Research on electronic stethoscope system and signal processing algorithm

Stethoscopes have an important role in non-invasive diagnosis of cardiovascular and respiratory diseases, digestive diseases, and other kinds of diseases. The emergence of high-end diagnostic devices and new diagnostic methods have caused the status of the stethoscope to decline. However, stethoscope has the advantages of simple operation, mature auscultation theory and low cost, and thus is still widely used in medical diagnosis. This paper first introduces the design and application of electronic stethoscope solutions based on contact sensors and air coupling sensors, and then introduces advanced algorithms for digital signal processing for the diagnosis and treatment of different diseases, including heart sound noise reduction algorithm, heart sound segmentation algorithm and heart sound feature extraction and recognition algorithm. Finally, this paper summarizes the application of the electronic stethoscope system in medical testing, and its future development direction. In summary, the electronic stethoscope system is a reliable medical testing tool, which can convert sound signals into digital signals through complex signal processing algorithms for more accurate detection of human physiological parameters. The research of this paper will be of great value to the research and application of electronic stethoscopes.


Introduction
In 2018, cardiovascular disease accounting for 46.66% and 43.81% of all mortalities in rural and urban area in China respectively, which ranked the first in the causes of all death [1].Cardiovascular diseases have long been a major factor affecting people's health, while finding the right diagnosis for them has always been difficult.Rene Laennec invented the first stethoscope in 1816.It has become one of the most common and practical tools used in clinical diagnosis for auscultation and saves many people's lives.However, as people attach more importance to their health and complicated diseases emerge one after another, stethoscopes with better accuracy and functions are in great demand.Currently, two types of stethoscopes are being used widely: electronic and acoustic [2].
Compared with an electronic one, an acoustic stethoscope is more demanding in the auscultation skills and abundant clinical experience of doctors and has great subjectivity.Doctors must be highly sensitive to the intensity of heart sounds and murmurs that are generally subtle and even below the threshold of hearing [2], making it difficult to give precise diagnoses.What's more, congenital factors and acquired environment differentiate human hearing, which means that different doctors may contradict with the same symptom when using acoustic stethoscope.
With the development of electronic and signal processing techniques, the introduction and the maturity of electronic stethoscope system has overcome some of the issues to a great extent.The heart sounds are collected and converted into electrical signals by analog/digital (A/D) converter.The converted signals are then amplified and processed by multiple modules integrated with algorithms to get specific results such as the frequency response and the signal-noise Ratio.Meanwhile, the signal processing algorithm is of great importance, because it is the key methodology to give correct information about the heart sounds which would be eventually visualized on the digital screen for doctors to interpret.Combining electronic stethoscope system with fast and accurate algorithm lowers doctors' requirement for clinical experience and hearing ability.The main purpose of this research is to introduce new types of electronic stethoscopes and relevant signal processing algorithms.The content can be divided into three parts: Section I provides solutions of electronic stethoscopes based on different types of sensors, including practical applications for different clinical situations and effects.Section II shows advanced digital signal processing algorithms used for diagnosis and treatment of different diseases.Section III makes conclusions about the topic and its future development.

Research on Hardware of Electronic Stethoscope System
To tackling the accuracy of the captured signals, the scientists turn to the electronics technology for help.The past decades have witnessed the unprecedent revolution of the electronics technology.The unmatched signal analysis capability that can distinct trivial differences in various signals meets people's expectation for a new generation of stethoscope.As a result, the electronic stethoscope was devised.In the following part, we will introduce the hardware of the electronic stethoscope.The hardware is comprised of the sensor segment and mechanical design.

The Sensor Segment
In the electronic segment, the sensor segment is the foremost one.We need to convert lung sound signals into corresponding electrical signals for lung sound monitoring devices.But because of the vibration amplitude of the signal on the skin surface is trivial, and in some cases noise interferes with it far more than the effective signal.Therefore, the sensor should have small size, light weight, low cost, high sensitivity, strong ability to resist external interference.In order to meeting this demand, the choice of sensing unit is very significant.In the design of electronic stethoscope, there are mainly two different sensitive methods [3][4][5]: one is the use of contact sensors (such as piezoelectric sensors), and the other is the use of air coupling sensors (such as moving coil microphone, electret capacitor microphone, etc.).From the Figure 1.we can see the general structure of the two different sensor types and their main differences.In the following part, we will introduce two different types of sensor segment.One is MEMS acoustic sensor based on air coupled sensor and the other is piezoelectric transducer (PTZ) voltage sensor based on contact sensor.

Figure 1.
The general structure of air coupled sensor and contact sensor structure [6].New types of MEMS electronic stethoscopes that use piezoelectric bimorph transducer have been used widely due to its outstanding ability in clinical diagnosis.To solve the problem that the traditional electronic stethoscope unable to perceive the signal of low-frequency vibration when collecting the heart sound, Chunxiu Kong developed a new type of MEMS electronic stethoscope system [7].Adding an inertial mass block to the vibration membrane can collect signals of both vibration and acoustic to gain more accurate information.The system is divided into 3 modules: module of main control, module of signal acquisition and module of signal processing.

Figure 2.
Picture of MEMS electronic stethoscope system structure [7].The main control module comprises sensor, core-control module and some signal preprocessing circuits.After the signal has been preprocessed.The signal acquisition module makes some transformations on the signal to be eventually sent to the signal processing module.A low-pass amplification filter circuit keeps the frequency from 0-1500Hz of cardiopulmonary vibration(see figure 2).It is calculated that the signal-to-noise ratio (SNR) of the developed stethoscope is 8.6dB, which is 24% higher comparing with American standard 3M electronic stethoscope, indicates that the collected auscultation signals meet the requirements of application [8].
The PTZ voltage sensor uses piezoelectric element to sample the vibration of the body and converts the vibration to the electronic signals.This sensor has various merits.Compared to the air coupled sensor, The contact sensor and the stethoscope shell are usually connected by mechanical coupling [6].When the piezoelectric sensor is attached to the surface of the human body, the mechanical motion generated by the contraction and expansion of the lung causes local strain in the active piezoelectric element, which causes the relative displacement of the corresponding internal charge on the surface of the material.Then it generates electrical output proportional to the displacement of the chest wall.Therefore, the chest wall vibration can be used in this way to directly sense the internal sound signal.
Although the signal output of the two sensing elements is related to the chest wall contact, the transmitted waveform's characteristics will differ due to the different coupling modes.For air-coupled sensors, the interference of the patch-diaphragm transmission path and the air conduction.medium is easy to cause the microphone to over-pick up the environmental noise, resulting in inefficient acoustic energy transmission.Piezoelectric sensors can be ideal for harsh environment monitoring.They not only have small energy loss in the process of acoustic transmission, but are very suitable for measuring small stress changes [9].In terms of sensitivity, they are also significantly better than aircoupled sensors.The contact piezoelectric sensor is a durable, highly sensitive, and low-noise effective solution that can be used as an ideal material for detecting vital signs such as lung sounds and respiratory rate of patients.
In order to ensuring the accuracy of the sampling vibration, we also need to choose the most congenial type of PTZ.At present, the most common PZT piezoelectric ceramics mainly include three kinds, namely PZT-4, PZT-5 and PZT-8.From the Table 1, we can see the parameters and differences of several material properties, and make a comparison.Apart from various parameters, we also need to understand the mechanism of the electromechanical coupling system model.Therefore, we can ensure the best choice of the PTZ piezoelectric ceramics.The σs represents the vibration signal transmitted from the lung area to the inner surface of the thorax, which is caused by the respiratory tract and lung gas.The piezoelectric sensors placed outside the chest wall surface, the relationship between the sensor output signal (V (t)) and strain (S (t)) can be expressed as: ε, Cp, tc and d 33 denote the dielectric constant, elastic constant, thickness, and piezoelectric strain constant, respectively.S (t) represents the internal signal σs effective stress mapped to the outer surface of the chest.
We can draw a conclusion from the TABLE 1 and the equation 1-1.The PZT-4 and PZT-8 are called the hard ceramics.They have high mechanical quality factor, low piezoelectric strain constant and low relative dielectric constant, which are especially suitable for input excitation of strong electric field and high mechanical amplitude, and are often used as emission transducers.Soft ceramics (PZT-5H and PZT-5A), with low mechanical quality factor, high electromechanical coupling coefficient and high piezoelectric strain constant, are very suitable for low power resonant sensors and non-resonant sensors, and are often used as receiving transducers.So, in designing the electronic and intelligent stethoscope, we decided to choose PZT-5H as an important sensitive element in this design because the element can be well used as a silent electrical conversion device.

The Mechanical Design of the Electronic Stethoscope
Based on the user centered design methodology [11], the mechanical design of the electronic stethoscope is constantly being improved to meet the requirements as the accuracy of the captured signals increases.The primary model has four screws to fix the PCB.From the Figure 3., we can see the structure more clearly.The microphone serving as the electronic part was placed at the center.Because of the lack of electronic elements and out-of-date style, this model was obsoleted.[12].Apart from those designs, there is also a modified prototype model.From the Figure 5., we can see the main structure of the modified prototype.This design was devised to meet some potential demands.It equips with dual-path.One path is an acoustic path that allows the doctor to listen to the sound; the other is the electronic sensing path, which makes the generation of electronic signals possible [12].In this way, while the generation of electronic signals occurs, the user can simultaneously obtain the heart sound by ears.Therefore, the users will have more available information to diagnose the potential diseases they may have.

Research Status of Signal Processing Algorithms at Home and Abroad
In recent years, in heart sound analysis, many scholars have used modern signal processing technology to study heart sound technology.This chapter focuses on three main links of an abnormal heart sound recognition system: heart sound noise reduction, heart sound segmentation, and heart sound recognition.

Heart Sound Noise Reduction Algorithm
The interference in collecting heart sounds mainly comes from the external environment noise, and environmental noise refers to all environmental noise entering from outside the stethoscope, such as hospital broadcast voice, human conversation and operating noise of medical equipment.Since the distribution of heart sounds and ambient noise in frequency is similar, traditional filtering algorithms cannot suppress noise while ensuring that heart sound signals are not distorted.Therefore, a dualchannel adaptive filtering method can be considered to achieve heart sound reduction: an additional microphone is used to collect ambient sounds, and a dual-channel signal is formed with the heart sound acquisition channel, and then a dual-microphone noise reduction algorithm is used to extract a pure cardiac sound signal.

Nature
The wind, the wind blowing leaves, the rain,the thunder..
After comparing the advantages, disadvantages, and scope of application of various noise reduction algorithms, Shunzhan He et al. chose the dual-microphone adaptive filtering algorithm as the filtering algorithm of the electronic stethoscope, and improved the algorithm according to the characteristics of the heart sound signal, accelerated its convergence speed and reduced the computational complexity, so as to improve the stability and noise reduction effect of the algorithm(see Table 2) [13].
In addition to the traditional dual-microphone adaptive NLMS filtering and noise reduction technology, other noise reduction methods include low-pass filtering and wavelet noise reduction.Zhuan He et al. compared the two algorithms of computer-assisted semen analysis (CASA) and normalized least-mean-square (NLMS), and proposed a CASA cardiopulmonary sound enhancement algorithm using dual microphones for the noise reduction of stethoscopes, which can directly select the target signal from the noisy signal by using IID characteristic information and masking principle [14].Compared with the traditional dual-microphone adaptive NLMS algorithm, this method has better subjective and objective effects on cardiopulmonary noise reduction, so it has greater practical application value.

Heart Sound Segmentation Algorithm
Generally, a heart sound signal sample is composed of multiple complete heart cycles, and each can be divided into four stages.The purpose of the heart sound segmentation algorithm is to segment a heart and heart sound analysis in recent years, and puts forward some solutions.It briefly introduces the system of electronic stethoscope based on MEMS and PZT sensors and its related mechanical structure and software design.It also introduces the three most important links in heart sound recognition system and their related algorithms, introducing the latest research achievements of domestic and foreign scholars.In short, electronic stethoscope is an advanced medical device.It can amplify the weak body sound signal collected by the sensor, and use computer and mobile intelligent devices to realize the analysis and processing of body sound signal, providing doctors with consultable diagnostic results.Its use can improve the diagnosis and treatment of patients, reduce medical costs and improve the satisfaction of patients.Although the development history of electronic stethoscope is short, noise performance is poor, operation speed is slow, reliability is poor, etc., it is expected that with the continuous development of artificial intelligence and 3D printing technology, more and more people will use electronic stethoscope to assist in the diagnosis and prevention of diseases.

Figure 3 .
Figure 3. Structure of the primary model[12].Then the advanced design with no screws was devised.More electronic components were integrated on the PCB board and protected by a nicely touched cover.Therefore, more electronic elements can improve the accuracy of the captured signals and the protect cover can prevent the stethoscope from interference of external environments.The Figure4. and Figure5.Can show the advanced model's main structure.

Figure 4 .
Figure 4. Structure of the advanced model[12].Apart from those designs, there is also a modified prototype model.From the Figure5., we can see the main structure of the modified prototype.This design was devised to meet some potential demands.It equips with dual-path.One path is an acoustic path that allows the doctor to listen to the sound; the other is the electronic sensing path, which makes the generation of electronic signals possible[12].In this way, while the generation of electronic signals occurs, the user can simultaneously obtain the heart sound by ears.Therefore, the users will have more available information to diagnose the potential diseases they may have.