In the Steven Spielberg film "Minority Report," futuristic billboards recognize customers on sight and tailor their sales pitches to an individual's shopping patterns.

As it turns out, the future is just around the corner for motorists in the Sacramento area.

Starting next month, two freeway billboards will be able to tell which radio stations passing cars are tuned to and then change the image on the sign to fit listeners' profiles…

Alaris President Tom Langeland said the sensors will be able to determine the radio station playing in 60 percent of the cars passing by. At Cal Expo, that would mean nearly 10,000 vehicles would have their radio stations scanned during the peak hour of traffic; at I-80 near the Roseville Auto Mall, it's about 7,700 vehicles. (Full text here (http://www.sacbee.com/content/news/story/5349258p-6338265c.html))Okay, geeks! I was under the assumption that a car radio was a passive listening device. How are they pulling this off?

Peace.

The majority of radios are not passive listening devices. A superheterodyne radio, the type most commonly used today, uses a local oscillator (LO) whose frequency is tuned by the listener. The local oscillator's signal is then mixed with the incoming RF, which mixes the information signal in the RF band down to audio baseband. The information is then frequency demodulated, decoded for stereo and what-not, and passed to amplifiers to power your speakers.

The local oscillator always radiates. Even if you're careful, the microstrips or traces on the PCB, along with other components like capacitors, can radiate a small amount of local oscillator energy away. These signals are faint, but a sensitive directional receiver mounted on a billboard can detect the faint LO emission coming from a passing car. The billboard would be equipped with a wide-band receiver with the same channels as the FM radio band -- and it would listen to see which channel picks up a bit more radiation than the background, and turn on the correct ad.

- Warren

I love these forums...
Thanks Warren!

Let's see what happens if I put all but the antennea in a farraday cage.

Originally posted by Gifted
Let's see what happens if I put all but the antennea in a farraday cage.
A little bit of LO emission may actually still sneak back out your antenna!

- Warren