Juliusz Chroboczek wrote on 2003-03-04 12:16 UTC: > MK> - Lowpass filtering the glyphs in horizontal direction > > Why the glyphs? Wouldn't you want to do that for everything that's > displayed?
Sure, if that's feasible to implement. Text is just the most interesting and most radio-readable part of the screen, because of its high-frequency content. You can also do the filtering in analog right after the DAC on the card of course. I connected a spectrum analyser to a couple of graphics cards and found significant differences in their harmonics output. VGA outputs of some Toshiba laptops go well above 1 GHz with their harmonics, whereas a Matrox cards that I tested seemed to filter quite properly. I personally believe that careful selection of the graphics cards adds far more protection than the font filtering along could ever provide. And if you want to make a PC easy to eavesdrop (sabotage), then just short-circuit the output filters after the DAC. The user might even thank you for sharper pixels ... at the cost of more radio/TV interference. > MK> - Replace the least few significant bits with pseudo-random bits for each > MK> usage of a glyph on the screen. > > Are you implying that what the eavesdroppers get is the derivative of > the signal rather than the signal itself? Not quite, but something similar. For analog displays (CRTs), they get a high-pass filtered version of the video signal, replicated throughout the spectrum by the discrete sampling. If you have a 100 MHz pixel clock, then any arbitrary 50 MHz band in the upper VHF or lower UHF range will contain a complete copy of a high-pass filtered version of the full pixel information. Therefore, continuous-tone images (JPEG photos, etc.) are practically impossible to see for an eavesdropper, unless they are dithered significantly. Markus -- Markus Kuhn, Computer Lab, Univ of Cambridge, GB http://www.cl.cam.ac.uk/~mgk25/ | __oo_O..O_oo__ _______________________________________________ Fonts mailing list [EMAIL PROTECTED] http://XFree86.Org/mailman/listinfo/fonts
