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They are, it's one of the reasons they [Nielsen] bought Arbitron back in 2013. The PPM devices use low-frequency tones encoded into the audio stream at the tim
by amitparikh 8y ago
They are, it's one of the reasons they [Nielsen] bought Arbitron back in 2013. The PPM devices use low-frequency tones encoded into the audio stream at the time of broadcast.
https://en.wikipedia.org/wiki/Portable_People_Meter https://en.wikipedia.org/wiki/Portable_People_Meter