A digital processing method for heart sound signals

1. Introduction

Cardiac auscultation is an important part of physical examination. The period of occurrence of heart murmurs is of great value to clinical diagnosis. For example, lighter murmurs during the systolic period are generally physiological, while murmurs during the diastolic period are mostly pathological. When auscultating the heart, it is necessary to be able to accurately distinguish the first and second heart sounds and identify the phase in which the murmur occurs. This has always been a difficult point in medical auscultation. The use of electronic information technology can effectively process heart sound signals, filter out irrelevant murmurs and environmental noise, and amplify useful sounds, providing doctors with stable and clear heart rate digital displays and good heart sound quality for clinical diagnosis. The performance of electronic stethoscopes based on this technology is far superior to traditional stethoscopes. Using this instrument, doctors can choose to examine a single organ and only listen to the sound of this organ without interference from the sound of neighboring organs, which can achieve the best diagnostic effect.

2. Composition of heart sound signal processing circuit

In recent years, electronic information technology has been widely used to process heart sound signals at home and abroad. In the early days, separate components and ordinary analog circuits were used to implement circuit design, but now it is mostly implemented with dedicated ICs and single-chip microcomputers. Figure 1 shows the typical structure of this circuit. After the heart sound sensor signal is amplified and filtered, one path is monitored by power amplification, and the other path is sent to the single-chip microcomputer for processing after pulse shaping. After the single-chip microcomputer timing, counting and data processing, the heart rate is digitally displayed. It has strong reliability, high measurement accuracy and conversion efficiency.

3. Implementation of Heart Sound Signal Processing Circuit

1. Signal acquisition and amplification circuit

Since the range of heart sound is 20Hz to 600Hz , and it is required to extract weak heart sound signals while minimizing the reception of external clutter and other signals, the heart sound sensor should be of high sensitivity and strong anti-interference ability. Based on our comparison of several sensors such as electret, dynamic, and capacitive, we chose the electret microphone as the initial heart sound collection sensor. [page]

Since the frequency signal received by the microphone is very weak and broadband, we need to amplify it and filter out the noise that is useless for auscultation, so high-precision amplification and filtering circuits are required.

In order to hear the heart sounds, a power amplifier circuit is needed to drive the speaker; in order to display the heart rate, the signal from the filter circuit needs to be sent to the comparator. Through comparison and shaping, the counter can receive a relatively regular pulse signal to facilitate the circuit operation. Then the pulse signal is sent to the microcontroller for processing, and the processed signal is sent to the display circuit. Figure 2 shows the signal acquisition and amplification circuit. R1 , R2 , R3 , and R4 form a resistance balance circuit. The purpose is to distribute the signal to the complementary and symmetrical dual-power amplifier to obtain a signal with sufficient power. The collected signal is sent to the input of U1A and U1B through R3 and R4 . U1A and U1B are two independent amplifiers in the integrated operational amplifier NE5532 .

In order to effectively suppress interference, we use the pulse processing chip 5G7650 to filter out interference from the output signals of U1A and U1B . 5G7650 is a fourth-generation integrated operational amplifier made using CMOS technology, also known as a chopper-stabilized zero operational amplifier. The output signals of U1A and U1B are connected to the 4th and 5th pins of 5G7650 to form an inverting amplifier. R7 and R8 are the input resistors of 5G7650 , R9 and R10 are feedback resistors, and the amplification factor of this circuit is R10/R7=200 times. In order to effectively filter out the tiny spike pulse interference caused by the internal clock chopping frequency of the 5G7650 circuit, an RC low -pass filter is connected to the output end, such as R8 and C3 take 1M Ω and 1µF respectively . In order to eliminate the output overload of 5G7650 , 200 Ω to 510 Ω current limiting resistors (such as R5 and R6 in the figure ) are connected in series at the positive and negative power supply ends to ensure that the circuit is not damaged. The dual in-line plug- in 5G7650 has two input protection terminals, pin 3 and pin 6 , which can easily form an input protection device. [page]

2. Filtering and shaping circuits

The specific circuit is shown in Figure 3. As mentioned above, the heart sounds are in the range of 20Hz to 600Hz , so other signals and noise must be filtered out. At the end of the amplifier circuit, a second-order voltage-controlled voltage source low-pass filter is used to filter out interference signals outside the heart sound range, and the crossover point is set to:

f0=1/ ( 2 × 3.14×RC ) =600Hz

In this way, the amplitude of the heart beat can be clearly separated.

U8B is a common-phase follower, which is used to enhance the load carrying capacity so that the signal has enough amplitude to provide to the comparator. U8A , R17 , R18 , R19 , R20 and two voltage-stabilizing diodes form a hysteresis comparator. When the input signal is disturbed or fluctuates up and down due to noise, the output voltage of the comparator can be prevented from repeatedly jumping between high and low levels by properly adjusting the values ​​of the two threshold levels of the hysteresis comparator according to the interference or noise level.

3. Single chip microcomputer processing circuit

The single-chip processing circuit is shown in Figure 4. The single-chip selects 89C51 . The waveform shaped by the comparator, that is, the pulse of the number of heart beats, is sent to the T1 pin of the single-chip. This pin counts the pulses. After the set timing time, the number of pulses is sent to the accumulator for processing, and then sent to the P2 port for display. The P2 port is used as an output, the P1 port is used as a bit selection, and is driven by 74LS07 . The digital tube uses a 7- segment common cathode digital tube. The level driven by 74LS07 provides power through a pull-up resistor and is added to the corresponding pin of the digital tube.

4. Hardware circuit debugging

During debugging, the signal generator outputs a sine wave of 0Hz to 2000Hz, and an oscilloscope is used to observe the output signal changes of each observation point of the hardware circuit. First, it can be observed that the signal collected by the sensor includes many frequency components, as shown in Figure 5. In the filter circuit, the cutoff frequency is close to 600Hz by adjusting R38 and R39 ; the Q value is close to 0.707 by adjusting R40 and R41 , until a relatively ideal output response is observed, that is, the amplitude of the heart beat is clearly highlighted and almost no clutter interference is seen, as shown in Figure 6. In the shaping circuit, as long as the potentiometer R19 is properly adjusted , that is, the threshold level of the hysteresis comparator is changed, a stable heart beat pulse can be obtained. Finally, the heart sound sensor is placed on the human body for actual measurement, and the heart rate measurement error is controlled within ± 1 time to meet the measurement requirements.

V. Summary

The stethoscope with this circuit design can amplify sound 14 times that of a conventional stethoscope . The reasonable filter circuit design greatly reduces the noise input from the edge of the auscultation area, and enables doctors to more effectively distinguish heart sounds from other physiological sounds. For doctors who see patients in a noisy environment, this processing circuit can achieve very ideal results .

References:

[1] Ding Yuanjie . Principles and Applications of Single-Chip Microcomputers, 2nd Edition [ M]. Beijing: Machinery Industry Press, 1999

[2] He Limin . Microcontroller Application Technology Selection 1st Edition [ M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 1996

[3] Huang Xianwu, Zheng Xiaoxia . Sensor Principles and Applications 6th Edition [M]. Chengdu: University of Electronic Science and Technology Press, 2001