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"Earlier this month Network World's Phil Hochmuth profiled Nielsen's efforts to figure out what people are watching on their iPods and other mobile devices. Nielsen, you may know, is the company that determines the ratings, i.e. audience size, of a particular TV show. As technology has progressed Nielsen has had to try and cope.Early on the Nielsen box would be bolted to your TV and hardwired into the electronics to know exactly what you are viewing, but since home electronics have changed so much this is no longer a viable option (no one is bolting a box to my iPod). Luckily, Nielsen has partnered with content creators to use psychoacoustic encoding to let Nielsen figure out what you're watching. Psychoacoustic encoding sounds scary, but it is simply an audio signal that is encoded into the television program. This way Nielsen's gear can look for this signal and find out what you're watching. They are working on a small device that one would plug in between your headphones and your iPod and monitor for these signals. When it hears one, it registers the show you're watching. At night, you charge the device and the data gets sent to Nielsen."^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.engadget.com/2006/10/25/nielsen-wants-to-know-what-youre-watching-on-your-ipod/>