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Technical Properties of the Arbitron PPM System
To one degree or another, all watermarking technologies use the well-known perceptual principle of "masking," which was first reported in the early 20th century and is a core technical basis for MP3, AAC, and a host of data-rate reduction schemes. In simple language, a loud burst of energy at one frequency will deafen the human auditory system to certain other audio components at nearby frequencies for a period of time before, during, and after the loud signal.
Consider the following illustration: A tone burst at 1.1kHz with an intensity of 0dB will hide (make imperceptible) an added signal at 1.11kHz with a level of -30dB for a period of about 10ms before the burst and as much as 50ms after the burst. However, modern signal-processing techniques can still detect the existence of this added 1.11kHz component even though the ear cannot. This is the basis of PPM and other similar watermarking technologies that use masking for determining the frequencies and intensity of the data that can be added for the station-identifying watermark.
The PPM system constructs 10 spectral channels in the region from 1.0kHz to 3.0kHz. The original program audio energy in each channel is evaluated for its ability to mask an added component. If that masking energy is insufficient, nothing is added. Conversely, if the energy in a channel is large enough, a tone is injected, chosen from one of four possible frequencies within the channel. For example, the channel centered at 1058Hz might have one of the following four frequencies injected: 1046, 1054, 1062, or 1070 Hz.
Each of the four frequencies represents 2 bits of information. If we assume that this process repeats at a 500ms rate, using all channels provides 40 bits per second or 2400 bits per minute of watermark code. Let's further assume that a radio station is credited for a listener if any code is correctly detected within a 3-minute interval. With the very large number of encoded bits generated in 3 minutes (2400 x 3 = 7200 bits) and a station's identification data needing perhaps only 50 bits, there is massive excess capacity for redundancy, error correction, and for audio that does not have enough high-frequency content for masking.
The system should work perfectly. But nobody, including this author and Arbitron, expects the PPM system to be perfect. Every technology has limits, which should be understood by users of a technology.
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