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Technical Properties of the Arbitron PPM System
Hidden Assumptions -- Encoding
While "most" audio, especially conventional music, produces a full band of energy above 1 kHz, some audio has little energy above 1kHz and some has virtually none. Obviously, silence is the extreme case, where there is no possible masking for inaudible watermarking.
When audio produces only a modest amount of energy above 1kHz, only a few channels will be available for carrying strong watermarking information. In such cases, the PPM system can encode watermarking only at very low levels, which makes it fragile when being received.
Examining the spectrograms of certain instruments sparse in musical overtones can provide predictive information on when the PPM will be stressed. Considering that a male fundamental pitch might be as low as 80Hz, some announcers may have a speaking style that is weak in high frequencies. Depending on the structure of the vocal cords and articulation style, there may or may not be any energy at the 12th harmonic of that pitch (which happens to be the center frequency of the first channel of the PPM encoder) for some announcer.
Because fricative phonemes (such as /s/, /z/, /th/, and /f/) contain a broadband hissing component that is like white noise, they can encode large amounts of data. But some announcers may have weak or rapid articulation of such fricatives. Consonants, although short in duration, are good for PPM; pregnant pauses and halting delivery are not. Speaking style matters.
While the typical radio program may produce perfect watermarking performance, and while the average reliability over the universe might be 99 percent, there are likely to be some announcer voices, vocal articulation styles, and specific genres of music that belong to the 1 percent failure cases. If a particular program on a particular station is one of the failure cases, that program might experience "bad luck" in its audience ratings.
Hidden Assumptions -- Decoding
Even though the PPM system may successfully encode sufficient bits into a radio program, the portable decoder may or may not be able to detect those bits if the listening environment deviates from the designer's assumptions. Even if the station's confidence decode monitor, operating with a wired connection to the encoder or a monitor tuner, confirms that there are sufficient bits embedded in the transmitted signal, there is no assurance that the portable monitor can extract those bits.
Specifically, the usable signal-to-noise ratio between 1.0 and 3.0kHz varies dramatically depending on the environment. Not only does the power in the watermarking depend on encoding channels and playback volume, but a listening environment may also be very quiet or extremely noisy. A hostile listening environment is less likely to have deleterious effects when all 10 spectral channels have watermarking data, as some channels will get through. But if only a few channels are encoded, consistent background noise may overpower them.
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