Whether it's 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 around for about 200 years, and electronic digital stethoscopes are new devices that have recently been developed.
The basic goal of a digital stethoscope is to maintain the look and feel of an acoustic stethoscope, but to improve the detection of sound signals. In addition, high-end digital stethoscopes offer new features such as recording and playback, as well as intuitive data display results and display on peripheral displays such as computer monitors. Advanced features enhance the diagnostic capabilities of physicians. While improving performance, digital stethoscopes also require the shape (same appearance and feel) of existing acoustic stethoscopes to be maintained, which requires a small, low-power solution.
Audio signal channelThe main units of digital stethoscopes are acoustic sensors, audio codecs and speakers. The acoustic sensor converts the sound into an analog voltage signal, which is the key to the signal link. It determines the diagnostic quality of the digital stethoscope and ensures that the user gets the same experience as an acoustic stethoscope.
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 cancellation function that requires another acoustic sensor or microphone to record ambient noise and then eliminate the noise through a digital output. In this scenario, two audio ADCs are required.
Once converted to digital signals, signal processing can be performed using a microcontroller unit (MCU) or digital signal processor (DSP), including ambient noise suppression and filtering, limiting the bandwidth range of heart and lung sound signals. The processed digital signal is then converted to an analog signal by an audio digital-to-analog converter (DAC) or an audio codec.
The audio signal is adjusted using a headphone amplifier or a speaker amplifier before being output to the speaker. A mono speaker can be placed at the tube fork of the stethoscope, and the amplified sound is transmitted to the ear through the double tube. Another option is to use a two-channel speaker with one at the end of each earpiece. Considering the low frequency of the sound signal here, the frequency response of the speaker is close to that of the subwoofer. Choose to use single or dual speaker amplifiers depending on the design.
The stethoscope must maintain a high sensitivity to the sound from the 20 Hz to 400 Hz heart and the sound signal from the 100 Hz to 1200 Hz lung. Note that different vendors may specify different frequency ranges and DSP algorithms can be used to filter out signals outside the frequency range.
Data storage and transmissionOnce the acquired sound signal is converted to 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 acquired sound signal can also be converted into a digital signal and then stored inside the stethoscope or in a removable non-volatile (NV) memory such as EEPROM or flash memory, which can be played back through the stethoscope of the stethoscope when needed, or transferred to a computer for further processing. analysis. Add a real-time clock (RTC) to record by time and date. The recording signal is transmitted via a wired interface (such as USB) or a wireless interface (such as Bluetooth? or other proprietary wireless interface).
Display and backlightMost digital stethoscopes can only be configured with small displays due to space limitations; some monitors only have buttons and LED indicators. Because the ambient brightness of the stethoscope is typically dark during operation, the display of the digital stethoscope requires backlighting. Small displays require one or two white light-emitting diodes (WLEDs) controlled by LED drivers or electroluminescent (EL) boards controlled by EL drivers. Of course, adding a touch screen and its controller can also remove the button user interface.
Power managementMost digital stethoscopes are powered by one or two AAA 1.5V primary batteries and are designed to boost the voltage to 3.0V or 5.0V with a boost or boost switching regulator, depending on the circuit.
If a single 1.5V battery is installed, the switching regulator may remain active and the system's low quiescent current is critical to extending battery life. The longer the battery life, the more convenient the digital stethoscope is, and the closer it is to the acoustic stethoscope.
When powered by two 1.5V series batteries, the switching regulator can remain active at all times or shut it down when not in use. If the circuit's operating range is from 3.6V to a minimum of 1.8V, switching regulators can be eliminated, further reducing cost and space. A low battery alarm is a must-have feature to ensure that no battery replacement is required during patient diagnosis.
Battery managementThe digital stethoscope can be powered by a rechargeable battery, preferably a single-cell Li+ battery. If you use a rechargeable battery, you need a battery charger in your digital stethoscope or charging stand. The fuel gauge can accurately measure the remaining battery capacity and calculate the working time that the battery can support. If the battery is detachable, it is necessary to increase the security authentication function to ensure safety and better manage the aftermarket.
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