Ray Dillinger wrote:
> If you get two sensitive microphones in a room, you
> should be able to do interferometry to get the exact locations
> on a keyboard of keystrokes from the sound of someone typing.
Interesting. Probably not the easiest way to snoop, but you might be
driven to it.
> I guess three would be better, but with some reasonable
> assumptions about keys being coplanar or on a surface of known
> curvature, two would do it. Interesting possibilities.
Interferometry like measuring the time delay between the two
microphones? Defines a hyperboloid, which when intersected with the
keyboard still isn't specific enough, so I think you need three mics.
> [A quick contemplation of the wavelength of the sounds in question
> would put an end to that speculation I suspect. --Perry]
You can localize to better than the shortest wavelength present, so
the spectrum isn't obviously a problem. Consider it under ideal
conditions -- anechoic, no transmission losses, omnidirectional
emission. Then the mics get the same signal (at different times), and
you can just find peak correlations between them.
The required accuracy is roughly a centimeter, or 30 usec of sound
travel, over one sample at audio rates; adjust that trigonometrically
for mics placed other than 60 degrees apart. Keystrokes are noisy and
should make decent correlation codes. Less-than-ideal conditions
might make the scheme impossible, but I don't know how to conclude
that without a lot more work.
I don't know the state of the art, but a little web searching appears
to say that people can localize speech in a videoconferencing room to
within one 44-kHz sample. http://www.ie.ncsu.edu/kay/msf/sound.htm
--
Eli Brandt | [EMAIL PROTECTED] | http://www.cs.cmu.edu/~eli/
