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Comment by SlowRobotAhead | Hacker News Reader

Which makes WAY more sense than a range outside of human hearing that OP implicated ("inaudible").

Expecting TV's to play tones outside the range people can hear is ridiculous.

TV channels already contain inaudible-sound identifiers. Nielsen has people put listeners in their homes, and uses the identifiers to track which channels are getting played.

Er... holy shit to the thought that (Alexa|Assistant|Siri) + Neilsen (or just by Google) could monitor advert viewing in millions of homes.

I thought a few apps had been discovered over the years that fingerprinted TV ads played near the phone and reported them?

https://www.howtogeek.com/338409/hundreds-of-smartphone-apps...

To scare you further, multiple brands and models of “SmartTVs” have the ability to fingerprint what is displayed on the screen and report back to a cloud service. Said services are also, not surprisingly, poorly secured.

> and uses the identifiers to track which channels are getting played.

I don't think they're using infrasound for that. I think they use a technology similar to Shazam where it just analyzes the sound to determine what's on.

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

You can't count on a TV being able to reproduce sounds below about 100Hz or above about 16kHz. The position of the speakers on the back of the TV means you're likely to get a lot of weird phase effects and many TVs have quite heavy audio DSP to compensate for the inadequacy of their speakers. Any hidden signals will need to be in-band and low bit rate with a high level of redundancy.

So, basically, steganography.

You can even less expect all the compression in the system that’s designed to leave out everything people can’t hear, to leave in this signal people can’t hear.

Sometimes they're fairly audible; try listening closely to re-runs of Arrested Development as an example, there are scenes with fairly loud high-pitched noises which I believe were listened for by some phone apps to try estimating viewership.

Interestingly, though, if psychoacoustic compression was used in when reuploading a video, that 3000-6000Hz range might actually be restored—it’s less information if it’s there than if it’s not.

Similarly, if the video is heard through a phone line, the call might be using a symbolic voice codec, which would also restore the range (in the sense that it’s not even storing sound, just phonemes.)

"Inaudible" doesn't necessarily mean a range outside of human hearing.

For example you could embed a 6kHz tone in a way that's inaudible to humans due to the other frequencies in the waveform.

And promptly lose it to lossy compression

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