[Cryptography] "The Visual Microphone: Passive Recovery of Sound from Video"

[Jerry Leichter]

On Aug 5, 2014, at 12:02 AM, Christian Huitema <huitema at huitema.net> wrote:
> Everything leaks.  Film a bag of chips through a soundproof window, decode
> its 
>> vibrations to listen to what's being said in the room.
>> 
>> https://www.youtube.com/watch?v=FKXOucXB4a8
>> 
>> I'll bet you could go a pretty good job of decoding keyclicks from a
>> "secure" room this way.
> 
> Very nice demo, but it requires a really high speed camera -- Nyquist and
> all that. 
Did you watch through to the end?  Starting at around 3 minutes in, they show how to use artifacts due to shutter motion to recover sound reasonably well using a standard camera - frequencies 5 times the frame rate of the camera.  The Nyquist Theorem is a valid result about Fourier transforms, but its real-world application can be subtle.
                                                        -- Jerry