"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*

Comments

Re:So how . . .

[spectecjr]

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

Re:Privacy?

[John Hasler]

> 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.

Re:Privacy?

[John Miles]

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.

Re:How does this work?

[Istealmymusic]

Okay, time for a clue. As I'm sure you know, your radio antenna receives all wavelengths simultaneously. The receiver has to filter out all but your tuned-in frequency. To do this, a so-called resistor-capacitor (the cap being your tuning knob) "RC tank circuit" is utilized to provide an oscillation to beat against the mish-mash of the received environmental waves. Local oscillators of this kind are powered by a solid-state Gunn oscillator in a Phase-Locked Loop (PLL).

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.

Re:Privacy?

[Lumpy]

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.