Anti-eavesdropping device and method

Abstract

This application provides an anti-eavesdropping device and method, belonging to the field of signal processing technology. The anti-eavesdropping device includes: a signal generation module for generating an ultrasonic carrier signal and a pseudo-voice noise signal, wherein the ultrasonic carrier signal matches the resonant frequency of a mobile phone microphone; a modulation module for modulating the pseudo-voice noise signal onto the ultrasonic carrier signal using AM modulation to generate a broadband modulated signal; and an ultrasonic transmission module for transmitting the broadband modulated signal. The broadband modulated signal is down-converted to a human voice baseband with a frequency range of 0Hz to 8kHz via nonlinear mixing by the mobile phone microphone and preamplifier, forming an interference signal that overlaps with the spectrum of a real human voice. This application achieves effective countermeasures against dual protection of digital spectrum filtering and ANC adaptive noise reduction without relying on high-power suppression or affecting the hearing of the person on site, ensuring the stability and reliability of the anti-eavesdropping effect.

Description

Technical Field

[0001] This application relates to the field of signal processing technology, and in particular to an anti-eavesdropping device and method. Background Technology

[0002] With the widespread use of portable recording devices such as smartphones and voice recorders, the use of their built-in microphones for covert eavesdropping and illegal recording has become increasingly rampant. To prevent such eavesdropping, various active anti-eavesdropping solutions have emerged in existing technologies. One common method is to emit ultrasonic signals into the protected area. Taking advantage of the fact that ultrasonic waves are imperceptible to the human ear, this interferes with the microphone of the recording device without affecting normal communication on site. For example, some solutions use continuous emission of a single frequency ultrasonic wave (such as 40kHz) to suppress the microphone's pickup capability through high-power ultrasonic signals, or to superimpose intolerable noise into the recording.

[0003] However, with the continuous improvement of smartphone signal processing capabilities, the aforementioned traditional ultrasonic interference solutions have gradually become ineffective. Specifically, modern smartphones generally integrate digital spectrum filtering technology (such as notch filters based on fast Fourier transform), which can accurately identify and remove ultrasonic interference signals with stable frequency points and narrowband characteristics. At the same time, the adaptive active noise cancellation (ANC) function, which is widely equipped in mobile phones, continuously collects ambient noise and generates inverse cancellation signals, thereby filtering out conventional, fixed-pattern ultrasonic interference as ambient noise.

[0004] In addition, some existing technologies attempt to suppress microphone response by increasing ultrasonic transmission power, but this not only leads to excessive power consumption of the device, making it difficult to integrate into portable or low-power products, but also poses a potential risk of ultrasonic radiation to the human body, and still cannot break through the dual protection of ANC and digital filtering.

[0005] In summary, existing anti-eavesdropping technologies cannot effectively counteract the digital spectrum filtering and ANC adaptive noise reduction functions of smartphones under low power consumption and imperceptible to the human ear, thus failing to achieve stable and reliable anti-eavesdropping effects. Therefore, there is an urgent need for an ultrasonic anti-eavesdropping solution that can circumvent digital spectrum filtering, counteract ANC adaptive noise reduction, and is feasible with low power consumption. Summary of the Invention

[0006] The purpose of this application is to provide an anti-eavesdropping device and method to solve the above-mentioned problems.

[0007] To achieve the above objectives, in a first aspect, this application proposes an anti-eavesdropping device, which includes: A signal generation module is used to generate an ultrasonic carrier signal and a pseudo-speech noise signal, wherein the ultrasonic carrier signal is matched to the resonant frequency of the mobile phone microphone; The modulation module is used to modulate the pseudo-speech noise signal onto the ultrasonic carrier signal in AM modulation mode to generate a broadband modulated signal; An ultrasonic transmitting module is used to transmit the broadband modulated signal; The broadband modulation signal is down-converted to the human voice baseband with a frequency range of 0Hz to 8kHz via the nonlinear mixing effect of the mobile phone microphone and preamplifier, forming an interference signal that overlaps with the spectrum of the real human voice.

[0008] In some embodiments, the center frequency of the ultrasonic carrier signal is 40 kHz; the frequency range of the pseudo-speech noise signal is 100 Hz to 8 kHz, used to simulate the breathing noise and / or formant texture of human voice.

[0009] In some embodiments, the anti-eavesdropping device further includes: The frequency hopping and sweeping module is used to perform chaotic frequency hopping and / or slow frequency sweeping processing on the broadband modulation signal generated by the modulation module; the frequency range of the chaotic frequency hopping is 38kHz to 42kHz, and the frequency hopping interval is 1 to 10 ms; the frequency range of the slow frequency sweeping is 36kHz to 44kHz, and the sweeping period is 1 to 3 s.

[0010] In some embodiments, the anti-eavesdropping device further includes: The waveform optimization module is used to convert the broadband modulated signal processed by the frequency hopping and sweeping module into a sawtooth-like wave with a duty cycle of 40% to 60% and a clipping degree of 5% to 15%, so as to introduce nonlinear harmonics and intermodulation products.

[0011] In some embodiments, the ultrasonic transmitting module is also used to transmit an auxiliary frequency band signal, the frequency range of which is 22kHz to 24kHz, and the transmitted sound pressure of the auxiliary frequency band signal is 20dB to 40dB lower than that of the broadband modulated signal.

[0012] In some embodiments, the signal generation module is further configured to generate at least two ultrasonic signals of different frequencies, wherein the at least two ultrasonic signals of different frequencies are located in the frequency range of 36 kHz to 44 kHz. The at least two ultrasonic signals of different frequencies are generated into a difference frequency signal of 0Hz to 8kHz through the nonlinear intermodulation of the mobile phone microphone and the preamplifier.

[0013] In some embodiments, the anti-eavesdropping device further includes: The control module is used to adjust the transmission power in stages within the range of 0.5W to 1.5W according to a preset power level table, and to set the modulation depth of the AM modulation to a preset level within the range of 30% to 50%.

[0014] Secondly, to achieve the above objectives, this application also proposes an anti-eavesdropping method based on the aforementioned anti-eavesdropping device, the method comprising: The generation module generates an ultrasonic carrier signal and a pseudo-speech noise signal, wherein the ultrasonic carrier signal is matched to the resonant frequency of the mobile phone microphone; The pseudo-speech noise signal is modulated onto the ultrasonic carrier signal using AM modulation to generate a broadband modulated signal; The broadband modulated signal is transmitted, wherein the broadband modulated signal is down-converted to the human voice baseband with a frequency range of 0Hz to 8kHz by the nonlinear mixing effect of the mobile phone microphone and preamplifier, forming an interference signal that overlaps with the spectrum of the real human voice.

[0015] In some embodiments, after modulating the pseudo-speech noise signal onto the ultrasonic carrier signal using AM modulation to generate a broadband modulated signal, the method further includes: The broadband modulated signal is subjected to chaotic frequency hopping in the range of 38kHz to 42kHz and / or slow frequency sweep processing in the range of 36kHz to 44kHz; and / or The broadband modulated signal processed by chaotic frequency hopping and / or slow frequency sweeping is converted into a sawtooth-like wave with a duty cycle of 40% to 60% and a clipping degree of 5% to 15%.

[0016] In some embodiments, the method further includes: Synchronous transmission of auxiliary frequency band signals in the range of 22kHz to 24kHz; and / or At least two ultrasonic signals of different frequencies are transmitted simultaneously. The at least two ultrasonic signals of different frequencies are located in the frequency range of 36kHz to 44kHz. The at least two ultrasonic signals of different frequencies generate a difference frequency signal of 0Hz to 8kHz through the nonlinear intermodulation of the mobile phone microphone and the preamplifier.

[0017] Compared with the prior art, the beneficial effects of this application include: By employing an ultrasonic carrier signal (e.g., 40kHz) that matches the resonant frequency of the mobile phone microphone, and combining it with AM modulation, a pseudo-speech noise signal is modulated onto the ultrasonic carrier signal, forming a broadband modulated signal with a continuous spectrum. When this broadband modulated signal is received by the mobile phone microphone, it does not directly interfere with the recording in the original ultrasonic form. Instead, it utilizes the inherent nonlinear characteristics of the microphone and preamplifier to downconvert the interference signal to the human voice baseband frequency range of 0Hz to 8kHz through nonlinear mixing. Furthermore, because the modulated signal itself highly simulates the human voice in terms of spectral characteristics, the interference signal generated after downconversion forms an indistinguishable high degree of overlap with the live human voice signal in the frequency domain. On the one hand, digital spectrum filtering based on Fast Fourier Transform (FFT) cannot identify and remove a clutter signal that is difficult to distinguish from normal speech and covers the entire baseband. On the other hand, the Adaptive Noise Cancellation (ANC) system can only generate inverse cancellation signals for fixed noise in the audible frequency band of the environment. However, for interference signals that have been mixed into the human voice baseband and highly overlap with the spectrum of human voice, ANC cannot identify them as noise that needs to be canceled, nor can it filter them out separately while preserving the human voice, thus forcing the ANC system to fail on the contaminated signal. Therefore, this application achieves effective countermeasures against the dual protection of digital spectrum filtering and ANC adaptive noise cancellation without relying on high-power suppression or affecting the hearing of the human ear on site, ensuring the stability and reliability of the anti-eavesdropping effect. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation on the scope of this application.

[0019] Figure 1 This is a schematic diagram of the spectrum of ultrasonic interference using a single frequency in the prior art. Figure 2 This is a schematic diagram illustrating the principle of adaptive noise reduction for a single frequency in existing ANC technology. Figure 3 This is a schematic diagram illustrating the working principle of an anti-eavesdropping device in one embodiment; Figure 4 This is a schematic diagram illustrating the principle that the ANC cannot filter interference signals generated by the anti-eavesdropping device in one embodiment. Figure 5 This is a schematic diagram of the frequency spectrum of the anti-eavesdropping device performing frequency hopping and sweeping in one embodiment; Figure 6 This is a waveform diagram of a sawtooth wave in one embodiment; Figure 7 This is a schematic diagram of the output signal of the waveform optimization module in one embodiment; Figure 8 A schematic diagram of the spectrum of nonlinear harmonics introduced in one embodiment; Figure 9 This is a waveform diagram of a multi-frequency ultrasonic signal introduced in one embodiment; Figure 10 This is a schematic diagram of the measured spectrum of a multi-frequency ultrasonic signal in one embodiment; Figure 11 This is a schematic diagram of the mixing of a mobile phone microphone and a preamplifier in one embodiment; Figure 12 This is a schematic diagram of the measured spectrum of the control module performing dynamic control in one embodiment; Figure 13 This is a flowchart illustrating an anti-eavesdropping method based on an anti-eavesdropping device in one embodiment. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0021] All terms used in this application (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0022] For example, the terms "first," "second," etc., used in this application may be used herein to describe various elements, but these elements are not limited by these terms. These terms are only used to distinguish the first element from another element.

[0023] For example, the terms "comprising" or "including" used in this application indicate the presence of features, steps, operations and / or components, but do not exclude the presence or addition of one or more other features, steps, operations or components.

[0024] As mentioned earlier, various active anti-eavesdropping solutions have emerged in the existing technology to prevent eavesdropping. One common method is to emit ultrasonic signals into the protected area. Taking advantage of the fact that ultrasonic waves are imperceptible to the human ear, this interferes with the microphone of the recording equipment without affecting normal communication. For example... Figure 1 As shown, some solutions use a single frequency of ultrasonic waves (such as 40kHz) to continuously transmit, suppressing the microphone's pickup capability with high-power ultrasonic signals, or adding unbearable noise to the recording.

[0025] However, with the continuous improvement of smartphone signal processing capabilities, the aforementioned traditional ultrasonic interference solutions have gradually become ineffective. Specifically, modern smartphones generally integrate digital spectrum filtering technology (such as notch filters based on Fast Fourier Transform), which can accurately identify and remove ultrasonic interference signals with stable frequency points and narrowband characteristics; at the same time, the adaptive active noise cancellation (ANC) function widely equipped in mobile phones continuously collects ambient noise and generates inverse cancellation signals, thereby filtering out conventional, fixed-pattern ultrasonic interference as ambient noise. For example, as... Figure 2 As shown, a single-frequency 40kHz ultrasonic signal is used as the interference source. Figure 2 (Note 1 in the text). When this signal is received by the phone's microphone, the phone's ANC adaptive noise reduction module can identify the fixed frequency characteristics of the ultrasonic interference and generate an inverse cancellation signal ( ). Figure 2 (Note 2 in the text). Because the cancellation signal generated by the ANC adaptive noise reduction module has a similar amplitude and opposite phase to the original ultrasonic interference, the two cancel each other out, resulting in the ultrasonic interference being unable to effectively affect the recording, ultimately causing the anti-eavesdropping to fail. Figure 2 (Note 3 in the text).

[0026] In addition, some existing technologies attempt to suppress microphone response by increasing ultrasonic transmission power, but this not only leads to excessive power consumption of the device, making it difficult to integrate into portable or low-power products, but also poses a potential risk of ultrasonic radiation to the human body, and still cannot break through the dual protection of ANC and digital filtering.

[0027] In summary, existing anti-eavesdropping technologies cannot effectively counteract the digital spectrum filtering and ANC adaptive noise reduction functions of smartphones under low power consumption and imperceptible to the human ear, thus failing to achieve stable and reliable anti-eavesdropping effects. Therefore, this application proposes an anti-eavesdropping device and method that can circumvent digital spectrum filtering, counteract ANC adaptive noise reduction, and is feasible with low power consumption.

[0028] like Figure 3 As shown, Figure 3This diagram illustrates the overall structure and working principle of the anti-eavesdropping device in this application embodiment, divided into a transmitter and a receiver. In one embodiment, this application provides an anti-eavesdropping device, which serves as the transmitter and includes: a signal generation module for generating an ultrasonic carrier signal and a pseudo-voice noise signal, wherein the ultrasonic carrier signal matches the resonant frequency of a mobile phone microphone; a modulation module for modulating the pseudo-voice noise signal onto the ultrasonic carrier signal using AM modulation to generate a broadband modulation signal; and an ultrasonic transmission module for transmitting the broadband modulation signal. The receiver (mobile phone) includes: a MEMS microphone (hereinafter referred to as the mobile phone microphone), a preamplifier, a digital spectrum filtering module, an ANC adaptive noise reduction module, and an ADC sampling and recording module. The broadband modulation signal generated by the transmitter propagates through space to the mobile phone microphone. Under the nonlinear mixing action of the preamplifier, it is down-converted to the human voice baseband with a frequency range of 0Hz to 8kHz, forming an interference signal that overlaps with the real human voice spectrum. This interference signal cannot be effectively filtered out by the digital spectrum filtering and ANC adaptive noise reduction, and is ultimately sampled and stored by the ADC to achieve anti-eavesdropping.

[0029] The core principle of this embodiment lies in generating a specially modulated ultrasonic carrier signal through a signal generation module. This ultrasonic carrier signal is selected to match the resonant frequency of the mobile phone microphone, thereby improving the microphone's sensitivity to receiving the interference signal. The pseudo-voice noise signal is used to simulate the spectral characteristics of human voice (such as breathing noise and formant texture). After the ultrasonic carrier signal and the pseudo-voice noise signal are modulated by the nonlinear interaction of the mobile phone microphone and preamplifier (mainly the mixing effect generated by the quadratic term), they are down-converted to the human voice baseband of 0Hz to 8kHz. Since the interference signal generated by this down-conversion completely overlaps with the spectrum of the real human voice, the mobile phone's digital spectrum filtering cannot distinguish and remove the interference, and ANC adaptive noise reduction cannot cancel it without destroying the real human voice. This achieves a stable and efficient anti-eavesdropping effect, and the entire interference process is completely imperceptible to the human ear.

[0030] Specifically, such as Figure 4 As shown, Figure 4 The marker 1 in the diagram points to the broadband modulation signal, indicating the broadband ultrasonic signal output by the transmitter that has been modulated by pseudo-speech noise. Figure 4 Mark 2 indicates that the interference signal overlaps with the spectrum of the live voice, meaning that after the broadband modulation signal is down-converted by the phone's microphone and preamplifier, the resulting interference signal completely overlaps with the live voice in the frequency domain and cannot be distinguished or filtered out by the phone's ANC adaptive noise reduction. Figure 4 Mark 3 indicates that because the interference signal is inseparable from the human voice spectrum, the mobile phone's ANC adaptive noise reduction module cannot cancel the interference without destroying the original sound, thus being forced to abandon filtering and achieve anti-eavesdropping.

[0031] It should be noted that the input-output characteristics of a mobile phone microphone and its preamplifier are not ideally linear; they can be expanded into a Taylor series near the operating point.

[0032] in, The total input signal received by the microphone (including the on-site human voice and the signal emitted by the anti-eavesdropping device in this embodiment of the application). This is the voltage output by the preamplifier. (Constant term) DC bias, For linear gain coefficients, and , These are nonlinear coefficients. In the above expansion, the squared term... Playing a crucial role, it mixes two frequency components in the input signal, generating a sum frequency and a difference frequency. Utilizing this physical effect, this application achieves down-conversion of broadband ultrasonic signals—transferring noise components originally located in the ultrasonic frequency band to the 0–8 kHz human voice baseband, forming clutter that completely overlaps with the spectrum of real human voice. Because this clutter has the same frequency domain characteristics as real speech, subsequent ANC adaptive noise reduction and digital spectrum filtering cannot filter it out without destroying the original sound.

[0033] For example, the broadband ultrasonic signal emitted by the anti-eavesdropping device in this embodiment can be represented as:

[0034] in, The signal consists of pseudo-speech noise in the frequency range of 100Hz to 8kHz, and the signal at 40kHz is an ultrasonic carrier signal.

[0035] The mobile phone microphone simultaneously picks up the real human voice at the scene and the ultrasonic signal emitted by this solution, that is: ,in, For the sound of the people in the audience. Substituting the squared terms from the aforementioned Taylor expansion After unfolding, it will appear The intersection terms. Among them, Includes a 40kHz carrier and its sidebands, and The product produces a frequency component of 40kHz ± the human voice frequency. The difference frequency is then calculated as 40kHz minus the human voice frequency. When the human voice frequency range is 0–8kHz, the difference frequency falls precisely within the 0–8kHz baseband. This process eliminates pseudo-speech noise. Its spectral characteristics are completely transferred to the baseband, forming noise pollution that is indistinguishable from human voice.

[0036] In some implementations, the center frequency of the ultrasonic carrier signal is 40kHz. This 40kHz carrier frequency is outside the audible range of the human ear (20Hz–20kHz), ensuring it is imperceptible. Furthermore, it precisely matches the acoustic resonance peak of current mainstream mobile phone microphones, enabling higher receiving sensitivity with lower transmission power. The pseudo-speech noise signal has a frequency range of 100Hz–8kHz and is used to simulate the breathing noise and / or formant texture of human voice. It completely covers the main energy concentration area of ​​human voice, and its simulated breathing noise, formant texture, and other characteristics make the down-converted interference signal highly similar to real human voice in the spectrum, further increasing the difficulty for ANC and digital filtering to distinguish interference from normal speech.

[0037] In some embodiments, the anti-eavesdropping device further includes: a frequency hopping and sweeping module, used to perform chaotic frequency hopping and / or slow frequency sweeping processing on the broadband modulation signal generated by the modulation module; the frequency range of the chaotic frequency hopping is 38kHz to 42kHz, and the frequency hopping interval is 1 to 10 ms (e.g., 5 ms); the frequency range of the slow frequency sweeping is 36kHz to 44kHz, and the sweeping period is 1 to 3 s (e.g., 2 s). Figure 5 As shown in the figure, the horizontal axis represents time (ms) and the vertical axis represents frequency (kHz). Smooth solid lines represent the slow frequency sweep baseline, and dotted solid lines represent chaotic frequency hopping with no fixed pattern. This embodiment introduces chaotic frequency hopping and slow frequency sweep processing to ensure that the transmitted ultrasonic interference signal does not have a fixed steady-state frequency. Chaotic frequency hopping employs an irregular hopping pattern with hopping intervals on the order of milliseconds, making it difficult for digital spectrum filtering based on Fast Fourier Transform to capture and lock onto the frequency points of the interference signal for notch removal. Slow frequency sweep drifts slowly across a wide range of 36kHz to 44kHz, further supplementing frequency band coverage and preventing interference dead zones caused by filtering specific frequencies. The two frequency sweeping methods can be used individually or in combination to maximize the avoidance of identification and removal by digital spectrum filtering.

[0038] In some implementations, such as Figure 6 and Figure 7 As shown, Figure 6 and Figure 7 The waveform is after clipping and duty cycle adjustment. The anti-eavesdropping device further includes a waveform optimization module, used to convert the broadband modulated signal processed by the frequency hopping and sweeping module into a sawtooth-like wave with a duty cycle of 40%–60% (e.g., 50%) and a clipping degree of 5%–15% (e.g., 10%), so as... Figure 8As shown, nonlinear harmonics (such as second and third harmonics) and intermodulation products are introduced to achieve full spectrum filling. In this embodiment, the originally relatively clean sine wave or modulated wave of the broadband modulation signal is converted into a sawtooth-like wave with slight clipping through a waveform optimization module (waveform shaping circuit). Additional second harmonics (such as 80kHz), third harmonics (such as 120kHz), and intermodulation products between these harmonics are deliberately introduced. These spurious components can fill the entire ultrasonic spectrum, making digital spectrum filtering face the dilemma of increasing spurious signals with each removal, making it difficult to effectively eliminate interference signals through modeling. At the same time, the clipping degree is strictly controlled between 5% and 15%, and the duty cycle is controlled between 40% and 60%, which ensures that the generated additional harmonics and intermodulation products are still in the ultrasonic frequency band (or although a small number of audible frequency components are generated, their energy is extremely low), and will not have a perceptible auditory impact on the human ear.

[0039] In some embodiments, the ultrasonic transmitting module is also used to transmit an auxiliary frequency band signal, the frequency range of which is 22kHz to 24kHz, and the transmitted sound pressure level of the auxiliary frequency band signal is 20dB to 40dB lower than that of the broadband modulation signal. For example, if the transmitted sound pressure level of the broadband modulation signal is 100dB, the transmitted sound pressure level of the auxiliary frequency band signal is 70dB. When the mobile phone's digital filter or anti-aliasing filter strongly attenuates the 40kHz main frequency band signal, the 22kHz to 24kHz auxiliary frequency band signal can still be received by the microphone and contaminate the human voice baseband through nonlinear downconversion. The sound pressure level of the auxiliary frequency band is set to 50% to 70% of that of the main frequency band to ensure that it has sufficient energy to produce effective downconversion products, while this frequency band is still outside the audible range of the human ear (beyond 20Hz-20kHz), and will not generate audible noise.

[0040] In some implementations, such as Figure 9 and Figure 10 As shown, the signal generation module is also used to generate at least two ultrasonic signals of different frequencies (see reference). Figure 9 and Figure 10The at least two ultrasonic signals of different frequencies (e.g., 38kHz, 40kHz, 42kHz, etc.) are located in the frequency range of 36kHz to 44kHz. These signals, through nonlinear intermodulation between the mobile phone microphone and preamplifier, generate a difference frequency signal ranging from 0Hz to 8kHz. In this embodiment, the signal generation module generates at least two ultrasonic signals of different frequencies (e.g., 38kHz, 40kHz, 42kHz, etc.), which simultaneously enter the mobile phone microphone and preamplifier. Utilizing the inherent nonlinear characteristics of the microphone and preamplifier (mainly the intermodulation effect generated by the quadratic term), the ultrasonic signals of different frequencies intermodulate, generating sum and difference frequency signals. The difference frequency signal (e.g., 40kHz - 35kHz = 5kHz) will directly fall into the 0Hz to 8kHz human voice baseband, causing interference. Figure 11 As shown, the frequency domain input signal includes three channels: the first channel is a 40kHz broadband modulated signal, covering a spectrum from 36kHz to 44kHz; the second channel is a multi-frequency ultrasonic signal near 40kHz, such as combinations of 38kHz and 42kHz, or 40kHz and 36kHz; the third channel is the live human voice, with a spectrum ranging from 0 to 8kHz. All three signals simultaneously enter the nonlinear stages of the microphone and preamplifier. The frequency domain output signal consists of two parts: first, the traditional carrier ± human voice down-conversion product, i.e., the interaction between the 40kHz broadband modulated signal and the live human voice, shifting pseudo-speech noise pollution to the 0–8kHz human voice baseband; second, the difference frequency signal, which can be directly generated without relying on pseudo-speech noise modulation. The difference frequency signal and the noise generated by the down-conversion of pseudo-speech noise together form dual baseband pollution, further enhancing the anti-digital spectrum filtering and anti-ANC effects. Furthermore, the difference frequency signal itself does not have a fixed frequency pattern, making it difficult for filtering algorithms to predict and eliminate.

[0041] In some embodiments, the anti-eavesdropping device further includes a control module, configured to adjust the transmission power in stages within the range of 0.5W to 1.5W according to a preset power level table, and to set the modulation depth of the AM modulation to a preset level (e.g., 40%) within the range of 30% to 50%. Figure 12As shown, under the control of the control module, the signal emitted by the anti-eavesdropping device is not only a multi-frequency superposition state, but its amplitude also changes dynamically. This embodiment achieves dynamic and precise control of the transmission power and modulation depth through the control module. The transmission power is controlled within the range of 0.5W to 1.5W. This ensures sufficient signal strength to cause the phone's microphone diaphragm to resonate and to induce slight saturation clipping in the preamplifier, thereby amplifying the nonlinear mixing effect. It also avoids increased power consumption and potential ultrasonic radiation risks due to excessive power. The modulation depth is set between 30% and 50%. Too deep a depth can easily lead to signal distortion and overflow spurious signals in the audible frequency band, while too shallow a depth weakens the down-conversion effect and makes it difficult to overcome ANC and digital filtering. Through tiered adjustments, it can adapt to the microphone sensitivity and filtering algorithms of different brands and models of mobile phones, improving the device's versatility and stability.

[0042] In the anti-eavesdropping device proposed in this application, an ultrasonic carrier signal (e.g., 40kHz) matching the resonant frequency of the mobile phone microphone is used, and a pseudo-voice noise signal is modulated onto the ultrasonic carrier signal using AM modulation to form a broadband modulated signal with a continuous spectrum. When the broadband modulated signal is received by the mobile phone microphone, it does not directly interfere with the recording in the original ultrasonic form. Instead, it utilizes the inherent nonlinear characteristics of the microphone and preamplifier to downconvert the interference signal to the human voice baseband frequency range of 0Hz to 8kHz through nonlinear mixing. Furthermore, since the modulated signal itself highly simulates a real human voice in terms of spectral characteristics, the interference signal generated after downconversion forms an indistinguishable high degree of overlap with the real human voice signal in the frequency domain. On the one hand, digital spectrum filtering based on Fast Fourier Transform (FFT) cannot identify and remove a clutter signal that is difficult to distinguish from normal speech and covers the entire baseband. On the other hand, the Adaptive Noise Cancellation (ANC) system can only generate inverse cancellation signals for fixed noise in the audible frequency band of the environment. However, for interference signals that have been mixed into the human voice baseband and highly overlap with the spectrum of human voice, ANC cannot identify them as noise that needs to be cancelled, nor can it filter them out separately while preserving human voice, thus forcing the ANC system to fail on the contaminated signal. Therefore, the embodiments of this application achieve effective countermeasures against the dual protection of digital spectrum filtering and ANC adaptive noise cancellation without relying on high-power suppression or affecting the hearing of the human ear on site, ensuring the stability and reliability of the anti-eavesdropping effect.

[0043] like Figure 13 As shown, in one embodiment, this application also proposes an anti-eavesdropping method based on the anti-eavesdropping device in the above embodiments, the method comprising: Step S10: Generate an ultrasonic carrier signal and a pseudo-speech noise signal, wherein the ultrasonic carrier signal is matched to the resonant frequency of the mobile phone microphone.

[0044] In this embodiment, the ultrasonic carrier signal refers to a sine wave or quasi-sine wave signal with a frequency higher than 20kHz (the upper limit of human hearing). The center frequency of the ultrasonic carrier signal in this embodiment is set to 40kHz, which matches the mechanical resonant frequency of most smartphone MEMS microphones. The pseudo-speech noise signal is an artificially synthesized broadband noise signal with the time-frequency characteristics of human voice. In this embodiment, the frequency range of the pseudo-speech noise signal is set to 100Hz–8kHz to simulate breathing noise, friction sounds, and formant textures in human voice. Pink noise (1 / f noise, where the energy of human voice increases as the frequency decreases), brown noise, or adaptive noise dynamically synthesized based on the real-time spectrum of human voice in the actual meeting environment can be used.

[0045] Step S20: Modulate the pseudo-speech noise signal onto the ultrasonic carrier signal using AM modulation to generate a broadband modulated signal.

[0046] Specifically, the pseudo-speech noise signal is modulated onto a 40kHz ultrasonic carrier with a modulation depth of 30% to 50% through an AM modulation circuit, generating a continuous broadband ultrasonic signal with a spectrum covering 36kHz to 44kHz, thus avoiding the filtering and removal of a single frequency point.

[0047] Step S30: Transmit the broadband modulation signal, wherein the broadband modulation signal is down-converted to the human voice baseband with a frequency range of 0Hz to 8kHz by the nonlinear mixing effect of the mobile phone microphone and preamplifier, forming an interference signal that overlaps with the spectrum of the real human voice.

[0048] In this embodiment, the transmitted 40kHz ultrasonic carrier signal is efficiently received by the mobile phone microphone (resonant frequency 40kHz). After nonlinear mixing by the microphone and preamplifier (mainly the mixing effect generated by the square term), the 40kHz ultrasonic carrier signal and the pseudo-speech noise signal intermodulate to generate continuous noise of 40kHz±100Hz~40kHz±8kHz. Among them, the 40kHz component minus the human voice frequency is downconverted to the 0Hz~8kHz human voice baseband, which completely overlaps with the spectrum of the real human voice. Digital spectrum filtering cannot separate it, and ANC adaptive noise reduction cannot cancel the interference without destroying the real human voice.

[0049] In some embodiments, after step S20, the method further includes: performing chaotic frequency hopping in the range of 38kHz to 42kHz and / or slow frequency sweep processing in the range of 36kHz to 44kHz on the broadband modulation signal; and / or converting the broadband modulation signal processed by chaotic frequency hopping and / or slow frequency sweep into a sawtooth-like wave with a duty cycle of 40% to 60% and a clipping degree of 5% to 15%. This embodiment avoids capturing steady-state frequency points using fast Fourier transform through chaotic frequency hopping (frequency hopping interval 1 to 10 ms, without a fixed pattern), supplements frequency band coverage to prevent interference dead zones through slow frequency sweep (sweep period 1 to 3 s), and introduces nonlinear spurious emissions by slightly clipping and shaping the waveform, further enhancing the anti-digital spectrum filtering and anti-ANC effects.

[0050] In some embodiments, the method further includes: synchronously transmitting an auxiliary frequency band signal with a frequency range of 22kHz to 24kHz; and / or synchronously transmitting at least two ultrasonic signals of different frequencies, wherein the at least two ultrasonic signals of different frequencies are located in the frequency range of 36kHz to 44kHz, wherein the at least two ultrasonic signals of different frequencies generate a difference frequency signal of 0Hz to 8kHz through the nonlinear intermodulation of a microphone and a preamplifier. This embodiment achieves fallback protection through dual-band superimposed transmission, and generates difference frequency interference that directly falls into the baseband through multi-frequency intermodulation, forming dual baseband pollution, thus ensuring the stability and reliability of the anti-eavesdropping effect.

[0051] In the anti-eavesdropping method proposed in this application, an ultrasonic carrier signal matching the resonant frequency of a mobile phone microphone and a pseudo-voice noise signal simulating human voice characteristics are generated. The pseudo-voice noise signal is then embedded into the ultrasonic carrier signal in a broadband form using AM modulation, forming a broadband modulated signal with a continuous spectrum. When this broadband modulated signal is received by the mobile phone microphone, it does not directly interfere with the recording in the original ultrasonic form. Instead, it utilizes the inherent nonlinear characteristics of the microphone and preamplifier to downconvert the interference signal to the human voice baseband frequency range of 0Hz to 8kHz through nonlinear mixing. Furthermore, because the modulated signal itself highly simulates a real human voice in its spectral characteristics, the interference signal generated after downconversion overlaps in the frequency domain with the actual human voice signal, creating an indistinguishable high degree of overlap. On the one hand, digital spectrum filtering based on Fast Fourier Transform (FFT) cannot identify and remove a clutter signal that is difficult to distinguish from normal speech and covers the entire baseband. On the other hand, the Adaptive Noise Cancellation (ANC) system can only generate inverse cancellation signals for fixed noise in the audible frequency band of the environment. However, for interference signals that have been mixed into the human voice baseband and highly overlap with the spectrum of human voice, ANC cannot identify them as noise that needs to be cancelled, nor can it filter them out separately while preserving human voice, thus forcing the ANC system to fail on the contaminated signal. Therefore, the embodiments of this application achieve effective countermeasures against the dual protection of digital spectrum filtering and ANC adaptive noise cancellation without relying on high-power suppression or affecting the hearing of the human ear on site, ensuring the stability and reliability of the anti-eavesdropping effect.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

[0053] Furthermore, those skilled in the art will understand that although some embodiments herein include certain features included in other embodiments but not others, combinations of features from different embodiments are intended to be within the scope of this application and form different embodiments. For example, any of the embodiments or implementations claimed above can be used in any combination. The information disclosed in this background section is intended only to enhance the understanding of the general background of this application and should not be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

Claims

1. An anti-eavesdropping device, characterized in that, The aforementioned anti-eavesdropping device includes: A signal generation module is used to generate an ultrasonic carrier signal and a pseudo-speech noise signal, wherein the ultrasonic carrier signal is matched to the resonant frequency of the mobile phone microphone; The modulation module is used to modulate the pseudo-speech noise signal onto the ultrasonic carrier signal in AM modulation mode to generate a broadband modulated signal; An ultrasonic transmitting module is used to transmit the broadband modulated signal; The broadband modulation signal is down-converted to the human voice baseband with a frequency range of 0Hz to 8kHz via the nonlinear mixing effect of the mobile phone microphone and preamplifier, forming an interference signal that overlaps with the spectrum of the real human voice.

2. The anti-eavesdropping device according to claim 1, characterized in that, The center frequency of the ultrasonic carrier signal is 40kHz; the frequency range of the pseudo-speech noise signal is 100Hz to 8kHz, used to simulate the breathing noise and / or formant texture of human voice.

3. The anti-eavesdropping device according to claim 1, characterized in that, The anti-eavesdropping device also includes: The frequency hopping and sweeping module is used to perform chaotic frequency hopping and / or slow frequency sweeping processing on the broadband modulation signal generated by the modulation module; the frequency range of the chaotic frequency hopping is 38kHz to 42kHz, and the frequency hopping interval is 1 to 10 ms; the frequency range of the slow frequency sweeping is 36kHz to 44kHz, and the sweeping period is 1 to 3 s.

4. The anti-eavesdropping device according to claim 3, characterized in that, The anti-eavesdropping device also includes: The waveform optimization module is used to convert the broadband modulated signal processed by the frequency hopping and sweeping module into a sawtooth-like wave with a duty cycle of 40% to 60% and a clipping degree of 5% to 15%, so as to introduce nonlinear harmonics and intermodulation products.

5. The anti-eavesdropping device according to claim 1, characterized in that, The ultrasonic transmitting module is also used to transmit an auxiliary frequency band signal, the frequency range of which is 22kHz to 24kHz, and the transmitted sound pressure of the auxiliary frequency band signal is 20dB to 40dB lower than that of the broadband modulated signal.

6. The anti-eavesdropping device according to claim 1, characterized in that, The signal generation module is also used to generate at least two ultrasonic signals of different frequencies, wherein the at least two ultrasonic signals of different frequencies are located in the frequency range of 36kHz to 44kHz. The at least two ultrasonic signals of different frequencies are generated into a difference frequency signal of 0Hz to 8kHz through the nonlinear intermodulation of the mobile phone microphone and the preamplifier.

7. The anti-eavesdropping device according to claim 1, characterized in that, The anti-eavesdropping device also includes: The control module is used to adjust the transmission power in stages within the range of 0.5W to 1.5W according to a preset power level table, and to set the modulation depth of the AM modulation to a preset level within the range of 30% to 50%.

8. A method for preventing eavesdropping based on the anti-eavesdropping device according to any one of claims 1 to 7, characterized in that, The method includes: The generation module generates an ultrasonic carrier signal and a pseudo-speech noise signal, wherein the ultrasonic carrier signal is matched to the resonant frequency of the mobile phone microphone; The pseudo-speech noise signal is modulated onto the ultrasonic carrier signal using AM modulation to generate a broadband modulated signal; The broadband modulated signal is transmitted, wherein the broadband modulated signal is down-converted to the human voice baseband with a frequency range of 0Hz to 8kHz by the nonlinear mixing effect of the mobile phone microphone and preamplifier, forming an interference signal that overlaps with the spectrum of the real human voice.

9. The anti-eavesdropping method according to claim 8, characterized in that, After modulating the pseudo-speech noise signal onto the ultrasonic carrier signal using AM modulation to generate a broadband modulated signal, the method further includes: The broadband modulated signal is subjected to chaotic frequency hopping in the range of 38kHz to 42kHz and / or slow frequency sweep processing in the range of 36kHz to 44kHz; and / or The broadband modulated signal processed by chaotic frequency hopping and / or slow frequency sweeping is converted into a sawtooth-like wave with a duty cycle of 40% to 60% and a clipping degree of 5% to 15%.

10. The anti-eavesdropping method according to claim 8, characterized in that, The method further includes: Synchronous transmission of auxiliary frequency band signals in the range of 22kHz to 24kHz; and / or At least two ultrasonic signals of different frequencies are transmitted simultaneously. The at least two ultrasonic signals of different frequencies are located in the frequency range of 36kHz to 44kHz. The at least two ultrasonic signals of different frequencies generate a difference frequency signal of 0Hz to 8kHz through the nonlinear intermodulation of the mobile phone microphone and the preamplifier.

Images