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Technical Properties of the Arbitron PPM System
To respond to these inquiries, I carefully read the relevant patents (U.S. 5,450,490, 5,764,763, 5,581,800, and 5,787,334) issued to Arbitron. In addition to the patents themselves, the file folders ("file wrappers") maintained by the U.S. Patent and Trademark Office (USPTO) containing correspondence between the inventors and the patent examiner during the application process are in the public domain and available to any interested party.
Since most of the relevant patents were filed and issued in the late 1990s, it is possible, and perhaps likely, that Arbitron has advanced the PPM technology beyond that described in its patents. Nevertheless, the fundamental basis of PPM as described in these patents is entirely consistent with Arbitron's public literature.
The PPM system is just one example of watermarking technology, which has been subject to extensive research for the past two decades. The Arbitron PPM system embeds watermarks with station identification codes into the audio program at the time of broadcast using an encoder in each individual radio station's transmission chain. Portable PPM decoders then identify which stations the wearers of these people meters are listening to.
In order to embed the digital bits that make up the identification code, watermarking modifies the original audio by adding new content or changing existing audio components. The goal of an ideal audio watermarking system is to be 100 percent reliable in terms of embedding and extracting the watermarking data in all "typical" listener scenarios while remaining 100 percent inaudible for all "typical" program material. These goals underscore a paradox: 100 percent encoding reliability requires audible watermarks. Conversely, to achieve total inaudibility, watermarks cannot be present on some material, which sacrifices reliability. Anecdotal reports from radio broadcasters say that Arbitron lowered the watermarking energy in response to complaints about the watermarking being audible in certain circumstances. Trade offs must always be made in audio watermarking systems to balance audibility and reliability.
Some radio broadcasters question whether the PPM watermarking system is as reliable as Arbitron claims. From my analysis, I conclude that the answer is both yes and no, depending on the definition of typical program material and typical listening environments.
Real-world conditions may not always match the assumptions made by the PPM system designers. This article explores potential consequences of station programming and listener behavior that deviate from the PPM system designers' assumptions of typical. After conducting a modest literature search, I found no published papers that describe the behavior of the PPM system in real-world conditions. Because of the absence of empirical data, this article also considers how an empirical evaluation might be conducted.
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