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"Smart" Billboards Debut in Sacramento
k0osh.CEOofCLIT writes "Remember the billboards in "Minority Report" that scanned your eyes and changed the advertisement based on your shopping preferences? The Sacramento Bee reports: "Soon, this sign along the Capital City Freeway will be able to change its message based on what radio stations motorists have tuned in.""Yeah, Chris can't spell. He and Rob should form a club. *grin*
So how exactly do these billboards figure out what radio stations people are listening to? Do radios emit EM signals that can be used to determine what they're tuned to (it's been a long time since I took a physics class, somebody help me out here)?
Yep - as do television sets.
It's called heterodyning, and is used to decode FM (frequency modulated) signals. Basically, you mix the signal coming in with the frequency you want to listen to, and the signal at that frequency gets amplified (due to the interference), and the outcome of that is rectified, amplified, and is ultimately what you listen to.
So the billboard picks up the frequency you're mixing the incoming signal with (because you need a frequency generator to create that frequency, and they will emit it -- there's not much you can do to stop it short of burying it in a completely metal box -- which kind of stops the incoming radio signal).
Simon
Coming soon - pyrogyra
> there's no passive way to do this at all.
Wrong. All they have to do is monitor the radiation from the local oscillator in your radio. The British government uses this to detect unlicensed radios and TVs. To stop them modify your radio to use a non-standard IF.
Warning: this article may contain humor, sarcasm, parody, and perhaps even irony. Read at your own risk.
Sure, there's a "way." Most FM radios in the US, presumably including the ones in most cars, do their analog signal processing work at an intermediate frequency (IF) of 10.7 MHz. To convert the station's frequency to the IF, the radio uses a local oscillator tuned to either Fincoming+10.7 MHz or Fincoming-10.7 MHz -- usually the former, since it means the range of the oscillator is smaller as a percentage of its output frequency. So if you're listening to a station at 95.5 MHz, your radio is emitting a very weak local-oscillator signal at 106.2 MHz. A receiver at the billboard's location only has to watch for the LO signals corresponding to the stations that are paying to advertise on it at the moment. Often you can demonstrate this yourself by putting two FM radios next to each other, tuning one to a blank spot on the dial near the high (or low) end of the band and sweeping the other one back and forth across the band until it appears to interfere with the first radio.
This is also how UK residents who operate their TV sets without the proper government license are ferreted out. A van cruises around the neighborhood listening for radiated TV local-oscillator signals from unlicensed households.
Dahlmann tightly grips the knife, which he may have no idea how to use, and steps out into the plain.
The output is fed through a low-power Schottkey diode to clamp the waveform and lock onto the desired frequency. I'm sure you can tell what I'm getting at: in order to receive frequency RF, one must generate frequency IF via local oscillations (LO), and IF directly corresponds to RF. Stephen Wolfram points out the relationship V[IF] = V[RF] + V[LO] for increasing and V[IF] = V[RF] - V[LO] for decreasing. Armed with this formula and decent knowledge of the radio's tank circuit, it is trivial to pick up the LO and IF frequencies your car radio transmits, albiet inadvertedly, and customize the billboard contents accordingly. Quite simple really.
"The lesson to be learned is not to take the comments on slashdot too literally." --Vinnie Falco, BearShare
OK, I know very little on the subject, so I want to know if it would work to shield the radio, but not the antenna. Would the internal frequency it still leak "back up" the antenna? Could you extend this in some way so that it wouldn't? (Second, unshielded receiver box, sending a "shielded" signal to the receiver/decoder/whatever.) I mean (given you're paranoid enough) you could probably make a box to encode the whole signal digitally and send it encrypted to a shielded box for digital processing. If you were desperate.
(And for those who say "who cares, why be so silly over such a small thing"... well, it might not matter now, when your radio station of preference is being monitored, but at some point, it will. That's when this knowledge becomes useful.)
Don't think of it as a flame---it's more like an argument that does 3d6 fire damage
It's a majority rules kind of thing... the strongest LO frequency it finds is converted back to the station it corresponds to, and that is what determines which of the four ads show. If there's no way to make any sense of the signals, then the board just remains in place showing whatever ad the last group of cars that it could make sense of indicated.
have you actually tried to detect that signal outside a vehicle? ok now do it to a MOVING TARGET.
and what you are talking about is not the case.. the BBC transmits a subcarrier with a tone on it that is easily detectable. It's the same detection scheme used by american cable TV companies to snif out people stealing cable tv. It's a simple device and putting the subcarrier there makes it air tight in court.. trying to say that "we detected what channel your tv is tuned to doesnt work in court... saying we detected our special signal we transmit to catch them.... does.
Do not look at laser with remaining good eye.