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Technical Properties of the Arbitron PPM System
Nevertheless, insuring that audio leaving the station is optimized for successful PPM encoding is every station's business. A program director who understands which audio makes the PPM system more reliable has an advantage over a competitor who does not. Knowledge that is not public has high monetary value, not unlike that used for insider trading in the stock market.
To avoid gaming the ratings, I believe all broadcast stations should have a deep understanding of how the PPM system works in the real world, not in the ideal world of the designers' laboratory. Stations should understand why their confidence decoders are kicking out failure indications.
Testing Your Radio Station
A radio station can test its programming as well as explore the listening assumptions in the world of PPM. There are two basic approaches to evaluating your environment.
First, the input to the PPM encoder can be subtracted from the output of the encoder to create a "mix-minus" of the encoding. The subtraction removes most of the original program and allows the channels to be seen clearly with a spectrogram analysis of the difference. This will indicate which channels are being used to encode data and measure the total power in the watermarking information. Given this measurement technique, every program can be evaluated for its watermarking robustness, showing the ability of the PPM system to carry watermarking information on specific audio samples without examining the encoding itself.
Moreover, the Arbitron decode confidence monitor provides an output that indicates if watermarking decoding was successful. If the encoding process is marginal because of weak channel encoding, then it may take longer for the decoder to find the embedded data, if it finds it at all. A long response time to acquire the station ID watermark serves as a warning that a non-ideal listening environment may result in a failure to detect the ID.
Second, the Arbitron confidence decoder monitor can be placed in a listening setting deemed by the station as typical of its real-world listeners. For a drive-time talk show, the environment might be a truck or automobile with the windows rolled down. For a late-night show, it might be a bedroom. The microphone used by the decoder device as its input source may be at some distance from a radio loudspeaker and there may be "environmental" obstacles, such as a chair blocking the high-frequency channels but not the low frequencies. Effectively, one can determine the required signal-to-noise ratio in the listening environment for successfully decoding a specific received program for that program's target audience.
Keep in mind yet another hidden assumption: There is no evidence that the performance of a station's confidence decoder monitor is the same as that of the portable PPM carried by listeners. The confidence monitor is fed by a direct-wired connection, has a large power budget, and is seemingly without constraints on the computational power dedicated to decoding. In contrast, the portable decoder may have severe constraints related to power consumption, volume, and computational capabilities. On the other hand, the portable version may have front-end analog processing and filtering to optimize the extraction of the acoustic signal. Obviously, the portable device is a better test vehicle since it mimics the real world.
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