components is supplied to the helical antenna  905  connected to an output terminal of the directional coupler  1105  (the terminal indicated as “OUT” in  FIG. 11 ). 
     The directional coupler  1105  is a widely known circuit element configured by a coil(s), a capacitor(s), and a resistor(s), and is used in a VSWR (Voltage Standing Wave Ratio) meter or the like. The directional coupler  1105  can respectively output an output signal proportional to the traveling wave and an output signal proportional to the reflected wave based on the traveling wave and the reflected wave included in a first transmission path. 
     The helical antenna  905  emits a radio wave having a plurality of frequencies based on the signal of the harmonic components. Further, the radio wave reflected by the object (such as a human body) is received by the helical antenna  905  to generate a standing wave in the directional coupler  1105 . 
     A signal proportional to the signal of the radio wave inputted from the output terminal of the directional coupler  1105  through the helical antenna  905  (i.e., the reflected wave) is outputted to an insulated terminal of the directional coupler  1105  (the terminal indicated as “INSULATED” in  FIG. 11 ). 
     A signal proportional to the signal of the harmonic components (i.e., the traveling wave) inputted to the input terminal of the directional coupler  1105  is outputted to a coupling terminal of the directional coupler  1105  (the terminal indicated as “COUPLED” in  FIG. 11 ). 
     The coupling terminal is connected to a ground node through a resistor R 1107 . The resistance of the resistor R 1107  is set to a value equal to the impedance of the directional coupler  1105  or helical antenna  905 . In most cases, the value of the resistor R 1107  is set to 50Ω or 75Ω. 
     The second RF amplifier  1108  amplifies the signal of the harmonic components passed through the BPF  1103  (i.e., the traveling wave). 
     The third RF amplifier  1109  amplifies the signal of the radio wave (i.e., the reflected wave) inputted from the output terminal through the helical antenna  905  and outputted from the insulated terminal of the directional coupler  1105 . 
     The output signal of the second RF amplifier  1108  is supplied to the fourth mixer  1112  through an inverting amplifier  1111 , as well as being supplied to the third mixer  1110 . 
     The output signal of the third RF amplifier  1109  is supplied to the third mixer  1110  through a buffer  1113 , as well as being supplied to the fourth mixer  1112 . Incidentally, even if the phase of the output signal of the second RF amplifier  1108  and the phase of the output signal of the third RF amplifier  1109  are different from each other, a desired signal can be obtained from the third mixer  1110  and the fourth mixer  1112 . Thus, a buffer (a non-inverting amplifier) can be used instead of the inverting amplifier  1111 . 
     Thus, the third mixer  1110  and the fourth mixer  1112  each output a multiplication signal of the traveling wave and the reflected wave. Here, a dual gate FET, for example, can be used as the third mixer  1110  and the fourth mixer  1112 . 
     The output signal of the third mixer  1110  is supplied to the third LPF  1114 . In the multiplication signal of the traveling wave and the reflected wave outputted from the third mixer  1110 , the third LPF  1114  outputs a difference signal between the frequency of the traveling wave and the frequency of the reflected wave. 
     Similarly, the output signal of the fourth mixer  1112  is supplied to the fourth LPF  1115 . In the multiplication signal of the traveling wave and the reflected wave outputted from the fourth mixer  1112 , the fourth LPF  1115  outputs a difference signal between the frequency of the traveling wave and the frequency of the reflected wave. 
     The output signal of the third LPF  1114  and the output signal of the fourth LPF  1115  are inputted to the differential amplifier  1116 . The differential amplifier  1116  (which is an operational amplifier) outputs a signal obtained by removing noise components from both the output signal of the third LPF  1114  and the output signal of the fourth LPF  1115 . 
     The output signal of the differential amplifier  1116  is supplied to the fifth LPF  1117 . The fifth LPF  1117  removes AC components having relatively high frequency from the output signal of the differential amplifier  1116 , and passes through a low frequency signal indicating the heart sound of the human body. 
     As described above, it is possible to achieve the blood pressure measuring system  101  capable of continuously measuring the blood pressure of a human body in a non-contact manner by supplying the heartbeat signal outputted by the biological signal detecting device  902  and the heart sound signal outputted by the heart sound detecting device  903  described in the present embodiment to the blood pressure measuring device  904  based on the technical content described in the first embodiment. 
     The embodiments described above include the following applications. 
     (1) In the biological signal detecting device  902  shown in  FIG. 10 , the second π/2 phase shifting circuit  1010  may also be connected to the subsequent stage of the second LPF  1011 , instead of being connected to the subsequent stage of the first LPF  1009 . At this time, the output signal of the first LPF  1009  is “USB+LSB” (wherein USB means “Upper Side Band”, and LSB means “Lower Side Band”) in the synchronous detection circuit, and the output signal of the second π/2 phase shifting circuit  1010  connected to the subsequent stage of the second LPF  1011  is “USB−LSB”. The differential amplifier  1012  outputs the signal of USB.
 
(2) The blood pressure measuring system  101  of the first embodiment and the in-vehicle blood pressure measuring system  901  of the second embodiment have the same configuration in the following components:
 
     the biological signal detecting device  902  that outputs the heartbeat signal of the subject  103 ; 
     the heart sound detecting device  903  that outputs the heart sound signal of the subject  103 ; and 
     the blood pressure measuring device  904  that measures the blood pressure of the subject  103  using the heartbeat signal outputted from the biological signal detecting device  902  and the heart sound signal outputted from the heart sound detecting device  903 . 
     The biological signal detecting device  902  of the first embodiment is the heartbeat sensor  104  attached to the outer ear  103   a  of the subject  103 . 
     The heart sound detecting device  903  of the first embodiment is the heart sound microphone  105  attached to the chest of the subject  103 . 
     (3) The blood pressure measuring method disclosed in Patent document 1 is based on a technique in which a table that includes data indicating the correspondence relation between the amplitude of the heart sound and the blood pressure is created based on a fact that the amplitude of the heart sound has correlation with the blood pressure, and the amplitude of the heart sound is converted into blood pressure data by referring to such table. By using such technical content, it is possible to prepare a table that includes data indicating the correspondence relation between the frequency of a signal constituting the heart sound, instead of the amplitude of the heart sound, and the blood pressure, and convert the frequency component of the heart sound into the blood pressure data. In such case, since the conversion from the frequency component of the heart sound to the blood pressure data is achieved by extracting information associated with the frequency of the signal constituting the heart sound from the inside of the noise removal processing section  411 , the DCT inverse transformation processing section  503  shown in  FIG. 5A  (or the FFT inverse transformation processing section  515  shown in  FIG. 5B ) and the position restoration processing section  413  shown in  FIG. 4  can be eliminated. 
     In the present embodiment, the blood pressure measuring system has been described. 
     In the blood pressure measuring system  101  of the first embodiment, particularly the signal processing section  302 , which can remove the noises mixed into the heart sound signal by using the heart sound signal and the heartbeat signal, has been described. The signal processing device  306  detects the R-wave of the heartbeat signal accompanied by periodic fluctuation to obtain the average value of the RR interval. Further, the waveform of the heart sound signal, which periodically fluctuates in synchronization with the heartbeat signal, is forcibly repositioned at an interval equivalent to the average value of the RR interval. After performing the reposition processing, the noise is removed by using the orthogonal transformation and the orthogonal inverse transformation, and the position of the obtained waveform is restored to its original position. As described above, it is possible to effectively remove the noise components not associated with the heartbeat cycle by performing the reposition processing before and after the noise removing processing. 
     In the in-vehicle blood pressure measuring system  901  of the second embodiment, the biological signal detecting device  902 , which detects the heartbeat signal of the subject in a non-contact manner by using a radio wave, has been described. The radio wave transmitted from the oscillation source  1001  through the first antenna  907  is received each by the second antenna  909  and the third antenna  910 , and a synchronous detection is performed. The variation in impedance of the subject, i.e., the variation in impedance of the human body, causes an effect equivalent to performing amplitude modulation with respect to the unmodulated radio wave, and the gain fluctuation of a received radio wave caused due to the existence of the human body is cancelled out by the differential amplifier  1012 . Thus, it is possible to detect the heartbeat signal of the subject in a non-contact manner by using synchronous detection. 
     The embodiments of the present invention are described as above; however, it is to be understood that the present invention is not limited to the embodiments described above, and various modifications and applications can be made without departing from the spirit described in the claims of the present invention. 
     For example, in the aforesaid embodiments, the configurations of the device and system are described in detail and concrete manner so that the present invention is easily understandable; however, the aforesaid configurations do not have to be fully included. Further, configurations of one embodiment can be partly substituted with configurations of another embodiment, and configurations of one embodiment can be added to a configuration(s) of another embodiment. Further, configurations of one embodiment can be partly omitted, or added with other configuration(s), or substituted with other configurations. 
     Further, the aforesaid each configuration, function, processor and the like can be partly or entirely achieved by hardware by being designed using an integrated circuit, for example. Further, the aforesaid each configuration, function and the like can be achieved by software whose processor explains and executes a program that achieves respective functions. Information such as the program, tables, files and the like for achieving each function can be stored in a volatile or non-volatile storage, such as a memory, a hard disk, a SSD (solid state drive) or the like, or a recording medium, such as an IC card, an optical disk or the like. 
     Further, a control line and an information line are shown because it is required for description, but the product does not necessarily show the control line and information line. It can be considered that almost all configurations are actually connected with each other. 
     REFERENCE SIGNS LIST 
     
         
         
           
               101  blood pressure measuring system 
               102  sensor driving device 
               103  subject 
               104  heartbeat sensor 
               150  heart sound microphone 
               106  information processing device 
               201  CPU 
               202  ROM 
               203  RAM 
               204  first A/D converter 
               205  second A/D converter 
               206  first buffer 
               207  second buffer 
               208  bus 
               209  LED 
               210  photodiode 
               213  first operational amplifier 
               214  second operational amplifier 
               215  short-range wireless communication section 
               221  CPU 
               222  ROM 
               223  RAM 
               224  nonvolatile storage 
               225  display 
               226  operating section 
               227  short-range wireless communication section 
               228  bus 
               229  touch panel display 
               301  light emission controller 
               302  signal processing section 
               303  blood pressure calculator 
               304  input/output controller 
               401  heartbeat buffer 
               402  delay 
               403  heart sound buffer 
               407  heartbeat detector 
               408  reposition processing section 
               409  reposition buffer 
               410  interpolation processing section 
               411  noise removal processing section 
               412  processed buffer 
               413  position restoration processing section 
               501  DCT transformation processing section 
               502  coefficient filter 
               503  DCT inverse transformation processing section 
               511  FFT transformation processing section 
               512  vector operation processing section 
               513  high-pass filter 
               514  inverse vector operation processing section 
               515  FFT inverse transformation processing section 
               901  in-vehicle blood pressure measuring system 
               902  biological signal detecting device 
               903  heart sound detecting device 
               904  blood pressure measuring device 
               905  helical antenna 
               906  driver 
               907  first antenna 
               908  driver seat 
               909  second antenna 
               910  third antenna 
               911  steering 
               912  AC resistance detector 
               913  BPF 
               1001  oscillation source 
               1006  first mixer 
               1007  second mixer 
               1008  first π/2 phase shifting circuit 
               1009  first LPF 
               1010  second π/2 phase shifting circuit 
               1011  second LPF 
               1012  differential amplifier 
               1102  pulse wave generator 
               1103  BPF 
               1104  first RF amplifier 
               1105  directional coupler 
               1108  second RF amplifier 
               1109  third RF amplifier 
               1110  third mixer 
               1111  inverting amplifier 
               1112  fourth mixer 
               1113  buffer 
               1114  third LPF 
               1115  fourth LPF 
               1116  differential amplifier 
               1117  fifth LPF