Blesser Analyzes Arbitron's PPM in Technical Paper

RADIO ONLINE | , , | :am CT

In a research paper titled "Technical Properties of Arbitron's PPM System," 25-Seven Systems Director of Engineering Dr. Barry Blesser says after analyzing public information, he has concluded that PPM appears to be "technically sophisticated and well conceived and is expected to be extremely reliable when tested under "typical" conditions. But, he says, a more "careful analysis" suggests that there may be real-world scenarios that dramatically degrade PPM's performance.

"Regardless of how much testing Arbitron performed before releasing the PPM system, the real world of thousands of radio stations exposes the system to an almost infinite variety of idiosyncratic properties of particular programs, speaking styles of announcers, and listening environments," writes Blesser.

He continued, "Radio stations that believe their listenership has been inaccurately reported by the PPM system may wish to determine if their context deviates from the assumptions upon which the PPM system is designed."

Blesser noted that he had been asked by several radio engineering colleagues to comment on the technical properties of PPM. Inquirers were particularly interested in possible technical explanations of why some program formats on some stations have received dramatically lower audience ratings compared to that of the old diary system.

The PPM system is just one example of "watermarking" technology, which has been subject to extensive research for the past two decades, explained. 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% reliable in terms of embedding and extracting the watermarking data in all "typical" listener scenarios while remaining 100% inaudible for all "typical" program material.

For these reason, he says that some radio broadcasters question whether the PPM watermarking system is as reliable as Arbitron claims. From his analysis, 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.

Unfortunately, 100% 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.

After conducting a "modest" literature search, Blesser found no published papers that describe the behavior of the PPM system in real-world conditions. "Because of the absence of empirical data," his paper considers how an empirical evaluation might be conducted. More information on his paper can be found at 25-seven.com.