CDR: Re: Burglar Politics, Tempesting PC's that watch TV and DVD regions

[jim bell]

----- Original Message -----
X-Loop: openpgp.net
From: Lucky Green <shamrock at cypherpunks.to>
Subject: RE: Burglar Politics, Tempesting PC's that watch TV and DVD regions


> Sunder wrote, quoting
> > http://www.theregister.co.uk/content/1/13863.html
> > <SNIP>
> [...]
> > It's my understanding that TV detector vans work by picking up
> > the radiation emitted by cathode ray tube TVs - which should mean
> > that, if you're rich enough to run an LCD monitor they'll never
> > know you're a secret Paxman admirer.
>
> A popular, but false, myth. The video cards radiate more than the CRT's.
> Laptops tend to be the worst offenders.
>
> --Lucky Green <shamrock at cypherpunks.to>

As to the video cards...
Sorry, Lucky, but you're going to have to support this a little better.
Emissions are a function of  the signal voltage in a conductor, and the
extent that this conductor is free to emit.  The latter is strongly related
to (among other things) the wavelength of the signal versus the conductor
length:  To emit "well" requires an "antenna"  length a significant fraction
of 1/4 wave, and that would be about 40 cm for 150 MHz video signals. (I am
assuming a signal velocity of 0.8C, since it's PC-board insulated.)   These
days it's hard to find a video card as long as 10 cm, and the path length
from the D/A to the output connector is probably 3 cm or so.  (And that is a
pc board trace which is on a 4 (or more) layer PC board, not exactly
conducive to unintended radiation.   Besides, the voltage on that signal is
probably around 1 volt, quite low.  And, remember that this video card is,
itself, stuck inside an enclosure which is at least intended to shield the
outside world from the signals, at least  more than it already is..

Compare this to the monitor:  Those low-level video signals are amplified a
few times to drive the electron guns, and they modulate the passage of
electrons over a 24,000-volt circuit, in a not-particularly-well-shielded
(and huge) vacuum tube.  The length of that tube is close to 50 cm, just
right to emit ca. 150 MHz.  The two ends of the tube must be insulated from
each other, of course.  Naturally, the case of that monitor is...plastic.

As for the laptops:  I am unfamiliar with the way laptops drive their LCD
displays, but if they don't possess a SVGA-type  (X/Y scanned) signal
internally, perhaps they can avoid emitting a useful signal.  CRT's "write"
with an electron beam and must time-multiplex their video information.
LCD's (particularly TFT's)COULD be implemented so that the actual display is
scanned on only one axis, making it virtually impossible to "read."  The
data being sent to that LCD driver would have to be some sort of parallel
digital, I suppose.

Jim Bell, N7IJS

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