Telescope and photodiode helped eavesdrop on speech in the trembling light

Israeli scientists have presented Lamphone — method for remote eavesdropping, which allows real-time decoding speech according to the vibration of the surface of normal ceiling light bulb upon exposure to sound waves. For such eavesdropping used a telescope connected to a photodiode, which converts the trapped light into an electrical signal, and the signal is then converted into speech spectrogram using algorithm, then it is already possible to extract the said. Using Lamphone the researchers were able to recognize and recover the speech and music from the room at a distance of 25 m from the observation point on the bridge. Read more the method described on the project website.

The fluctuations of the objects that appear when the surface facing the sound wave, observer is not visible. To recognize them, of course, still possible, but this requires a camera with very high frame rate, and the fluctuations even at the slow motion video will be barely visible. In 2017, the developers of the Massachusetts Institute of technology, this, however, was enough to transform the barely visible vibrations of objects in the sound that is played beside them: change the pixels on the picture of the plant, shot on a camera with a frequency of 2200 fps, they managed to restore the whole melody.

Such a method, even though it works quite well, has serious limitations. First, a camera with a great frame rate is quite expensive (however, the method works even for videos with 60 frames per second). Second, the resolution of images when shooting with a high frame rate obtained is high and requires much time for processing and analysis, therefore, this method difficult to use in real time. In addition, it is also more difficult to work with long distances, and the technology which converts vibration into sound, and more for discreet wiretapping, including — from a distance.

Ben Nissi (Ben Nassi) of the University named after Ben-Gurion in the Negev and colleagues decided to get rid of all these limitations and developed a new method of tapping on the vibrations of the vibrating object which serves as the usual hanging from the ceiling light. Instead of the camera with high frame rate for the registration of the oscillations is used for the photodiode — photosensitive semiconductor, which converts illuminating its photons into electric current. The light from the bulb onto the photodiode flows through an Amateur telescope, that can be placed far enough away from the bulb.

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