System function and design considerations of digital stethoscope

Abstract: This application note discusses the basic operation and design considerations of digital stethoscopes, and introduces the similarities between digital stethoscopes and older generation acoustic stethoscopes. This article also briefly introduces the latest technology of digital design, including recording and playback functions. When discussing the design considerations of the digital stethoscope, the important functions of the audio signal circuit are introduced in detail, the design points of the audio codec and the frequency requirements of the audio signals of the heart and lungs are given. This article also describes the sub-functions of the system, including data storage and transmission, display and backlight, power management and battery management.

Overview

Whether it is an acoustic stethoscope or a digital stethoscope, the main function is as an auxiliary diagnostic tool for the heart and lungs, by listening to the sound signal for diagnosis. Acoustic stethoscopes have been used for about 200 years, and electronic digital stethoscopes are new devices that have been recently developed.

The basic goal of a digital stethoscope is to maintain the look and feel of an acoustic stethoscope, but to improve the ability to detect sound signals. In addition, high-end digital stethoscopes also provide some new functions, such as recording and playback, and can also provide intuitive data display results, and display on peripheral displays, such as computer monitors. Advanced functions enhance the diagnosis ability of physicians. While improving the performance of the digital stethoscope, it is also required to maintain the shape (same look and feel) of the existing acoustic stethoscope, which requires the use of a solution with a small size and low power consumption.

Audio signal channel

The main units of the digital stethoscope are acoustic sensors, audio codecs and speakers. Acoustic sensors convert sound into analog voltage signals, which is the key to the signal link. It determines the diagnostic quality of digital stethoscopes, and ensures that users get the same experience as acoustic stethoscopes.

The analog voltage needs to be conditioned and converted to a digital signal by an audio analog-to-digital converter (ADC) or audio codec. Some digital stethoscopes have a noise canceling function, which requires another acoustic sensor or microphone to record the environmental noise, and then eliminate the noise through the digital output. In this scheme, two audio ADCs need to be used.

Functional block diagram of digital stethoscope.

Once converted into a digital signal, it can be processed with a microcontroller unit (MCU) or digital signal processor (DSP), including environmental noise suppression and filtering, limiting the bandwidth of the heart and lung sound signals. The processed digital signal is converted into an analog signal through an audio digital-to-analog converter (DAC) or audio codec.

Before outputting to the speaker, use the headphone amplifier or speaker amplifier to adjust the audio signal. A mono speaker can be placed at the tube fork of the stethoscope, and the amplified sound is transmitted to the ear through the dual sound tube. Another solution is to use two-channel speakers, one at the end of each earpiece. Considering the low frequency of the sound signal here, the frequency response characteristics of the speaker are close to the subwoofer. Choose to use single-channel or two-channel speaker amplifiers according to different design schemes.

The stethoscope must maintain a high sensitivity to the sound signals from the heart of 20 Hz to 400 Hz and the sound signals from the lungs of 100 Hz to 1200 Hz. Note that different manufacturers may specify different frequency ranges, and DSP algorithms can be used to filter out signals outside the frequency range.

Data storage and transmission

Once the collected sound signal is converted into an analog voltage, the voltage signal can be output through an audio jack and can be played back on a computer or digital stethoscope. The collected sound signal can also be converted into a digital signal, and then stored in the stethoscope or removable non-volatile (NV) memory, such as EEPROM or flash memory, when needed, can be played back through the stethoscope speaker, or can be transmitted to the computer for further analysis. Add a real-time clock (RTC), you can mark records by time and date. The recording signal is transmitted through a wired interface (such as USB) or a wireless interface (such as Bluetooth® or other proprietary wireless interface).

Display and backlight

Most digital stethoscopes can only be equipped with small displays due to limited space; some displays only have buttons and LED instructions. Because the ambient brightness of the stethoscope is usually dark during operation, the display of the digital stethoscope needs to be backlit. Small displays require 1 to 2 white light-emitting diodes (WLEDs) controlled by LED drivers or electroluminescence (EL) panels controlled by EL drivers. Of course, adding a touch screen and its controller can also remove the key user interface.

Power management

Most digital stethoscopes are powered by one or two AAA 1.5V primary batteries. The design requires a boost or boost switching regulator to boost the voltage to 3.0V or 5.0V, depending on the specific circuit.

If a single 1.5V battery is installed, the switching regulator may always be effective, and the low quiescent current of the system becomes the key to extending the battery life. The longer the battery life, the more convenient the digital stethoscope is to use, and the closer it is to the acoustic stethoscope.

When using two 1.5V series batteries for power supply, the switching regulator can always remain active or be turned off when not in use. If the operating range of the circuit is from 3.6V to a minimum of 1.8V, the switching regulator can be eliminated, thereby further reducing costs and saving space. The low battery alarm is a must-have feature to ensure that the battery does not need to be replaced during the diagnosis of the patient.

Battery management

The digital stethoscope can be powered by a rechargeable battery. It is best to choose a single Li + battery. If rechargeable batteries are used, a battery charger is required in the digital stethoscope or charging stand. The fuel gauge can accurately measure the remaining capacity of the battery and calculate the working time that the battery can support. If the battery is removable, in order to ensure safety and better manage the after-sales market, it is necessary to add anti-counterfeiting authentication function.

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