Your TV can see through walls (2025)
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标准电视信号竟然可以成为一种出人意料的"运动探测器"，甚至能透过墙壁捕捉到人的移动。通过使用像 Ettus USRP B200 这类设备，对本地电视台信号的频谱图进行分析，就能看到信号与环境之间的相互作用。直接且无遮挡的信号会呈现为一条强烈、稳定不变的线；而当有人走到信号传播路径附近时，频谱图上就会出现清晰可见的扰动。
这些扰动源于选择性衰减：强反射会把直达信号抵消。由于反射的相位随信号频率变化，这种抵消并不均匀，因而在频谱图上会形成具有辨识度的特征图案。这种状态极为敏感，哪怕是家里很轻微的移动，也会在频谱图中表现得非常明显。
当人开始走动，比如从椅子上起身或在相邻房间里行走时，频谱图会显示出醒目的条纹状图案。这些条纹会以精确的间隔反复出现，因为当传播路径长度每改变一个波长时，反射就会恰好把残余信号再次抵消。通过把这些图案出现的时间与信号"瀑布图"的刷新速率对应起来，甚至可以推算出人的移动速度。
在这个例子里，计算结果显示行走速度约为每秒 1.3 米，也就是每小时 4.6 公里。由于受试者的运动方向与直达信号路径大致垂直，因此不需要进行复杂的几何修正就能得到这个数值。这说明，一套相对简单的无线电接收系统也能以相当高的精度追踪运动。
尽管电视信号能把这一效应直观地呈现出来，但它们并不是唯一可用的媒介。其他宽带无线电信号（例如 WiFi 和 LTE）同样可以被用来提取类似信息。虽然这些信号在频谱图上可能呈现得更"杂乱"，但底层原理依旧相同，使得在不同的无线电频率环境下，都能利用这种方式穿透墙壁探测运动。
A standard television signal can act as a surprising tool for detecting movement, even through walls. By analyzing the spectrogram of a local TV station signal using equipment like an Ettus USRP B200, one can observe how the signal interacts with the environment. While a direct, unobstructed signal appears as a strong and unchanging line, the introduction of a person walking near the signal path creates visible disturbances.
These disturbances are caused by selective fading, a phenomenon where a direct signal is canceled out by a strong reflection. Because the phase of this reflection is dependent on the signal frequency, the cancellation effect is not uniform, resulting in distinctive patterns on a spectrogram. This state is remarkably sensitive, reacting clearly to even minor movements within a home.
When a person moves around, such as getting up from a chair or walking in an adjacent room, the spectrogram displays clear, striped patterns. These stripes appear at precise intervals because the reflection perfectly cancels the residual signal every time the path length changes by one wavelength. By calculating the timing of these patterns against the update rate of the signal waterfall, one can even determine the speed of the person's movement.
In this instance, the math revealed a walking speed of approximately 1.3 meters per second, or 4.6 kilometers per hour. Because the subject was walking perpendicular to the direct signal path, no complex geometric adjustments were required to arrive at this figure. This demonstrates that a relatively simple radio receiver is capable of tracking motion with significant accuracy.
While television signals provide a clear visual demonstration of this effect, they are not the only medium that can be used for such surveillance. Other wideband radio signals, including WiFi and LTE, can also be exploited to extract similar data. While these signals might produce a messier visual output on a spectrogram, the underlying principle remains the same, allowing for the detection of movement through walls across various radio frequency environments.
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• 声波可能使光纤电缆振动，从而以可重建音频的方式改变穿行其中的光信号。此类漏洞通常需要对特定线路进行物理接入。
• 远程窃听技术经历了从范·埃克（Van Eck）式的"偷看"发展：早期可通过墙壁重建 CRT 显示图像；如今则借助激光多普勒测振、射频信号和 Wi‑Fi 信号来检测振动或人的存在。
• 利用光信号进行音频监听在历史上已有先例，例如苏联时期的"装置"（The Thing）。它并非依赖传统光学激光，而是使用无线电信号来调制音频，从而凸显射频监视与光学波段监视之间的区别。
• 现代 Wi‑Fi 和蜂窝网络正越来越具备充当"空间智能"传感器的能力，能够实现存在探测与活动监测，并绕过传统的视觉监控。类似功能也体现在如 IEEE 802.11bf 等新兴标准中。
• 虽然存在高端监控工具，但不少追踪方法门槛很低，研究者和爱好者已经证明，用普通的 Wi‑Fi 与无线电设备也能估算交通流量、识别人体运动，并绘制家庭环境。
• "智能"设备的普及制造了这样的局面：家庭硬件从安防摄像头、智能电视到支持 Zigbee 的灯泡，都可能被重新利用来采集遥测数据或音频数据，往往还能形成一种持续、自动化的监控网格。
• 在现代社会，要实现完全匿名越来越困难，因为互联设备密度高、政府拥有的大规模监控基础设施存在，同时广告追踪网络普遍会把独特标识符与个人关联起来。
• 欺诈性的"诈骗库"兴起，其中一些声称能借助普通 Wi‑Fi 信号实现由 AI 驱动的高级人体运动追踪，反映了公众目前对这类技术的焦虑，以及谣言传播门槛之低。
• 从硬件层面采取的安全措施，比如关闭天线或使用"哑"监视器，只能提供有限的保护，因为监控技术可能依赖外部发射的信号，或依赖已被集成进现代电子设备标准运行方式中的信号。
• 长期暴露在射频能量与数字追踪之下，是现代生活的基本组成部分。由此引发的讨论也从"彻底避免被监控"转向威胁建模，认识到监控往往是自动化完成的，而非由人工直接指挥。
整体讨论反映出一个持续的张力：无线感知技术的普及程度不断提高，而在高度互联的环境中维护个人隐私却愈发困难。参与者强调，尽管这些监控方法在技术上可能相当复杂，但它们背后往往有深厚的类比时代窃听与射频监控历史渊源。普遍共识是：尽管高端的"国家级"能力依然强大，真正的变化在于借助路由器、智能家电和移动设备等廉价商用硬件实现了追踪的自动化。最终，这场讨论表明：虽然确实存在一些技术层面的缓解手段，但在现代数字基础设施高度密集、日常设备又被广泛赋予传感能力的情况下，想要实现完全匿名几乎不可能。 • Sound waves can cause fiber-optic cables to vibrate, altering the light signal passing through them in a way that allows for audio reconstruction. This vulnerability typically requires physical access to a specific line.
• Techniques for remote eavesdropping have evolved from Van Eck phreaking, which reconstructs CRT display images through walls, to modern methods utilizing laser Doppler vibrometry, radio waves, and Wi-Fi signals to detect vibrations or human presence.
• Audio monitoring via light-based techniques has historical precedence, such as the Soviet-era "The Thing" device, which used radio signals to modulate audio rather than traditional optical lasers, highlighting a distinction between radio-frequency and optical-wavelength surveillance.
• Modern Wi-Fi and cellular networks are increasingly capable of acting as sensors for "spatial intelligence," enabling presence detection and activity monitoring that bypasses traditional visual surveillance, as seen in emerging standards like IEEE 802.11bf.
• While high-end surveillance tools exist, many tracking methods are accessible at a low cost, with researchers and hobbyists demonstrating that commodity Wi-Fi and radio equipment can be used to estimate traffic flow, detect human motion, and map home environments.
• The proliferation of "smart" devices creates an environment where household hardware—from security cameras and smart TVs to Zigbee-enabled light bulbs—can be repurposed to collect telemetry or audio data, often functioning as a persistent, automated surveillance grid.
• Total anonymity in modern society is increasingly difficult to achieve due to the density of interconnected devices, government-owned surveillance infrastructure, and the pervasive nature of ad-tracking networks that associate unique identifiers with individuals.
• The rise of fraudulent "scam" repositories—which claim to use commodity Wi-Fi signals for advanced AI-driven human motion tracking—illustrates the current public anxiety surrounding these technologies and the ease with which misinformation can be propagated.
• Hardware-level security, such as disabling antennas or using "dumb" monitors, offers only limited protection, as surveillance techniques may rely on signals broadcast from external sources or signals already integrated into the standard operation of modern electronics.
• The constant exposure to radio frequency energy and digital tracking is a fundamental aspect of modern life, shifting the conversation from avoiding surveillance entirely to threat modeling and recognizing that surveillance is often automated rather than human-directed.
The discussion highlights an ongoing tension between the increasing ubiquity of wireless sensing technologies and the difficulty of maintaining individual privacy in a hyper-connected environment. Participants emphasize that many of these surveillance methods, while technically sophisticated, have deep historical roots in analog eavesdropping and radio-frequency monitoring. There is a broad consensus that while high-end "nation-state" capabilities remain formidable, the real shift has occurred in the automation of tracking through commodity hardware like routers, smart appliances, and mobile devices. Ultimately, the conversation underscores that while technical mitigations exist, achieving complete anonymity is nearly impossible given the density of modern digital infrastructure and the extent to which everyday devices are now inherently capable of acting as sensors.