Several list members have mentioned methods to detect hardwired
microphone systems that have been concealed in walls, floors, and ceilings.
The first issue that we must consider is the nature of the signal
that is passing through the cable or wire that we are seeking to
detect. A cable designed to carry base-band audio can be detected
with the same equipment used to detect wire cable, but the sweep
method is slightly different.
To detect a hardwired microphone we use a generate white noise
(hissing sound) into the area we are going to check. Then we pass a
short 3-6 inch high permeability, high nickel content, magnetic loop
stick antenna across the surfaces being examined. The antenna is
tuned for the voice band between 300 Hz and 6 kHz, cleaned up by a
high pass filter and then amplified by 40 dB of so. The voltage out
put from this circuit is that used to modulate the output of 50 MHz
oscillator and then drop it into a CPM-700 or similar product which
will let your hear the white noise though the headphones (this works
surprisingly well). This assumed that the wire is terminated at a
microphone, and that the microphone is listening to room audio. You
will have optimal results using a FFT based audio spectrum analyzer,
but try the CPM-700 method first.
Video cabling is detected through similar means, but you will want
less nickel in the antenna, and will want to be about 12 inches long.
On the other hand video is easier to detect because of the fast
transition times of the timing signals and the color burst for
non-monochrome signals. If you are merely looking for the sync pulses
then a signal loop stick can be tuned and filtered from 14 kHz to 32
kHz. The magnetic leakage probe for the CPM-700 does a nice job of
this, but you may want to consider installing a trim cap and tuning
it for maximum sensitivity at 16 kHz or so. A shielded non-ferrite
core small loop antenna will also give you favorable results, and an
extended detection distance but you may have trouble filtering out
stuff are not interested in.
If you have a hardwires microphone or camera system, but the
microphone is inactive, or the camera is turned off or disabled then
you will have to inject a broad spectrum of pulsed signals via a comb
oscillator and then detect those same signals a few feet away with a
tuned loop stick. The signal coming off the pickup antenna is gated
based on the distance between the transmit and receive antenna, and
because of this the two antenna can be place on a non-metallic rod a
foot or two apart and this delay easily calculated. This signals can
also be modulated at 50 MHz and injected into a CPM-700 or similar product.
Just remember that for the base band audio wiring you need a HIGH
NICKEL and COPPER bar, but for the video wiring, you will need a
slightly different bar. Either way you will need several hundred
loops of wire, and a tuning capacitor and you may want to combine
multiple bars instead of using longer bars.
IMHO, everything you can do with a Bloodhound you can also do with
CPM-700 (for a fraction of the price), and you will actually have
better results with the CPM-700 due to the extended frequency range
of the equipment.
-jma
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Received on Sat Mar 02 2024 - 00:57:27 CST