Slashdot Mirror

← Back to Stories (view on slashdot.org)

Smart Billboards

Posted by michael on from the your-dog-wants-steak dept.

djdanlib writes "The New York Times ran this story Sunday about the Mobiltrak smart billboard system. It works by detecting what radio station you're listening to as you pass by a billboard, then displaying advertisements targeted at that station's demographic. It's kind of like a real-time Nielsen Ratings system for radio. And it's entirely passive, requiring no special hardware in your car - it receives the faint tuning signal generated by your radio." We've mentioned these before.

5 of 313 comments (clear)

  1. The google link by Albanach · · Score: 4, Informative

    here

  2. People seem to be misunderstanding by jerw134 · · Score: 5, Informative

    This sign doesn't react to what it detects. It simply logs what stations people are listening to, so that the advertiser on the board can look at that info and decide who to target during different parts of the day. It works exactly like Nielsen ratings. The info gets collected and people look at it to make decisions, it's not reacted to immediately.

  3. Re:Some questions by cryptor3 · · Score: 5, Informative
    Why does every radio emit a signal? Is it inefficiency? Is it really every radio or only old ones?

    Your radio has a component in it (an oscillator) that vibrates at the frequency of the station you're listening to. This is "tuning" into the station. This vibration is what emits the signal.

    Is this signal broadcasted back through your antenna or is this just a faint signal inside your radio and they have really good receivers in their billboards? Definitely a result of good receivers in the billboard. Though I think the antenna helps.

    Has anybody tried to create a radio that doesn't emit this signal?

    Not that I know of. I don't think it's really been a major issue worth pursuing in the consumer market. The best way to do it would probably be to shield the box. But since you've got to have an antenna linking the oscillator with the emag signal, you can never completely isolate it.

    Is this only something with FM radio, or also with AM?

    Both AM and FM. You've got to have an oscillator to tune into either one.

  4. Re:Some questions by mellon · · Score: 4, Informative
    Most FM radios use superheterodyne receivers (actually, I'm not sure how true this is anymore, what with software-defined radios, but it certainly has been historically true). There's a pretty good quick explanation of how this works at this site. Look at the text around the first diagram.

    The key is that the frequency of the local oscillator varies, so if you can detect the output of the local oscillator, you can tell to what frequency the radio is tuned. I'm oversimplifying greatly, and the article I've referred to is a pretty rough overview - if you really want to know how this stuff works, you need to do some serious studying. :'}

  5. Not quite right by wowbagger · · Score: 5, Informative

    You are almost right, but not quite.

    The way a modern superheterodyne radio works is like this:

    You want to listen to a radio frequency at F1.
    The radio creates a local oscillator (or LO) frequency of F2, such that abs(F2-F1) = F3, where F3 is a fixed, intermediate frequency (or IF). A common IF for FM radios is 10.7 MHz, and a common IF for AM radios is 455 kHz. By pulling the signal to a fixed frequency the rest of the radio's design can be better optimized and simpler.

    Now, F2 can be either F1+F3 or F1-F3, it make little difference. So one way to confuse the system would be to retune the radio so that it uses "the other IF" - i.e. if the radio is using F2=F3+F1, retune the guts so that it uses F2=F1-F3.

    Alternatively, replace the IF strip to change F2, and then retune the radio appropriately - if the sign's systems assume an IF of 10.7 MHz, and you are using an IF of 9.7 MHz, that will confuse the sign. The difficulty there is getting components designed for a non-standard frequency. If the radio is using the old "tuned slug" design this isn't so bad, but if the radio is using a crystal filter you are looking at custom crystals.

    However, there is no need for the LO to be coupled to the world - the first stages of the radio can amplify the RF and decouple the first LO mixer from the world. It just takes a bit more work on the sheilding of the radio - you use a milled block of aluminum rather than foil sheilds. I know, since that is what I do for a living - design radios (well, radio test gear, which is a special case of the class Radio)

    However, building a jamming oscillator at the needed frequencies to scramble this sign, while completely illegal, is also trivial - buy a cheap FM transmitter kit and retune it slightly. Of course, by causing interference you are in violation of FCC part 15 rules, and will get nailed if you get caught, so don't, 'mkay?

    If it bothers you, just don't listen to the radio.

    --
    www.eFax.com are spammers