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

[Peter Todd]

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On 5 August 2014 16:59:17 GMT-07:00, Jerry Leichter <leichter at lrw.com> wrote:
>Stories about listening to conversations in a room by bouncing a laser
>of the window have made the rounds for years.  Someone - maybe
>MythBusters - set out to test whether this actually worked.  And they
>found it didn't - all they got was noise.
>
>But as we know from this bit of work, they were missing something
>fundamental:  Just because the unaided human ear - or even the human
>eye, looking at an oscilloscope - can't pick out the signal doesn't
>mean it isn't there.  Had they done the theoretical work and the signal
>processing it pointed to, they would undoubtedly have reached the
>conclusion that the "laser and window" microphone is a great way to
>listen in on a room.  After all, the signal was undoubtedly *much*
>better than even the high-speed camera image - much less the low-speed,
>edge-of-the-scan image.

I can tell you it certainly works: I've personally seen the technique used in a insect biology lab to record the sounds a species of spider makes during mating. They simply bounced the laser off the glass plate the spiders were placed on and did some kind of demodulation; the whole system was a commercial product aimed at the insect biology industry.
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