Re: Optical CRT eavesdropping from diffusely reflected light
From: Markus Kuhn (n05W28+mgk25_at_cl.cam.ac.uk)
Date: 12 Jul 2005 08:35:36 GMT
Jan Panteltje <pNaonStpealmtje@yahoo.com> writes:
|> > http://www.cl.cam.ac.uk/~mgk25/ieee02-optical.pdf
|> Is that the best picture in that pdf that you got?
|> There is still a significant error (smear) in the LF range.
|> Line based clamp (DC component restor) + perhaps some LF compensation
|> would further improve things?
|> This sort of LF error can be compensated a lot by RC filters.
I use already something far more sophisticated than analog
RC filters, namely I properly deconvolve the signal (via FFT and a
complex division in the fourier domain) with a close model of
the exact impulse response. Any combination of analog filters can
only ever be a crude approximation of that. There are certainly
ways to improve the quality even further by
- even more acurate modeling of the phosphor impulse response
(I simply stopped tweaking it when I was impressed enough
with the readability of the result);
- use of a filter wheel in front of the photomultiplier, and
modeling the phosphor impulse response for narrower bandwidths;
this will not only give you colour information, but it will
also take into account that the impulse response can look rather
different at each wavelength.
-- Markus Kuhn, Computer Laboratory, University of Cambridge http://www.cl.cam.ac.uk/~mgk25/ || CB3 0FD, Great Britain