Researchers have developed an optical microphone that uses cameras to record the vibrations of objects. This system makes it possible to isolate the sound of a single instrument in an orchestra, and eliminates any effect of the acoustics of the place.
Isolating the sound of a single instrument in an orchestra, or recording the music being played without ambient noise, is an extremely complicated task, even with the best of microphones. To resolve this problem, from Carnegie-Mellon University in the United States have created a optics capable of seeing of an object and thus capture only the sound produced by a single instrument.
Such a system using only a camera would need to zoom in on an object to be able to pick up tinyand should be at very high to detect the frequency of these . This type of device would be extremely expensive and complicated to set up.
A 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 awhich is pointed at a surface, such as an instrument. When the laser is reflected, the create or a speckle, 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 with a frequency that can. The trick is to combine images from two different sources.
Explanation and demonstration of the optical microphone. Only in English, activate automatic translation of subtitles. © Mark Sheinin
Two cameras to record at 63 kHz
The first camera has aoverall (or global shutter) which records the scene in one take. The second uses a rolling shutter (or rolling shutter) which saves the image line by line on the with a very slight delay. This delay, which can create distortions in the photo of a moving object, is used here to obtain information about high frequency vibrations. The lines are thus recorded with a frequency of 63,000 .
The researchers then use an algorithm to put the two images together and recreate the sound. Their system works directly on the instrument, likeor a violin, and even on other objects which vibrate by , like a packet of crisps placed in front of a loudspeaker. It is even 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 theothers. It entirely removes the effect of acoustics from the recording room by effectively eliminating . The system could even be used to monitor every machine in a noisy factory to spot when one needs maintenance.