Wearable audio transmission system based on cross-medium bidirectional electromagnetic induction

Abstract

The invention discloses a wearable audio transmission system based on cross-medium bidirectional electromagnetic induction, and belongs to the technical field of underwater magnetic induction communication technology and wearable audio equipment.The wearable audio transmission system comprises a shore transmitting end module and an underwater earphone receiving end module, and a transmitting end converts an audio signal into a digital signal through an audio acquisition and preprocessing unit; the modulation and amplification unit modulates the signals into carrier signals and amplifies the carrier signals; the carrier signal is converted into an alternating magnetic field through the transmitting coil unit; a receiving end induces the alternating magnetic field by using a three-dimensional orthogonal receiving coil unit and converts the alternating magnetic field into a weak electric signal; the signal processing unit carries out amplification, filtering, demodulation and adaptive gain control on the weak electric signal to restore a binary digital signal; and finally, the audio signal is recovered into an audio signal through the audio decoding and sounding unit. According to the system, through impedance matching and three-dimensional coil design, high-fidelity, low-power-consumption, stable and reliable wearable audio transmission from ashore to underwater is realized.

Description

Technical Field

[0001] This invention belongs to the field of underwater magnetic induction communication technology and wearable audio device technology, specifically relating to a wearable audio transmission system based on cross-medium bidirectional electromagnetic induction. Background Technology

[0002] Driven by both the national fitness boom and the refined development of professional sports training, the demand for underwater audio transmission in swimming pool settings has exploded (e.g., coach instructions, background music playback). However, existing underwater audio transmission technologies are limited and cannot meet the demands of practical applications. Specific limitations are as follows: (1) Wired headphones: Wired headphones connect to the audio source via waterproof cables, enabling basic audio transmission over short distances. However, they have several drawbacks: First, the drag force of the cable restricts the swimmer's freedom of movement, especially during high-speed strokes like freestyle and butterfly. The cable can easily become entangled in the body or be pulled by the water current, affecting swimming posture and speed, and potentially causing safety hazards. Second, the connection between the cable and the headphones is a weak point in terms of sealing. Underwater pressure increases with depth, easily causing the seal at the connection to fail. Statistics show that the average trouble-free operating time for such wired headphones is generally less than 50 hours. Frequent seal failures not only affect the user experience but may also damage the internal circuitry due to water ingress. (2) Radio frequency wireless headphones: Currently, most radio frequency (RF) wireless headphones rely on 2.4GHz Bluetooth and other RF signals for communication, but RF signals have extremely poor transmission characteristics in water: water has high conductivity. This results in a very small skin depth for the radio frequency signal, which can be calculated using the following formula: , In the formula, To reach skin depth, For signal frequency, The permeability of free space, Relative permeability; when hour, The effective transmission distance is less than 0.3m, which cannot meet the standard lane length of swimming pools; (3) Optical communication headset: Optical communication headsets utilize the straight-line propagation characteristic of optical signals to transmit audio. The propagation loss of optical signals in water is significantly affected by the turbidity of the water. The turbidity of chlorinated swimming pools is usually >5 NTU, which leads to a surge in optical signal scattering loss and a communication interruption rate >20%. Low-frequency magnetic induction communication utilizes alternating magnetic fields to penetrate water; its magnetic field strength attenuation model is as follows: ,

[0003] In the formula, The permeability of free space, The number of coil turns. For current, The area of ​​the coil, For transmission distance; This characteristic gives low-frequency magnetic fields a natural advantage in resisting interference in turbid water. However, existing low-frequency magnetic induction communication technology still faces technical bottlenecks when adapting to wearable audio devices: problems such as strong coil direction sensitivity (single coil receiving angle range <30°), low transmission rate (<500bps), and large device size (coil diameter >10cm) make it difficult to adapt to the lightweight requirements of headphone devices. In summary, all existing underwater audio transmission technologies have significant drawbacks, and existing low-frequency magnetic induction communication technology has not yet effectively solved the compatibility problem for wearable devices. Therefore, developing an underwater wearable audio transmission system with omnidirectional reception, high-speed transmission, lightweight design, and high reliability is crucial to meeting the actual needs of swimming pool scenarios. Summary of the Invention

[0004] To address the shortcomings of the existing technology, the present invention aims to propose a wearable audio transmission system based on cross-medium bidirectional electromagnetic induction, consisting of a shore-based transmitter module and an underwater earphone receiver module, realizing wireless audio transmission across air and water. The shore-based transmitter module performs PCM acquisition and preprocessing on the audio signal, followed by 2FSK modulation and amplification, and then uses a transmitting coil to resonate and radiate an alternating magnetic field. The underwater earphone receiver module uses a three-dimensional orthogonal coil to induce a magnetic field, and after signal amplification, filtering, demodulation, and adaptive gain control, restores the binary digital signal and decodes and outputs the audio signal. The system employs resonant matching and impedance matching to improve energy efficiency, orthogonal coils to eliminate attitude blind spots, and adaptive gain to ensure transmission stability. This enables high-fidelity, low-power, stable, and reliable wearable audio transmission from shore to underwater, meeting the real-time voice communication needs of portable underwater scenarios.

[0005] The technical solution adopted in this invention: A wearable audio transmission system based on cross-medium bidirectional electromagnetic induction includes the following: The wearable audio transmission system includes a shore-based transmitter module and an underwater earphone receiver module. The shore-based transmitter module and the underwater earphone receiver module achieve wireless audio transmission through signals. The shore-based transmitter module includes an audio acquisition and preprocessing unit, a modulation and amplification unit, and a transmitting coil unit. The underwater earphone receiver module includes a three-dimensional orthogonal receiving coil unit, a signal processing unit, and an audio decoding and sound generation unit. The audio acquisition and preprocessing unit is used to sample, quantize, encode and perform redundancy checks on the input audio signal to generate a binary digital signal with a frame structure. The modulation and amplification unit is used to modulate the binary digital signal into a carrier signal of a specific frequency, amplify the carrier signal, and output current. The transmitting coil unit is used to convert the amplified carrier signal into an alternating magnetic field; The three-dimensional orthogonal receiving coil unit is used to sense the alternating magnetic field and generate an induced electromotive force signal. The signal processing unit is used to amplify, filter, demodulate, and adaptively control the received induced electromotive force signal to obtain a digital signal. The audio decoding and sound generation unit is used to convert the demodulated digital signal back into an audio signal via a D / A converter and drive the speaker to produce sound.

[0006] Preferably, the audio acquisition and preprocessing unit performs pulse code modulation processing on the acquired audio signal to generate discrete binary digital signals. Redundant information is added using a CRC checksum to obtain a binary digital signal with a frame structure of "preamble + data segment + checksum".

[0007] Preferably, the modulation and amplification unit employs binary frequency shift keying modulation, using the frame-structured binary digital signal as the baseband signal. In this process, "0" in the binary digital signal is mapped to a 95kHz carrier signal, and "1" is mapped to a 105kHz carrier signal, generating alternating 95kHz and 105kHz carrier signals. The calculation formula is:

[0008] In the formula, For fixed signal amplitude, For the center frequency, For time, For frequency offset, It is a baseband signal; The carrier signal is then amplified and output as current.

[0009] Preferably, the transmitting coil unit adopts a hollow cylindrical transmitting coil with a coil inductance value of The calculation formula is:

[0010] In the formula, The permeability of free space, Number of turns Where is the coil radius, Given the diameter of the conductor, the calculation is as follows: ; Based on the coil inductance value, the transmitting coil and the resonant capacitor are connected in series to form a coil resonant matching circuit; the resonant capacitor The formula is:

[0011] In the formula, The resonant frequency; Next, the current output from the modulation and amplification unit is fed into the transmitting coil, which generates an alternating magnetic field around the transmitting coil.

[0012] Preferably, the three-dimensional orthogonal receiving coil unit adopts a design using... The receiving coil, composed of three mutually orthogonal axes, has an inductance value calculated using the formula for coil inductance. Combined with the resonant frequency The resonant capacitance of the receiving coil is obtained from the formula for calculating the resonant capacitance. and will Connected in series with the corresponding receiving coil; When the alternating magnetic field passes through the receiving coil, the receiving coil generates an induced electromotive force signal. The formula is:

[0013] In the formula The number of coil turns. The area of ​​the coil, For the first Magnetic field components of the axis, Corresponding to Magnetic field components in mutually orthogonal directions of the three axes.

[0014] Preferably, the signal processing unit includes a preamplifier, a bandpass filter, a demodulation circuit, and adaptive gain control, specifically: Preamplification: The induced electromotive force signal is amplified by an amplifier; the gain of the amplifier is dynamically adjusted within the range of 40~100dB by automatic gain control, and the amplified induced electromotive force signal is obtained after amplification. Bandpass filtering: The amplified induced electromotive force signal is filtered to output a modulated carrier signal with a high signal-to-noise ratio; Demodulation circuit: Performs FSK demodulation on the modulated carrier signal; restores the modulated carrier signal to a binary digital bit stream, identifies the frequency mapping relationship of 95kHz corresponding to "0" and 105kHz corresponding to "1", and outputs a binary digital signal; Adaptive gain control: This involves dynamically adjusting the amplifier's gain based on the strength of the received binary digital signal, achieving dynamic gain adjustment. The formula is as follows:

[0015] In the formula, For the first Amplifier gain at time, For the first Amplifier gain at time, Step size factor For the target voltage, For the first The input voltage at any given time.

[0016] Compared with the prior art, this invention proposes a wearable audio transmission system based on cross-medium bidirectional electromagnetic induction. The beneficial effects of this system are as follows: (1) The omnidirectional receiving system adopts a three-dimensional orthogonal magnetic induction coil array to achieve a receiving angle coverage of ±90°, effectively solving the problem of traditional single-axis receiving coils being sensitive to attitude and prone to communication blind spots; when the swimmer’s head changes attitude within ±30° of pitch / yaw, it can still stably capture magnetic field signals, with a communication interruption rate of <0.1%, significantly improving the communication reliability in underwater dynamic scenarios.

[0017] (2) Strong anti-interference performance: The system uses a low-frequency alternating magnetic field with a center frequency of 100kHz as the transmission carrier, and is unaffected by factors such as water turbidity (<20NTU), suspended bubbles, water flow disturbance, and chlorinated swimming pool environment; within a transmission distance of 15m, the system's bit error rate is low. It is far superior to traditional underwater transmission solutions such as visible light communication (bit error rate > 100%). It exhibits higher transmission stability in complex aquatic environments.

[0018] (3) High-efficiency energy coupling efficiency: The resonant matching circuit enables the receiving coil to exhibit a purely resistive resonance state at the operating frequency. Under the miniaturized structure with a receiving coil diameter of only 2cm, the energy coupling efficiency is increased to 65%, which is 40% higher than the traditional non-resonant receiving scheme. While ensuring the miniaturization of the equipment, the signal receiving strength and energy utilization rate are greatly improved.

[0019] (4) Lightweight structure and highly practical underwater headphones: The whole unit weighs only 55g and adopts IP68 waterproof packaging technology. It can work stably for a long time in a water depth of 2m. The structure is compact and comfortable to wear. It can be adapted to long-term underwater wearing scenarios such as swimming training and underwater operations. It combines miniaturization, lightweight and high reliability.

[0020] (5) Low power consumption and long battery life: The receiver adopts low power devices and an efficient power management architecture with an average power consumption of <50mW. Under standard power supply conditions, it can achieve a continuous working time of ≥6 hours, which can fully meet the battery life requirements of underwater long-term training, leisure swimming and other scenarios, and improve the practicality of the device and user experience. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the overall structure of a wearable audio transmission system based on cross-medium bidirectional electromagnetic induction according to the present invention. Figure 2 This is a schematic diagram illustrating an application scenario of the present invention; Figure 3 This is a schematic diagram of the underwater earphone structure of the present invention; Figure 4 This is a schematic diagram of an embodiment of the present invention. Detailed Implementation

[0022] The technical solutions of the embodiments of this application will be further described clearly and completely below with reference to the accompanying drawings. It should be noted that the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0023] To make the inventive objectives, technical solutions, and advantages of this application clearer, the embodiments of this application will be further described in detail below with reference to the accompanying drawings: In order to better understand the above-mentioned objectives, features, and advantages of this invention, the advantages of this invention will be further illustrated below by comparing the embodiments with the accompanying drawings and specific implementation methods.

[0024] This invention proposes a wearable audio transmission system based on cross-medium bidirectional electromagnetic induction, such as... Figure 1 As shown, the detailed description of the system is as follows: The wearable audio transmission system includes a shore-based transmitter module and an underwater earphone receiver module. The shore-based transmitter module and the underwater earphone receiver module achieve wireless audio transmission through signals. The shore-based transmitter module includes an audio acquisition and preprocessing unit, a modulation and amplification unit, and a transmitting coil unit. The underwater earphone receiver module includes a three-dimensional orthogonal receiving coil unit, a signal processing unit, and an audio decoding and sound generation unit. The audio acquisition and preprocessing unit is used to sample, quantize, encode and perform redundancy checks on the input audio signal to generate a binary digital signal with a frame structure. The modulation and amplification unit is used to modulate the binary digital signal into a carrier signal of a specific frequency, amplify the carrier signal, and output current. The transmitting coil unit is used to convert the amplified carrier signal into an alternating magnetic field; The three-dimensional orthogonal receiving coil unit is used to sense alternating magnetic fields and generate induced electromotive force signals, while improving magnetic field receiving efficiency through resonant matching. The signal processing unit is used to amplify, filter, demodulate, and adaptively control the received induced electromotive force signal to obtain a digital signal. The audio decoding and sound generation unit is used to convert the demodulated digital signal back into an audio signal via a D / A converter and drive the speaker to produce sound.

[0025] Specifically, the audio acquisition and preprocessing unit performs Pulse Code Modulation (PCM) processing on the acquired audio signal, uses an 8kHz sampling rate to perform discretization sampling, and then uses 8-bit quantization precision to perform discretization encoding, generating discrete binary digital signals. Redundant information is added through CRC checksum to obtain a binary digital signal with a frame structure of "preamble (16 bits) + data segment (128 bits) + checksum (8 bits)".

[0026] Specifically, the modulation and amplification unit adopts binary frequency shift keying (2FSK) modulation, and the modulation process is implemented through a Xilinx Spartan-7 series FPGA: the binary digital signal of the frame structure is used as the baseband signal. In this process, "0" in the binary digital signal is mapped to a 95kHz carrier signal, and "1" is mapped to a 105kHz carrier signal, thus generating an alternating 95kHz and 105kHz carrier signal. The calculation formula is:

[0027] In the formula, For fixed signal amplitude, For the center frequency, For time, For frequency offset, It is a baseband signal; when hour, ,when hour, ; The FPGA outputs a carrier signal via a digital-to-analog converter (DAC) at a clock frequency of 100MHz. Subsequently, the carrier signal is input to a Class-E power amplifier, which amplifies the carrier signal with an energy conversion efficiency of over 85% and outputs a current of 50mA. This effectively reduces energy loss in high-power transmission scenarios and provides a stable power source for subsequent applications.

[0028] Specifically, the transmitting coil unit adopts a hollow cylindrical structure transmitting coil, wherein the diameter of the transmitting coil is 5cm, the diameter of the enameled copper wire (conductor) is 0.2mm, the number of turns is 50, and its coil inductance value is... The calculation formula is:

[0029] In the formula, The permeability of free space, Number of turns Where is the coil radius, Given the diameter of the conductor, the calculation is as follows: ; Based on the coil inductance value, the transmitting coil and the resonant capacitor are connected in series to form a coil resonant matching circuit, which serves as the basic resonant network. The resonant capacitor... The formula is:

[0030] In the formula, For the resonant frequency, we get ; When a carrier signal with a center frequency of 100kHz passes through the fundamental resonant network, the fundamental resonant network needs to have the minimum impedance to achieve resonance at the 100kHz center frequency, thus minimizing energy loss. Furthermore, the fundamental resonant network generates impedance; when this impedance matches the 50Ω standard impedance of the Class-E power amplifier, the circuit impedance is minimized. This eliminates reflection loss of the carrier signal during transmission, allowing the 50mA current output by the Class-E power amplifier to be injected into the transmitting coil without loss, significantly reducing energy transmission loss. Then, the 50mA current output by the Class-E power amplifier is fed into the transmitting coil. According to the law of electromagnetic induction (a magnetic field is generated around a current, and a changing current generates a changing magnetic field), an alternating magnetic field is generated around the coil. This alternating magnetic field propagates from the transmitting coil into the surrounding space, both in the air and penetrating the air-water interface to propagate underwater. Finally, it is sensed by the three-dimensional orthogonal receiving coil at the underwater earphone receiver, completing the "shore to underwater" signal transmission. This significantly improves the energy transmission efficiency of the entire transmission link, increasing it to 72%.

[0031] Specifically, the three-dimensional orthogonal receiving coil unit adopts a design using... A miniature receiving coil composed of three mutually orthogonal axes has a diameter of 2 cm, 30 turns, and a wire diameter of 0.1 mm. The inductance value of the receiving coil is calculated using the formula for coil inductance. Combined with the resonant frequency The resonant capacitance of the receiving coil is calculated using the formula for calculating the resonant capacitance. and will By connecting it in series with the corresponding receiving coil, the receiving coil resonant matching circuit achieves series resonance at 100kHz, thus exhibiting purely resistive characteristics and improving magnetic field induction efficiency. 3D orthogonal layout coverage The receiving angle effectively avoids signal attenuation or dead zones caused by receiving attitude deviation; when an alternating magnetic field passes through the receiving coil, a weak induced electromotive force signal is generated in the receiving coil according to the law of electromagnetic induction. The formula is:

[0032] In the formula, The number of coil turns. The area of ​​the coil, For the first Magnetic field components of the axis, Corresponding to Magnetic field components in mutually orthogonal directions of the three axes.

[0033] Specifically, the signal processing unit includes a preamplifier, a bandpass filter, a demodulation circuit, and adaptive gain control, the specific contents of which include: Pre-amplification: The weak induced electromotive force signal is amplified by the low-noise operational amplifier AD8605. This low-noise operational amplifier has a noise figure of only 1.8dB, which can amplify weak signals in the millivolt range to the volt range with minimal noise. The gain of this low-noise operational amplifier is dynamically adjusted in the range of 40~100dB through an automatic gain control (AGC) circuit to ensure that the weak signal is amplified to an appropriate amplitude, ensuring that the signal amplitude in subsequent processing stages is within the optimal range. After amplification, the signal-to-noise ratio of the signal is improved, providing a stable input basis for subsequent filtering and demodulation, resulting in the amplified induced electromotive force signal. Bandpass filtering: The amplified induced electromotive force signal is filtered by a second-order Butterworth bandpass filter. This second-order Butterworth bandpass filter has a center frequency of 100kHz and a 3dB bandwidth of 20kHz, effectively suppressing 50Hz power frequency interference and high-frequency noise, while retaining the 95kHz~105kHz signal containing 2FSK modulation information. The characteristics of the second-order Butterworth filter ensure that the amplitude-frequency response of the signal within the passband is the flattest, avoiding signal distortion and outputting a high signal-to-noise ratio modulated carrier signal. This step is a key prerequisite for subsequent demodulation, ensuring that the demodulation chip can accurately identify frequency changes. Demodulation circuit: The filtered modulated carrier signal is demodulated using the TI CC1101 RF chip via FSK. The TI CC1101 RF chip has a demodulation sensitivity of ≤-110dBm and supports a baud rate of 1200bps. It restores the frequency-changing modulated carrier signal to a binary digital bit stream, identifies the frequency mapping relationship of 95kHz corresponding to "0" and 105kHz corresponding to "1", and outputs a binary digital signal. This enables the conversion of the modulated carrier signal to a binary digital signal, laying the foundation for subsequent audio signal restoration. Adaptive gain control: To address signal fluctuations caused by changes in underwater magnetic field strength with distance and attitude, the gain of the low-noise operational amplifier is dynamically adjusted based on the received signal strength (RSSI) to achieve dynamic gain regulation. The formula is as follows:

[0034] In the formula, For the first Low-noise operational amplifier gain at any given time. For the first Low-noise operational amplifier gain at any given time. This is the step size factor, used to control the rate of gain adjustment. For the target voltage, For the first The input voltage at a given moment, i.e., the current input voltage; By comparing the difference between the current input voltage and the target voltage in real time, the gain of the preamplifier is dynamically adjusted to ensure that the signal amplitude input to the demodulation chip is always stable within the optimal range. This effectively avoids saturation distortion caused by excessively strong signals or an increase in the bit error rate caused by excessively weak signals, further improving the robustness of the system.

[0035] Specifically, the audio decoding and sound generation unit includes: The demodulated digital signal is converted back into an audio signal by a D / A converter (PCM5102), while efficient power management ensures long battery life. The audio signal is output through a speaker (impedance 8Ω, output power 0.5W), with total harmonic distortion (THD) <1%, ensuring high-fidelity audio output. The power supply uses a 3.7V lithium polymer battery (capacity 600mAh), which provides a stable DC voltage to each module through an LDO (low dropout linear regulator). The average power consumption at the receiver is <50mW. Combined with the battery capacity, it can achieve ≥6 hours of continuous operation, meeting the needs of portable underwater use.

[0036] To facilitate understanding of the specific embodiments and practical application process of this invention, the following is combined with the shore The complete working process of water-to-medium audio transmission, such as Figure 2 ,3 As shown in Figure 4, the specific implementation steps of this system are explained in detail below: I. System Hardware Deployment (1) Installation of onshore transmitter module Transmitter Fixing and Position Calibration: Fix the onshore transmitter module 1.2m above the pool wall, ensuring the center of the transmitting coil is horizontally aligned with the pool surface and the coil axis is perpendicular to the pool wall. A level must be used for calibration during installation, with a deviation not exceeding ±1° to avoid uneven magnetic field distribution due to angular misalignment. The onshore transmitter module housing is designed with an IP65 waterproof rating and is connected to the pool wall via expansion bolts, with a load-bearing capacity ≥5kg, capable of withstanding the humid environment of the swimming pool.

[0037] Power and signal input: It uses a 220V AC power supply, which is converted to 12V DC via an AC-DC converter to power the Class-E power amplifier and FPGA. Audio signal input supports both a 3.5mm audio jack and Bluetooth 5.0. The former can directly connect to the coach's audio equipment, while the latter can receive wireless audio signals from mobile devices such as phones and tablets. The input audio signal amplitude must be controlled within the range of 0.5-2Vpp to avoid signal overload.

[0038] (2) Underwater receiving earphone assembly Coil array encapsulation: The X / Y / Z axis magnetic induction coils are encapsulated with epoxy resin, with encapsulated dimensions of 30mm×30mm×30mm and an encapsulation layer thickness of 1.5mm. This ensures both fixed coil position (vertical deviation of each axis ≤0.5°) and waterproof and impact-resistant performance. After encapsulation, inductance testing is required to ensure that the single-axis inductance value is within the range of 15±0.5μH. If the deviation exceeds this range, the number of coil turns or the wire spacing needs to be readjusted.

[0039] Circuit Integration and Housing Design: The signal processing unit (including AD8605 op-amp, CC1101 chip, and PCM5102 DAC) and power management are integrated on a 25mm × 40mm PCB board. The PCB board uses an immersion gold process to enhance corrosion resistance. The earphone housing is made of food-grade silicone, conforming to the ear contour and weighing ≤20g. A pressure relief valve is installed on the housing to balance internal and external pressure within a 2m water depth range, preventing housing deformation. The speaker is embedded inside the housing, with a contact area of ​​≥150mm² with the ear skin to ensure efficient audio transmission.

[0040] II. System Debugging Process (1) Debugging of onshore transmitter module Resonant network calibration: Use an LCR meter to measure the inductance of the transmitting coil. If the actual value deviates from the design value (18.7μH) by more than 5%, the series resonant capacitor (original design value 140pF) needs to be replaced. During debugging, connect the audio signal generator to the transmitting end and output a 100kHz sine wave signal. Observe the phase difference between the voltage and current across the coil using an oscilloscope. When the phase difference is 0°, record the actual value of the resonant capacitor to ensure that the input impedance of the resonant network is matched to 50Ω.

[0041] Modulation signal verification: A 2FSK modulated signal was generated using an FPGA (Xilinx Spartan-7). The carrier frequency deviation between the "0" code (95kHz) and the "1" code (105kHz) was measured using a spectrum analyzer, and it needed to be controlled within ±100Hz. Simultaneously, the output current of the Class-E power amplifier was measured, and the amplifier bias voltage was adjusted to stabilize the output current at 1.2±0.1A, achieving an efficiency of over 85%.

[0042] Data frame structure test: The audio signal (8kHz sampling rate, 8-bit quantization) is input into the audio preprocessing unit. The encoded data frame is captured by the logic analyzer to verify the structural integrity of the preamble (16-bit), data segment (128-bit), and checksum (8-bit), ensuring that the CRC checksum can correctly detect and correct 1-bit erroneous data.

[0043] (2) Earphone debugging Coil Array Sensitivity Test: Place a standard magnetic field generator (outputting a 100kHz, 1mT magnetic field) 1m away from the receiving earphone. Adjust the relative angle between the magnetic field generator and the earphone (0°-360°) and measure the induced electromotive force (EMF) of each axis coil using an oscilloscope. At any angle, at least one axis should have an induced EMF ≥ 5mV. If this is not met, check the number of coil turns or the resonant capacitance value, and recalibrate the resonant frequency to 100kHz.

[0044] Signal processing unit performance debugging: Preamplifier and AGC circuit: Input a 100kHz signal of -110dBm, adjust the AGC circuit step factor α (initial value set to 0.05), measure the output voltage with a multimeter, and ensure the output voltage is stable at 1.0±0.1V (target voltage). The gain adjustment range covers 40-100dB, and the adjustment response time is ≤100ms.

[0045] Bandpass filter test: The input contains an induced electromotive force signal containing 50Hz power frequency noise, 100kHz useful signal, and 200kHz high frequency noise. The filtered signal is observed using a spectrum analyzer. The attenuation of the 50Hz and 200kHz signals should be ≥40dB, and the attenuation of the 100kHz signal should be ≤1dB.

[0046] Demodulation and audio restoration: The 2FSK signal output by the transmitter module is transmitted underwater (1m depth, 15m distance). After demodulation by the receiving headphones, the audio signal output by the bone conduction speaker is measured by an audio analyzer. The total harmonic distortion (THD) must be <1%, and the sampling rate must be kept at 8kHz to ensure clear and uninterrupted voice.

[0047] Power management test: Connect a 3.7V 600mAh lithium polymer battery, enable all headphone functions, and measure the average power consumption using an ammeter. The power consumption must be <50mW. In the continuous discharge test, record the battery life. The battery life must be ≥6 hours, and the headphone must not experience any power loss or performance degradation before the battery voltage drops to 3.0V.

[0048] III. System Performance Verification (1) Transmission distance and bit error rate test Test environment setup: In a standard indoor swimming pool (water temperature 25±1℃, turbidity 10NTU, chlorine content 0.5mg / L), the onshore transmitter module was fixed to the pool wall, and the underwater receiver earphone was worn by the test personnel. Tests were conducted at distances of 5m, 10m, and 15m from the transmitter module, with each distance point lasting 5 minutes.

[0049] Data Acquisition and Analysis: The bit error rate at the receiving end is recorded using host computer software, with a requirement that the bit error rate be within a 15m range. Simultaneously, observe the communication status of the headphones when the swimmer is pitching or yawing ±30°, and record the number of communication interruptions. The interruption rate must be <0.1%.

[0050] (2) Anti-interference performance test Water turbidity interference test: Turbidity regulators were added to the swimming pool to achieve turbidity levels of 5 NTU, 10 NTU, and 20 NTU, respectively. The bit error rate was tested at a transmission distance of 15 m, requiring no significant increase in the bit error rate (still ≤). ), compared with optical communication schemes (bit error rate > 5 NTU when turbidity > 5 NTU) This verifies the anti-turbidity advantage of low-frequency magnetic fields.

[0051] Electromagnetic Interference Test: Place a working 2.4GHz Bluetooth device (such as a mobile phone or smart bracelet) and a 50Hz power frequency device (such as a water pump) near the transmitting module, and test the bit error rate and audio output quality at a distance of 15m. The bit error rate should be ≤ The audio was free of noise and disconnection, verifying the anti-interference effect of bandpass filtering and adaptive gain control.

[0052] (3) Practicality and reliability testing Lightweight and wearing comfort test: Ten testers with different head circumferences (54-60cm) were invited to wear the headphones and conduct 30 minutes of swimming training (including freestyle, breaststroke and backstroke). The number of times the headphones fell off and the discomfort score (1-5 points, 1 point for no discomfort and 5 points for severe discomfort) were recorded. The number of times the headphones fell off was required to be ≤1 time / person and the average discomfort score was required to be ≤2 points.

[0053] Waterproofing and durability testing: The earphones were placed in a pressure tank at a depth of 2 meters and submerged for 24 hours. After removal, the shell was checked for sealing, and no water ingress was observed. Subsequently, 100 repeated wearing and removing operations were performed to test the durability of the coil encapsulation and interface. After completion, the inductance value and communication performance were retested to ensure no performance degradation.

[0054] IV. System Maintenance and Troubleshooting (1) Routine maintenance Onshore transmitter module: Clean the module casing weekly to remove dust and water stains; check the sealing of the power and audio input interfaces monthly; replace the sealing rings or interface components promptly if aging or loosening is found. Test the resonant network impedance and power amplifier efficiency quarterly to ensure that the transmission efficiency remains above 72%.

[0055] Underwater receiver headphones: After each use, rinse the outer shell with clean water and dry it to avoid residual chlorine corroding the silicone material. Check the bone conduction speaker outlet for blockage monthly; if blocked, clean it with cotton swabs. Disassemble the headphones every six months (performed by a professional) to check the coil encapsulation for integrity and the PCB board for corrosion; replace aging batteries or capacitors if necessary.

[0056] (2) Troubleshooting common faults Communication interruption failure: If the transmitter indicator light is normal (green light is always on) but the receiver has no audio output, first check if the receiver headphone coil is misaligned (by testing the inductance value). If the coil inductance value deviation is >10%, it needs to be re-refilled and calibrated. If the transmitter indicator light flashes (fault indication), check if the power supply voltage is 12V and if the Class-E amplifier output current is normal. If the current is <1.0A, adjust the amplifier bias voltage or replace the power transistor.

[0057] Audio distortion problem: If there is noise in the audio, check if the bandpass filter is faulty (measure the filtering effect with a spectrum analyzer). If the attenuation of 50Hz or high-frequency noise is insufficient, replace the filter capacitor or resistor. If the audio is choppy, check the response time of the AGC circuit. If the response time is >100ms, adjust the step factor α (increase it to 0.08-0.1), or replace it with a low-noise op-amp AD8605.

[0058] Battery life reduction issue: If the earphone's battery life is less than 5 hours, first check the battery capacity (using a battery tester). If the capacity decay is greater than 20%, replace the lithium polymer battery. If the battery capacity is normal, check the power consumption of each module. If the power consumption of a certain module is >15mW, check whether there is a short circuit or component aging problem in that module, and repair or replace the component in time.

[0059] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0060] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A wearable audio transmission system based on cross-medium bidirectional electromagnetic induction, characterized in that, The wearable audio transmission system includes a shore-based transmitter module and an underwater earphone receiver module. The shore-based transmitter module and the underwater earphone receiver module achieve wireless audio transmission through signals. The shore-based transmitter module includes an audio acquisition and preprocessing unit, a modulation and amplification unit, and a transmitting coil unit. The underwater earphone receiver module includes a three-dimensional orthogonal receiving coil unit, a signal processing unit, and an audio decoding and sound generation unit. The audio acquisition and preprocessing unit is used to sample, quantize, encode and perform redundancy checks on the input audio signal to generate a binary digital signal with a frame structure. The modulation and amplification unit is used to modulate the binary digital signal into a carrier signal of a specific frequency, amplify the carrier signal, and output current. The transmitting coil unit is used to convert the amplified carrier signal into an alternating magnetic field; The transmitting coil unit adopts a hollow cylindrical structure transmitting coil with a coil inductance value of The calculation formula is: ； In the formula, The permeability of free space, Number of turns Where is the coil radius, Given the diameter of the conductor, the calculation is as follows: ; Based on the coil inductance value, the transmitting coil and the resonant capacitor are connected in series to form a coil resonant matching circuit; the resonant capacitor The formula is: ； In the formula, The resonant frequency; Next, the current output from the modulation and amplification unit is fed into the transmitting coil, which generates an alternating magnetic field around the transmitting coil. The three-dimensional orthogonal receiving coil unit is used to sense the alternating magnetic field and generate an induced electromotive force signal. The signal processing unit is used to amplify, filter, demodulate, and adaptively control the received induced electromotive force signal to obtain a digital signal. The audio decoding and sound generation unit is used to convert the demodulated digital signal back into an audio signal via a D / A converter and drive the speaker to produce sound.

2. The wearable audio transmission system based on cross-medium bidirectional electromagnetic induction according to claim 1, characterized in that, The audio acquisition and preprocessing unit performs pulse code modulation on the acquired audio signal to generate discrete binary digital signals. Redundant information is added using CRC checksums to obtain a binary digital signal with a frame structure of "preamble + data segment + checksum".

3. The wearable audio transmission system based on cross-medium bidirectional electromagnetic induction according to claim 1, characterized in that, The modulation and amplification unit employs binary frequency shift keying modulation, using the frame-structured binary digital signal as the baseband signal. In this process, "0" in the binary digital signal is mapped to a 95kHz carrier signal, and "1" is mapped to a 105kHz carrier signal, generating alternating 95kHz and 105kHz carrier signals. The calculation formula is: ； In the formula, For fixed signal amplitude, For the center frequency, For time, For frequency offset, It is a baseband signal; The carrier signal is then amplified and output as current.

4. A wearable audio transmission system based on cross-medium bidirectional electromagnetic induction according to claim 1, characterized in that, The three-dimensional orthogonal receiving coil unit employs a receiving coil composed of mutually orthogonal X, Y, and Z axes. The inductance value of the receiving coil is obtained based on the formula for calculating coil inductance. Combined with the resonant frequency The resonant capacitance of the receiving coil is obtained from the formula for calculating the resonant capacitance. and will Connected in series with the corresponding receiving coil; When the alternating magnetic field passes through the receiving coil, the receiving coil generates an induced electromotive force signal. The formula is: ； In the formula, The number of coil turns. The area of ​​the coil, For the first Magnetic field components of the axis, Corresponding to Magnetic field components in mutually orthogonal directions of the three axes.

5. A wearable audio transmission system based on cross-medium bidirectional electromagnetic induction according to claim 1, characterized in that, The signal processing unit includes a preamplifier, a bandpass filter, a demodulation circuit, and adaptive gain control, specifically: Preamplification: The induced electromotive force signal is amplified by an amplifier; the gain of the amplifier is dynamically adjusted within the range of 40~100dB by automatic gain control, and the amplified induced electromotive force signal is obtained after amplification. Bandpass filtering: The amplified induced electromotive force signal is filtered to output a modulated carrier signal with a high signal-to-noise ratio; Demodulation circuit: performs FSK demodulation on the modulated carrier signal; The modulated carrier signal is restored to a binary digital bit stream. After identification, the frequency mapping relationship of "0" corresponding to 95kHz and "1" corresponding to 105kHz is obtained, and the binary digital signal is output. Adaptive gain control: This involves dynamically adjusting the amplifier gain based on the strength of the received binary digital signal, achieving dynamic gain adjustment. The formula is as follows: ； In the formula, For the first Amplifier gain at time, For the first Amplifier gain at time, Step size factor For the target voltage, For the first The input voltage at any given time.

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