more on Advertisers Tune In to New Radio Gauge

[David Farber <dave () farber net>]

Begin forwarded message:

From: Rusty Hodge <rusty () hodge com>
Date: November 10, 2004 4:22:45 PM EST
To: dave () farber net
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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