Re: End of Top of the Pops

In article <t7kgc29k3bemk4kmkg9pgql2penbq82525@xxxxxxx>, Jc wrote:
Indeed. The tempest stuff relies on the fact that the signal fed to
the display are still effectively RF. If you think of a monitor
refreshed at 50 Hz the line sync is around 15 KHz and higher refresh
rates give higher component frequencies. All the other signals used to
drive the display are radiated to some degree in the monitors cable
and circuits. The challenge is to remotely amplify and separate all
these low frequency signals, then lock the sync. Once you've done that
the image could be displayed on a conventional screen.

Th 15.625kHz signal is a voltage pulse, so of course it produces a series
of harmonics spaced 15.625kHz apart, well up into and beyond the medium
wave broadcasting band, which you can easily verify by tuning a MW radio
across the band anywhere in the vicinity of a CRT display. However, this
signal is only used to scan the electron beam and contains no video
information at all.

To recreate what is displayed, you would need a signal related to the
amplitude of the video signal or the brightness of the screen. Optical
pickup of this probably works quite well in suitable circumstances, but
unless the video signal is deliberately modulated onto an RF carrier, I
don't see how enough of it could be picked up even from a few yards away.

The Wikipedia article you refer to in your other message says that
someone called Van Eck is said to have picked up video signals from
electromagnetic emissions from a monitor, but it does not say from how
far away, and it is extremely vague about exactly what signal is supposed
to have been picked up, and which part of the monitor's circuitry is
supposed to be radiating it.

I don't understand the bit about the "electromagnetic lens" containing
video information, because video modulation is applied to the control
grid and only affects beam current. If the focusing fields (usually
electrostatic rather than electromagnetic in TV type displays), which
constitute the electron lens have any modulation at all, it is only
scan-related to allow for differences in focus requirements between
centre and edge, and does not contain any video information.

I can only guess that technical articles can be repeated and embellished
without reference to the original in the same way as ordinary gossip,
eventually creating nonsense. Sort of "technical Chinese whispers".

Rod.

.

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