Recovering Audio from Video – But Not How You Might Expect…

 In The Art of Sound – Algorithmic Foley Artists?, we saw how researchers from MIT’s CSAIL Lab were able to train a system to try to recreate the sound of a silently videoed object being hit by a drumstick using a model based on video+sound recordings of lots of different sorts of objects being hit by a drumstick. In this post, we’ll see another way of recovering audio information from a purely visual capture of a visual scene, also developed at CSAIL.

Fans of Hollywood thrillers or surveillance-themed TV series may be familiar with the idea of laser microphones, in which laser light projected onto and reflected from a window can be used to track the vibrations of the window pane and record the audio of people talking behind the window.

Once the preserve of surveillance agencies, such devices can today be cobbled together in your garage using components retrieved from commodity electronics devices.

The technique used by the laser microphone is based on measuring vibrations caused by sound waves relating to the sound you want to record. Which suggests that if you can find other ways of tracking the vibrations, you should similarly be able to retrieve the audio. Which is exactly what the MIT CSAIL researchers did: by analysing video footage of objects that vibrated in sympathy (albeit minutely) to sounds in their environment, they were able to generate a recovered audio signal.

As the video shows, in the case of capturing a background musical track, whilst the audio was not necessarily the highest fidelity, by feeding the input into another application – such as Shazam, an application capable of recognising music tracks – the researchers were at least able to identify it automatically.

So not only can we create videos from still photographs, as described  in Hyper-reality Offline – Creating Videos from Photos, we can also recover audio from otherwise silent videos.

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