Patent Publication Number: US-2022233112-A1

Title: Method, computer programming product and electronic device for calculating blood oxygen saturation

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of Taiwan Application No. 110102936, filed on Jan. 27, 2021, the entirety of which is incorporated by reference herein. 
     FIELD OF THE DISCLOSURE 
     The disclosure is related to a signal processing method, and in particular it is related to a method, a computer programming product, and an electronic device for calculating blood oxygen saturation. 
     DESCRIPTION OF THE RELATED ART 
     Ear-hook oximeters are set in the patient&#39;s ear, and use photoplethysmography (PPG) signals reflected by the skin of the ear to measure physiological information in the human body. In general, a PPG signal can be received by a light sensor that can receive green light, red light, and infrared light. The ear-hook oximeter uses the PPG signal to analyze and calculate data including heart rate, blood oxygen saturation, blood perfusion index (PI), etc. 
     However, if the user&#39;s ear structure is not easy to fit the ear-hook oximeter, or the signal light source does not touch the ear blood vessels, the quality of the PPG signal may be poor, and the physiological measurement data will be misjudged. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     In order to resolve the issue described above, the present disclosure provides a method for calculating blood oxygen saturation. The method for calculating blood oxygen saturation includes defining the extreme value point of a non-red light signal as the extreme value point of a red light signal; and calculating the blood oxygen saturation according to the extreme value point of the red light signal. 
     According to the method disclosed above, the method further includes filtering the red light signal to eliminate outliers of the red light signal. 
     According to the method disclosed above, the method further includes obtaining a peak value when the red light signal has a first wave crest; obtaining a first valley value and a second valley value when the red light signal has two wave troughs adjacent to the first wave crest; calculating the average value of the first valley value and the second valley value; and calculating the difference between the peak value and the average value to obtain the AC value of the red light signal. 
     According to the method disclosed above, the step of filtering the red light signal includes eliminating the peak value, wherein the peak value is less than the first valley value or the second valley value. 
     According to the method disclosed above, the step of filtering the red light signal includes eliminating an intermediate value, wherein the intermediate value corresponds to the red light signal between the first wave crest and one of the two troughs adjacent to the first wave crest; wherein the intermediate value is larger than the peak value. 
     According to the method disclosed above, the step of filtering the red light signal includes eliminating the AC value, wherein the AC value corresponds to the AC value at the first wave crest and the AC value is more than 1.5 times the average AC value of the previous three wave crests in the red light signal earlier than the first wave crest. 
     According to the method disclosed above, the step of filtering the red light signal includes eliminating the AC value, wherein the AC value corresponds to the AC value at the first wave crest and the AC value is less than 0.67 times the average AC value of the previous three wave crests in the red light signal earlier than the first wave crest. 
     According to the method disclosed above, the method further includes obtaining the DC value of the red light signal according to the average value of the first valley value and the second valley value; and calculating the blood oxygen saturation according to the AC value and the DC value. 
     According to the method disclosed above, a time point when a wave crest of the non-red light signal occurs is a time point when a wave crest of a green light signal occurs; a time point when a wave trough of the non-red light signal occurs is a time point when a wave trough of an infrared light signal occurs; wherein the green light signal has dicrotic pulses, and the red light signal and the infrared light signal do not have dicrotic pulses. 
     According to the method disclosed above, the red light signal and the non-red light signal are photoplethysmography (PPG) signals. 
     The present disclosure also provides a computer programing product. The computer programing product is suitable for being loaded by an electronic device with a processor and executing a method for calculating blood oxygen saturation. The computer programing product includes a crest-to-trough alignment instruction, and a calculation instruction. The crest-to-trough alignment instruction allows the processor to define the extreme value point of a non-red light signal as the extreme value point of a red light signal. The calculation instruction allows the processor to calculate the blood oxygen saturation according to the extreme value point of the red light signal. 
     According to the computer programing product disclosed above, the computer programing product further includes a filtering instruction. The filtering instruction allows the processor to filter the red light signal to eliminate outliers of the red light signal. 
     According to the computer programing product disclosed above, the computer programing product further includes a wave crest alignment instruction, a trough alignment instruction, a wave trough average instruction, and an AC value calculation instruction. The wave crest alignment instruction allows the processor to obtain a peak value, wherein the red light signal has a first wave crest. The trough alignment instruction allows the processor to obtain a first valley value and a second valley value, wherein the red light signal has two wave troughs adjacent to the first wave crest. The trough average instruction allows the processor to calculate the average value of the first valley value and the second valley value. The AC value calculation instruction allows the processor to calculate the difference between the peak value and the average value to obtain the AC value of the red light signal. 
     The present disclosure also provides an electronic device. The electronic device includes a light emitting unit, a light sensor, and a processor. The light emitting unit is configured to emit a red light signal and a non-red light signal to a skin. The light sensor is configured to receive the red light signal and the non-red light signal reflected from the skin. The processor is configured to execute instructions and calculations, including defining the extreme value point of a non-red light signal as the extreme value point of a red light signal, and calculating the blood oxygen saturation according to the extreme value point of the red light signal. 
     According to the electronic device disclosed above, the processor is configured to execute further instructions and calculations, including: obtaining the peak value when the red light signal has a first wave crest; obtaining the first valley value and the second valley value when the red light signal has two wave troughs adjacent to the first wave crest; calculating the average value of the first valley value and the second valley value; and calculating the difference between the peak value and the average value to obtain the AC value of the red light signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure can be more fully understood by reading the subsequent detailed description with references made to the accompanying figures. It should be understood that the figures are not drawn to scale in accordance with standard practice in the industry. In fact, it is allowed to arbitrarily enlarge or reduce the size of components for clear illustration. This means that many specific details, relationships and methods are disclosed to provide a complete understanding of the disclosure. 
         FIG. 1  is a schematic diagram of the absorption rate of oxyhemoglobin and hemoglobin in the blood to light of different wavelengths. 
         FIG. 2  is a flow chart of a method for calculating blood oxygen saturation in accordance with some embodiments of the disclosure. 
         FIG. 3  is a waveform diagram of green light signal  300 , red light signal  302 , and infrared light signal  304  in accordance with some embodiments of the disclosure. 
         FIG. 4  is a schematic diagram of calculating the AC value and DC value of a PPG signal  400  in accordance with some embodiments of the disclosure. 
         FIG. 5A  is a schematic diagram of a scene  1  of filtering a PPG signal  500  in accordance with some embodiments of the disclosure. 
         FIG. 5B  is a schematic diagram of a scene  2  of filtering a PPG signal  510  in accordance with some embodiments of the disclosure. 
         FIG. 5C  is a schematic diagram of a scene  3  of filtering a PPG signal  520  in accordance with some embodiments of the disclosure. 
         FIG. 6  is a block diagram of an electronic device in accordance with some embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     In order to make the above purposes, features, and advantages of some embodiments of the present disclosure more comprehensible, the following is a detailed description in conjunction with the accompanying drawings. 
     It should be understood that the words “comprise” and “include” used in the present disclosure are used to indicate the existence of specific technical features, values, method steps, operations, units and/or components. However, it does not exclude that more technical features, numerical values, method steps, work processes, units, components, or any combination of the above can be added. 
     The words “first” and “second” are used to describe components, they are not used to indicate the priority order of or advance relationship, but only to distinguish components with the same name. 
     The principle of photoplethysmography (PPG) is to pass through human tissue through a light source, and to receive continuous light signals from human tissue through a light sensor. When the light passes through human tissues, it is absorbed and attenuated by different human tissues. Therefore, the PPG signal is divided into two parts: direct current (DC) and alternating current (AC). Assuming that the composition of the human body is fixed, and the attenuation of light is also fixed, the DC value of the PPG signal is the absorbed part. The AC value of the PPG signal is a signal that is varied with changes of the volume of blood vessel. 
     The PPG signal mainly has three light sources: green light, red light and infrared light. Different light sources have different penetration depths of human tissues. For example, the penetration depth of green light is the shallowest, the penetration depth of red light is second deepest, and the penetration depth of infrared is the deepest.  FIG. 1  is a schematic diagram of an absorption rate of oxyhemoglobin and hemoglobin in the blood to light of different wavelengths. As shown in  FIG. 1 , curve  100  represents the absorption rate of hemoglobin in the blood to light of different wavelengths. Curve  102  represents the absorption rate of oxyhemoglobin in the blood to light of different wavelengths. Most of the green light with a wavelength of 530 nanometers is absorbed by red blood cells, so it can cause the attenuation of its light intensity and is suitable for calculating heart rate. Since hemoglobin and oxyhemoglobin have the most different absorption rate for red light with a wavelength of 660 nanometers and infrared light with a wavelength of 940 nanometers, for example, the absorption rate of hemoglobin to red light is higher than the absorption rate of oxyhemoglobin to red light, but the absorption rate of hemoglobin to infrared light is lower than the absorption rate of oxyhemoglobin to infrared light, red light and infrared light are suitable for calculating blood oxygen saturation. 
       FIG. 2  is a flow chart of a method for calculating blood oxygen saturation in accordance with some embodiments of the disclosure. As shown in  FIG. 2 , the method for calculating blood oxygen saturation of the embodiment of the disclosure first obtains a red light signal and a non-red light signal reflected from the skin. In some embodiments, the obtained red light signal and the obtained non-red light signal are reflected from the skin in the ear, but the present disclosure is not limited thereto. In step S 200 , the method for calculating blood oxygen saturation of the embodiment of the disclosure defines an extreme value point (for example, the wave crest or the wave trough) of the non-red light signal as an extreme value point of the red light signal. After that, in step S 202 , the method for calculating blood oxygen saturation of the embodiment of the disclosure calculates the blood oxygen saturation according to the extreme value point of the red light signal. 
     In some embodiments, the non-red light signal is a green light signal and/or an infrared light signal. In some embodiments, the red light signal and the non-red light signal are emitted by a light emitting unit of an ear-hook electronic device. For example, the light emitting unit of the ear-hook electronic device includes a green LED, a red LED, and an infrared LED to respectively emit corresponding green light signals, red light signals and infrared light signals.  FIG. 3  is a waveform diagram of green light signal  300 , red light signal  302 , and infrared light signal  304  in accordance with some embodiments of the disclosure. In some embodiments of  FIG. 3 , the horizontal axis is the number of samples, and the vertical axis is the amplitude (μV), wherein the number of samples can correspond to time. In some embodiments, the waveform diagram in  FIG. 3  is read from a light sensor of an ear-hook electronic device. The light sensor is used to receive the green light signal  300 , the red light signal  302 , and the infrared light signal  304  reflected from the skin. 
     As shown in  FIG. 3 , the green light signal  300 , the red light signal  302 , and the infrared light signal  304  all have peaks at time point t 1 , time point t 2 , time point t 3 , and time point t 4 . In other words, since the wave crest of the green light signal  300 , the wave crest of the red light signal  302 , and the wave crest of the infrared light signal  304  are aligned with each other at the time point t 1 , the time point t 2 , the time point t 3 , and the time point t 4 , the method for calculating the blood oxygen saturation in the embodiment of the present disclosure can use a time point when the green light signal  300  or the infrared light signal  304  has a wave crest, as a time point when the red light signal  302  has a wave crest. After that, the red light signal  302  and the infrared light signal  304  have wave troughs at time point t 5 , time point t 6 , time point t 7 , and the time point t 8 , but the green light  300  does not have wave troughs until time point t 5 ′, time point t 6 ′, time point t 7 ′, and time point t 8 ′. In other words, the wave trough of the red light signal  302  is aligned with the wave trough of the infrared light signal  304  at the time points t 5 , t 6 , t 7 , and d 8 , but the wave trough of the red light signal  302  is not aligned with the wave trough of the green light signal  300  at the time points t 5 , t 6 , t 7 , and d 8 . 
     In some embodiments, since the red light signal  302  and the infrared light signal  304  have dicrotic pulses of the physiological characteristic from the human body from the time points t 5  to t 5 ′, time points t 6  to t 6 ′, time points t 7  to t 7 ′ and time points t 8  to t 8 ′ (for example, dicrotic pulses  310  and  312  at time point t 8  to t 8 ′), the wave trough of the red light signal  302  and the wave trough of the infrared light signal  304  cannot be aligned with the wave trough of the green light signal  300 . In some embodiments, the dicrotic pulses come from the reflected shock wave formed by the blood hitting the aortic valve caused by the aortic blood pressure. Therefore, the method for calculating the blood oxygen saturation of the present disclosure uses the time point (for example, time points t 1 -t 4 ) when the non-red light signal (for example, the green light signal  300  or the infrared light signal  304 ) has a wave crest, as the time point when the red light signal  302  has a wave crest. Furthermore, the method for calculating the blood oxygen saturation of the embodiment of the disclosure uses the time point (for example, time points t 5 -t 8 ) when the non-red light signal (for example, the infrared light  304 ) has a wave trough, as the time point when the red light signal  302  has a wave trough. 
     In some embodiments, the green light signal  300  has a short wavelength and a shallow penetration depth, so the green light signal  300  may be directly absorbed by blood vessels. In other words, since human tissue has a good absorption rate of the green light signal  300 , the changes in blood flow that can be reflected by the green light signal  300  are more obvious, the amplitude of the green light signal  300  becomes larger (for example, the amplitude of the green light signal  300  in  FIG. 3  is greater than that of the red light signal  302  and the infrared light signal  304 ), and the interference may be smaller (that is, the higher signal-to-noise ratio (SNR)). The red light signal  302  and infrared light signal  304  have longer wavelengths, and their penetration depths are deeper than the penetration depth of the green light signal  300 , making them easier to hit bones and other human tissues. Thus, the red light signal  302  and the infrared light signal  304  may be more sensitive and more susceptible to interference. Therefore, the method for calculating the blood oxygen saturation of the embodiment of the disclosure uses the time point when the green light signal  300  has a wave crest, as the time point when the red light signal  302  has a wave crest. 
     In some embodiments, the light emitting unit of the ear-hook electronic device does not emit three light sources (such as the green light signal  300 , the red light signal  302 , and the infrared light signal  304 ) at the same time, but emits three light sources one by one according to the design of the firmware. However, since the switching time for the light emitting unit of the ear-hook electronic device to switch out the three light sources is very short (that is, the light source switching speed is very fast), the method for calculating the blood oxygen saturation in the embodiment of the present disclosure can achieve the waveform alignment method of step S 200 . The method for calculating the blood oxygen saturation in the embodiment of the disclosure first retrieves an extreme value of the PPG signal (for example, the green light signal  300 , the red light signal  302 , and the infrared light signal  304 ) to calculate the physiological measurement. For example, the wave crest of the green light signal  300  is used to calculate the heart rate, the wave crest and wave trough of the infrared light signal  304  are used to calculate the blood perfusion index, and the wave crest and wave trough of the red light signal  302  and the infrared light signal  304  are used to calculate the blood oxygen saturation. 
     Table 1 is a statistical table of embodiments of the method for calculating the blood oxygen saturation of the present disclosure using the time point of the wave crest or wave trough of auxiliary signals (for example, the green light signal  300  and the infrared light signal  304  in  FIG. 1 ) as the time point of the wave crest or wave trough of the red light signal  302  in  FIG. 1 . 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Auxiliary signals selection 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Wave crest 
                 Wave trough 
                 Calculation 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Good 
                 Worse 
                 Middle 
                 Good 
                 Worse 
                 Middle 
                 result of 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Extreme value 
                 (green 
                 (red 
                 (infrared 
                 (green 
                 (red 
                 (infrared 
                 blood oxygen 
               
               
                 Absorption rate 
                 light) 
                 light) 
                 light) 
                 light) 
                 light) 
                 light) 
                 saturation 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 embodiments 
                 1 
                 ✓ 
                   
                   
                 ✓ 
                   
                   
                 91% 
               
               
                   
                 2 
                 ✓ 
                   
                   
                   
                 ✓ 
                   
                 95% 
               
               
                   
                 3 
                 ✓ 
                   
                   
                   
                   
                 ✓ 
                 98% 
               
               
                   
                 4 
                   
                 ✓ 
                   
                 ✓ 
                   
                   
                 88% 
               
               
                   
                 5 
                   
                 ✓ 
                   
                   
                 ✓ 
                   
                 95% 
               
               
                   
                 6 
                   
                 ✓ 
                   
                   
                   
                 ✓ 
                 96% 
               
               
                   
                 7 
                   
                   
                 ✓ 
                 ✓ 
                   
                   
                 89% 
               
               
                   
                 8 
                   
                   
                 ✓ 
                   
                 ✓ 
                   
                 93% 
               
               
                   
                 9 
                   
                   
                 ✓ 
                   
                   
                 ✓ 
                 99% 
               
               
                   
               
               
                 Remarks: 
               
               
                 The blood oxygen saturation is calculated by averaging the data from measurement device for wearing 30 seconds. 
               
            
           
         
       
     
     The reference standard for the blood oxygen saturation in Table 1 is a fingertip pulse oximeter, which is a medical product. As shown in Table 1, in an embodiment 1, the method for calculating the blood oxygen saturation of the present disclosure defines the time point when the green light signal  300  has a wave crest, as the time point when the red light signal  302  has a wave crest in step S 200 , and defines the time point when the green light signal  300  has a wave trough, as the time point when the red light signal  302  has a wave trough in step  200 . According to  FIG. 3 , since the wave trough of the green light signal  300  is not aligned with the wave trough of the red light signal  302 , the accuracy rate of the blood oxygen saturation calculated in step S 202  of the method for calculating the blood oxygen saturation of the present disclosure is 91%. In an embodiment 2, the method for calculating the blood oxygen saturation of the present disclosure defines the time point when the green light signal  300  has a wave crest, as the time point when the red light signal  302  has a wave crest in step S 200 , but any time point of auxiliary signals is not used as the time point when the red light signal  302  has a wave trough, that is, the time point when the red light signal  302  has the wave trough is directly used. Therefore, the accuracy rate of the blood oxygen saturation calculated in step S 202  of the method for calculating the blood oxygen saturation of the present disclosure is 95%. 
     In an embodiment 3, the method for calculating the blood oxygen saturation of the present disclosure defines the time point when the green light signal  300  has a wave crest, as the time point when the red light signal  302  has a wave crest in step S 200 , and defines the time point when the infrared light signal  304  has a wave trough, as the time point when the red light signal  302  has a wave trough in step  200 . Therefore, the accuracy rate of the blood oxygen saturation calculated in step S 202  of the method for calculating the blood oxygen saturation of the present disclosure is 98%. In an embodiment 4, the method for calculating the blood oxygen saturation of the present disclosure does not define any time point of auxiliary signals, as the time point when the red light signal  302  has a wave crest, and defines the time point when the green light signal  300  has a wave trough, as the time point when the red light signal  302  has a wave trough. According to  FIG. 3 , since the wave trough of the green light signal  300  is not aligned with the wave trough of the red light signal  302 , the accuracy rate of the blood oxygen saturation calculated in step S 202  of the method for calculating the blood oxygen saturation of the present disclosure is 88%. 
     In an embodiment 5, the method for calculating the blood oxygen saturation of the present disclosure does not define any time point of auxiliary signals, as the time point when the red light signal  302  has a wave crest, and also does not use any time point of auxiliary signals, as the time point when the red light signal  302  has a wave trough. In other words, the method for calculating the blood oxygen saturation of the present disclosure directly calculates the blood oxygen saturation according to the wave crest and the wave trough of the red light signal  302 , and the accuracy rate of the calculated blood oxygen saturation is 95%. In an embodiment 6, the method for calculating the blood oxygen saturation of the present disclosure does not define any time point of auxiliary signals, as the time point when the red light signal  302  has a wave crest, and defines the time point when the infrared light signal  304  has a wave trough, as the time point when the red light signal  302  has a wave trough in step  200 . According to  FIG. 3 , since the wave trough of the infrared light signal  304  is aligned with the wave trough of the red light signal  302 , the accuracy rate of the blood oxygen saturation calculated in step S 202  of the method for calculating the blood oxygen saturation of the present disclosure is 95%. 
     In an embodiment 7, the method for calculating the blood oxygen saturation of the present disclosure defines the time point when the infrared light signal  304  has a wave crest, as the time point when the red light signal  302  has a wave crest in step S 200 , and defines the time point when the green light signal  300  has a wave trough, as the time point when the red light signal  302  has a wave trough in step S 200 . According to  FIG. 3 , since the wave trough of the green light signal  300  is not aligned with the wave trough of the red light signal  302 , the accuracy rate of the blood oxygen saturation calculated in step S 202  of the method for calculating the blood oxygen saturation of the present disclosure is 89%. In an embodiment 8, the method for calculating the blood oxygen saturation of the present disclosure defines the time point when the infrared light signal  304  has a wave crest, as the time point when the red light signal  302  has a wave crest in step S 200 , but does not define any time point of auxiliary signals, as the time point when the red light signal  302  has a wave trough, that is, the time point when the red light signal  302  has the wave trough is directly used. Therefore, the accuracy rate of the blood oxygen saturation calculated in step S 202  of the method for calculating the blood oxygen saturation of the present disclosure is 93%. 
     In an embodiment 9, the method for calculating the blood oxygen saturation of the present disclosure defines the time point when the infrared light signal  304  has a wave crest, as the time point when the red light signal  302  has a wave crest in step S 200 , and defines the time point when the infrared light signal  304  has a wave trough, as the time point when the red light signal  302  has a wave trough in step S 200 . According to  FIG. 3 , since the wave crest and the wave trough of the infrared light signal  304  are all aligned with the wave crest and the wave trough of the red light signal  302 , the accuracy rate of the blood oxygen saturation calculated in step S 202  of the method for calculating the blood oxygen saturation of the present disclosure is 99%. According to the embodiments 1-9, the method for calculating the blood oxygen saturation of the present disclosure can use a light source with a better absorption rate (for example, the green light signal  300  and/or the infrared light signal  304 ) to assist a light source with a poor absorption rate (for example, the red light signal  302 ). 
       FIG. 4  is a schematic diagram of calculating the AC value and DC value of a PPG signal  400  in accordance with some embodiments of the disclosure. As shown in  FIG. 4 , in order to obtain the AC value and the DC value of the PPG signal  400  (for example, the green light signal  300 , the red light signal  302 , and the infrared light signal  304  in  FIG. 1 ), the method for calculating the blood oxygen saturation of the present disclosure first obtains a peak value (X P1 , Y P1 ) of the PPG signal  400  at a wave crest {circle around (1)}. Then, the method for calculating the blood oxygen saturation of the present disclosure obtains a valley value (X V1 , Y V1 ) of the PPG signal  400  at a wave trough {circle around (2)} and a valley value (X V2 , Y V2 ) of the PPG signal  400  at a wave trough {circle around (3)}. After that, the method for calculating the blood oxygen saturation of the present disclosure calculates the average value (X′, Y′) of the valley value (X V1 , Y V1 ) of the PPG signal  400  at the wave trough {circle around (2)} and the valley value (X V2 , Y V2 ) of the PPG signal  400  at the wave trough {circle around (3)}. In some embodiments, the method for calculating the blood oxygen saturation of the present disclosure uses equation 1 to calculate a slope M from the valley value (X V1 , Y V1 ) to the valley value (X V2 , Y V2 ). 
     
       
         
           
             
               
                 
                   M 
                   = 
                   
                     
                       
                         Y 
                         
                           V 
                           ⁢ 
                           2 
                         
                       
                       - 
                       
                         Y 
                         
                           V 
                           ⁢ 
                           1 
                         
                       
                     
                     
                       
                         X 
                         
                           V 
                           ⁢ 
                           2 
                         
                       
                       - 
                       
                         X 
                         
                           V 
                           ⁢ 
                           1 
                         
                       
                     
                   
                 
               
               
                 
                   equation 
                   ⁢ 
                       
                   1 
                 
               
             
           
         
       
     
     Referring to  FIG. 4 , since X′=X P1 , Y′ can be obtained through the slope M. Finally, the method for calculating the blood oxygen saturation of the present disclosure calculates the difference between the peak value (X P1 , Y P1 ) and the average value (X′, Y′) to obtain the AC value of the PPG signal  400 , for example, the AC value is equal to Y P1 −Y′. Then, the method for calculating the blood oxygen saturation of the present disclosure calculates the difference between the average value (X′, Y′) and the X axis (that is y=0) to obtain a DC value of the PPG signal  400 , for example, the DC value is equal to Y′. 
     In some embodiments, the method for calculating the blood oxygen saturation of the present disclosure uses equation 2 to calculate the blood oxygen saturation (SpO2). 
     
       
         
           
             
               
                 
                   
                     blood 
                     ⁢ 
                         
                     oxygen 
                     ⁢ 
                         
                     saturation 
                     ⁢ 
                         
                     
                       ( 
                       
                         Sp 
                         ⁢ 
                         O 
                         ⁢ 
                         2 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       1 
                       ⁢ 
                       1 
                       ⁢ 
                       5 
                     
                     - 
                     
                       2 
                       ⁢ 
                       5 
                       × 
                       
                         
                           
                             R 
                             
                               a 
                               ⁢ 
                               c 
                             
                           
                           
                             R 
                             
                               d 
                               ⁢ 
                               c 
                             
                           
                         
                         
                           
                             IR 
                             
                               a 
                               ⁢ 
                               c 
                             
                           
                           
                             IR 
                             
                               d 
                               ⁢ 
                               c 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   equation 
                   ⁢ 
                       
                   2 
                 
               
             
           
         
       
     
     In equation 2, R ac  is the AC value of the red light signal  302 , R dc  is the DC value of the red light signal  302 , IR ac  is the AC value of the infrared light signal  304 , and IR dc  is the DC value of the infrared light signal  304 . 
     In some embodiments, the method for calculating the blood oxygen saturation of the present disclosure uses equation 3 to calculate blood perfusion index. The blood perfusion index is an index used to observe blood flow. 
     
       
         
           
             
               
                 
                   
                     blood 
                     ⁢ 
                         
                     perfusion 
                     ⁢ 
                         
                     index 
                   
                   = 
                   
                     
                       IR 
                       
                         a 
                         ⁢ 
                         c 
                       
                     
                     
                       IR 
                       
                         d 
                         ⁢ 
                         c 
                       
                     
                   
                 
               
               
                 
                   equation 
                   ⁢ 
                       
                   3 
                 
               
             
           
         
       
     
     In equation 3, IR ac  is the AC value of the infrared light signal  304 , and IR dc  is the DC value of the infrared light signal  304 . 
     After step S 200  is completed and before step S 202  is performed, the method for calculating the blood oxygen saturation of the present disclosure further filters the red light signal  302  to eliminate outliers of the red light signal  302 .  FIG. 5A  is a schematic diagram of a scene  1  of filtering a PPG signal  500  in accordance with some embodiments of the disclosure. As shown in  FIG. 5A , the PPG signal  500  (for example, the red light signal  302 ) in the embodiment of the present disclosure has a wave crest  502 , a wave crest  504 , a wave crest  506 , and a wave crest  508 . At the wave crest  502 , the PPG signal  500  has an AC value (AC 1 ). At the wave crest  504 , the PPG signal  500  has an AC value (AC 2 ). At the wave crest  506 , the PPG signal  500  has an AC value (AC 3 ). At the wave crest  508 , the PPG signal  500  has an AC value (AC). When the AC value (AC) of the PPG signal  500  at the wave crest  508  is more than 1.5 times the average value (for example, the average value is equal to (AC 1 +AC 2 +AC 3 )/3) of the AC value (AC 1 ) at the wave crest  502 , the AC value (AC 2 ) at the wave crest  504 , and the AC value (AC 3 ) at the wave crest  506 , the method for calculating the blood oxygen saturation of the present disclosure eliminates the AC value (AC) at the wave crest  508 . In other words, the method for calculating the blood oxygen saturation of the present disclosure does not use the AC value (AC) at the wave crest  508  as the basis for calculating the blood oxygen saturation in step S 202 . 
       FIG. 5B  is a schematic diagram of a scene  2  of filtering a PPG signal  510  in accordance with some embodiments of the disclosure. As shown in  FIG. 5B , the PPG signal  500  (for example, the red light signal  302 ) in the embodiment of the present disclosure has a wave crest  512 , a wave crest  514 , a wave crest  516 , and a wave crest  518 . At the wave crest  512 , the PPG signal  500  has an AC value (AC 1 ). At the wave crest  514 , the PPG signal  500  has an AC value (AC 2 ). At the wave crest  516 , the PPG signal  500  has an AC value (AC 3 ). At the wave crest  518 , the PPG signal  500  has an AC value (AC). When the AC value (AC) of the PPG signal  500  at the wave crest  518  is less than 0.67 times the average value (for example, the average value is equal to (AC 1 +AC 2 +AC 3 )/3) of the AC value (AC 1 ) at the wave crest  512 , the AC value (AC 2 ) at the wave crest  514 , and the AC value (AC 3 ) at the wave crest  516 , the method for calculating the blood oxygen saturation of the present disclosure eliminates the AC value (AC) at the wave crest  518 . In other words, the method for calculating the blood oxygen saturation of the present disclosure does not use the AC value (AC) at the wave crest  518  as the basis for calculating the blood oxygen saturation in step S 202 . 
       FIG. 5C  is a schematic diagram of a scene  3  of filtering a PPG signal  520  in accordance with some embodiments of the disclosure. As shown in  FIG. 5C , the PPG signal  520  (for example, the red light signal  302 ) in the embodiment of the present disclosure has a wave crest  522 , a wave trough  524 , and a wave trough  526 . When the AC value of the PPG signal  520  at a point  528  between the wave crest  522  and the wave trough  524  is greater than the AC value of the PPG signal  520  at the wave crest  522  (that is, the peak value), or the AC value of the PPG signal  520  at a point  530  between the wave crest  522  and the wave trough  526  is greater than the AC value of the PPG signal  520  at the wave crest  522 , the method for calculating the blood oxygen saturation of the present disclosure eliminates the AC value at the point  528  or the point  530  in the PPG signal  520 . In other words, the method for calculating the blood oxygen saturation of the present disclosure does not use the AC value at the point  528  or the point  530  as the basis for calculating the blood oxygen saturation in step S 202 . 
     In some embodiments, as shown in  FIG. 5C , when the AC value at the wave crest  522  (that is, the peak value) is less than or equal to the AC value at the wave trough  524  or the AC value at the wave trough  526 , the AC value at the wave crest  522  is eliminated. In other words, the method for calculating the blood oxygen saturation of the present disclosure does not use the AC value at the wave crest  522  as the basis for calculating the blood oxygen saturation in step S 202 . 
     The embodiment of the present disclosure also discloses a computer programming product suitable for an electronic device with a processor. In some embodiments, the electronic device executes a crest-to-trough alignment instruction, and a calculation instruction. The crest-to-trough alignment instruction enables the processor to execute step S 200  in  FIG. 2 . The calculation instruction enables the processor to execute step S 202  in  FIG. 2 . After the execution of the crest-to-trough alignment instruction is completed, and before the calculation instruction is executed, the electronic device of the embodiment of the present disclosure further executes a filtering instruction. The filtering instruction enables the processor to filter the red light signal  302  in the manner of scene  1  in  FIG. 5A , scene  2  in  FIG. 5B  and scene  3  in  FIG. 5C , so as to eliminate outliers of the red light signal  302 . 
     In some embodiments, the electronic device of the embodiment of the disclosure further executers a wave crest alignment instruction, a wave trough alignment instruction, a wave trough average instruction, and an AC value calculation instruction. The wave crest alignment instruction enables the processor to obtain a peak value when the red light signal has a first wave crest (for example, the peak value (X P1 , Y P1 ) of the PPG signal  400  at the wave crest {circle around (1)} in  FIG. 4 ). The wave trough alignment instruction enables the processor to obtain a first valley value and a second valley value when the red light signal has two wave troughs adjacent to the first wave crest (for example, the valley value (X V1 , Y V1 ) of the PPG signal  400  at the wave trough {circle around (2)} and the valley value (X V2 , Y V2 ) of the PPG signal  400  at the wave trough {circle around (3)} in  FIG. 4 ). The wave trough average instruction enables the processor to calculate the average value of the first valley value and the second valley value (for example, calculating the average value (X′, Y′) of the valley value (X V1 , Y V1 ) of the PPG signal  400  at the wave trough {circle around (2)} and the valley value (X V2 , Y V2 ) of the PPG signal  400  at the wave trough {circle around (3)} in  FIG. 4 ). The AC value calculation instruction enables the processor to calculate the difference between the peak value and the average value to obtain the AC value of the red light signal (for example, calculating the difference between the peak value (X P1 , Y P1 ) and the average value (X′, Y′) to obtain the AC value of the PPG signal  400  in  FIG. 4 ). 
     The embodiment of the present disclosure also discloses an electronic device.  FIG. 6  is a block diagram of an electronic device  600  in accordance with some embodiments of the disclosure. In some embodiments, the electronic device  600  is an ear-hook electronic device. The electronic device  600  includes a light emitting unit  604 , a light sensor  606 , and a processor  602 . The light emitting unit  604  is configured to emit a red light signal  610  and at least one non-red light signal  612  to a skin  620 . In some embodiments, the light emitting unit  604  includes a green LED, a red LED, and an infrared LED (not shown), but the present disclosure is not limited thereto. The light sensor  606  is configured to receive the red light signal  610 ′ (for example, the red light signal  302 ) and the at least one non-red light signal  612 ′ (for example, the green light signal  300  and/or the infrared light signal  304 ) reflected from the skin  620 . In some embodiments, the light sensor  606  transmits the received light signals to the processor  602  in digital form. Then, the processor  602  executes steps S 200  and S 202  in  FIG. 2 . In some embodiments, after the processor  602  completes step S 200  and before step S 202 , the processor  602  further filters the reflected red light signal  610 ′ in the manner of scene  1  in  FIG. 5A , scene  2  in  FIG. 5B , and scene  3  in  FIG. 5C  to eliminate outliers of the reflected red light signal  610 ′. 
     In some embodiments, the processor  602  executes further tasks including: obtaining a peak value when the red light signal  610 ′ has a first wave crest (for example, the peak value (X P1 , Y P1 ) of the PPG signal  400  at the wave crest {circle around (1)} in  FIG. 4 ); obtaining a first valley value and a second valley value when the red light signal  610 ′ has two wave troughs adjacent to the first wave crest (for example, the valley value (X V1 , Y V1 ) of the PPG signal  400  at the wave trough {circle around (2)} and the valley value (X V2 , Y V2 ) of the PPG signal  400  at the wave trough {circle around (3)} in  FIG. 4 ); calculating the average value of the first valley value and the second valley value (for example, calculating the average value (X′, Y′) of the valley value (X V1 , Y V1 ) of the PPG signal  400  at the wave trough {circle around (2)} and the valley value (X V2 , Y V2 ) of the PPG signal  400  at the wave trough {circle around (3)} in  FIG. 4 ); and calculating the difference between the peak value and the average value to obtain the AC value of the red light signal  610 ′ (for example, calculating the difference between the peak value (X P1 , Y P1 ) and the average value (X′, Y′) to obtain the AC value of the PPG signal  400  in  FIG. 4 ). 
     In some embodiments, before executing step S 200 , the processor  602  of the electronic device  600  of the present disclosure further executes a wearing stability judgment to detect whether the user is in a static state. If the user is not in the static state (for example, the user&#39;s movement is too large), the processor  602  may not perform step S 200 . In some embodiments, the processor  602  of the electronic device  600  of the present disclosure executes a dynamic timing warping (DTW). In short, the processor  602  first receives a template waveform from the PPG signal, and uses the template waveform as a reference. After that, when the processor  602  receives the subsequent PPG signal from the light sensor  606 , the processor  602  may match the waveform of each subsequent PPG signal with the template waveform to determine whether each PPG signal is a good signal. 
     The embodiments of the present disclosure are disclosed above, but they are not used to limit the scope of the present disclosure. A person skilled in the art can make some changes and retouches without departing from the spirit and scope of the embodiments of the present disclosure. Therefore, the scope of protection in the present disclosure shall be deemed as defined by the scope of the attached claims.