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Revolutionary, this optical microphone records instruments separately

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Researchers have developed an optical microphone that uses cameras to record the vibrations of objects. This system allows you to isolate the sound of a single instrument in an orchestra, and eliminates any effect of the acoustics of the place.

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Isolating the sound of a single instrument in an orchestra, or recording the music played without ambient noise, is an extremely complicated task, even with the best microphones. To solve this problem, researchers of Carnegie-Mellon University in the United States have created a microphone optics capable of seeing vibrations of an object and thus only capture the sound produced by a single instrument.

Such a system that uses only a camera would have to zoom in on an object in order to capture it tiny vibrations caused by soundand it should be very high speed to detect its frequency movements. This type of device would be extremely expensive and complicated to set up.

One laser and two consumer cameras

Instead, the researchers found a solution using much cheaper and more readily available hardware. The optical microphone is based on a laser which is pointed at a surface, such as a tool. When the laser is reflected, the vibration of the object create scabsor a speck, in other words a pattern that can be recorded with cameras.

Their system then uses two cameras that record at just 60 frames per second. This may seem insufficient for sounds as often as it can reach 20,000 hertz. The trick is to combine images from two different sources.

Explanation and demonstration of the optical microphone. In English only, turn on automatic subtitle translation. © Mark Sheinin

Two cameras to record at 63 kHz

The first camera has a shutter overall (o global shutter) which records the scene in one take. The second uses a roller shutter (o gate valve) which saves the image line by line to the file sensor with a very slight delay. This delay, which can create distortions in the photo of a moving object, is used here to obtain information on high-frequency vibrations. The lines are thus recorded with a frequency of 63,000 hertz.

The researchers then use an algorithm to put the two images together and recreate the sound. Their system works directly on the instrument, like a guitar or a violin, and also on other objects that vibrate resonance, like a packet of potato chips placed in front of a loudspeaker. It is also able to compensate for the movements of the musician.

With this system, sound engineers could track each instrument individually in a band or orchestra, without theinterference other. It completely removes the acoustic effect from the recording room by effectively eliminating it all eco. The system could also be used to monitor every machine in a noisy factory to pinpoint when maintenance is needed.

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