[IP] more on Advertisers Tune In to New Radio Gauge
Begin forwarded message:
From: Rusty Hodge <rusty@xxxxxxxxx>
Date: November 10, 2004 4:22:45 PM EST
To: dave@xxxxxxxxxx
Subject: Re: [IP] Advertisers Tune In to New Radio Gauge
The device, the size of a shoe box, is tough to spot unless you make
a habit of looking skyward to inspect utility poles while driving.
This is truly an aggregate data device unless used on a low use road
that only has one car at a time within a few hundred feet.
Since almost all FM radios today use local oscillator 10.7 mhz higher
than the frequency you are receiving, which is then combined with the
incoming signal to generate a signal at 10.7mhz (the IF or
intermediate frequency) which is then decoded to produce audio.
It's that local oscillator which gives off a weak signal that
corresponds to the station you're listening to, which MobilTrak then
listens for.
It would be fairly easy to jam or fake out these devices. Just get a
transmitter running 10.7 mhz higher than the station you want it to
appear that lots of people are listening to, and the signal will fake
out the Mobiltrak receiver. :-)
They have a decent explanation of their technology (which is nothing
new, they're just taking an old theory and applying it):
http://www.mobiltrak.com/about/index.cfm?loc=materials/faq.cfm
Where in the radio circuitry is all this coming from?
Basically, (in frequency modulated radios), there is a mixing going
on. A radio takes a specific known frequency (a local oscillator) and
mixes it with the incoming radio frequency, which then produces (or,
demodulates) the radio information - it produces the audio. At
MOBILTRAK, we are paying attention to the local oscillator frequency,
which is always 10.7 above the station frequency. Now, digital
presents its specific issues, since their local oscillator will be
different. Top
But I thought radios were only receivers, NOT transmitters? If that's
the case, how do you pick anything up from a radio?
Technically, radios are NOT geared to transmit. However, all
electronic devices; portable stereo, clock, cell phones, computer -
emit electronic radio signals and frequencies. It just so happens that
radios emit a unique signal for each station they are tuned to.
Granted, these emissions are tremendously weak, but that is what makes
MOBILTRAK's patented technology so exciting - we can detect those weak
emissions.
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