Jim Choate wrote on 1998-02-09 23:16 UTC:
The screen grid is where the dot clock goes to modulate the e-beam, or is your claim we're going to modulate the filament directly?
Ok, now I understand what you where talking about. Sorry, this was just a very silly language misunderstanding (my knowledge of CRTs is based on German vocabulary, so I mixed up "screen grid" and "mask" and was surprised to read that you seemed to claim that the per-pixel on-off modulation that van Eck described for his old-style terminals in fig 8c of his C&S paper is still there in the form of current interruptions caused by mask holes ... I hope you can understand my surprise ... ;-). Forget everything I wrote about "screen grid modulation" in my last reply, I fully agreed with you here.
If your getting your signal off the harmonics you're doing it the hard way. Go back and re-read your texts on Fourier Transforms and then do a power-spectrum analysis on the signals to the tube; what you will find is that the primary frequencies get the majority of the signal (eg 1st harmonic of a square wave (ie a dot clock) only gets, at best, 1/3 of the energy of the primary).
But this is not necessarily, where the the monitor resonates nicely. Van Eck has reported very similar results in his paper: His VDU had a dot clock of 11 MHz and he got nice resonance peaks near 125 and 210 MHz.
A very effective method to confuse Van Eck is to have several monitors sitting next to each other with different displays. A more active display is much more effective than one that is static (eg. such as a person typing in an email to cypherpunks).
If you have only a van Eck style receiver, yes. But as soon as you record the reception over some time and observe the images phases to drift only slightly against each other, you might be able to separate them using similar processing techniques as used in computer tomography. Markus -- Markus G. Kuhn, Security Group, Computer Lab, Cambridge University, UK email: mkuhn at acm.org, home page: http://www.cl.cam.ac.uk/~mgk25/