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Comment by xattt | Hacker News Reader

I’m curious how folks who have the know-how to dissect a phenomenon “miss the forest for the trees” when they stumble upon a “mysterious” signal.

Alternate data streams in FM like RDS, IBOC audio and FM time are not some new-fangled tech. This would/should be the first place to go to if you saw a signal that’s not modulated to analog audio.

Of course, the whole mystery aspect is just a hook and helps move the story along.

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Oona (the author) knew about the existence of such things; it wasn't a mystery. The only reason it was strange is that because her radio had been knocked about during a move, it was strangely down-shifting the data channel ... she was not monitoring the FM (from what I could make out), but the actual audio from the radio.

This was just ('just' for her, impressive for me) an exercise of going down the rabbit hole, and then curating that tour for us.

Rather, I don't think the data channel was actually down-shifted, but that it wasn't filtered out (probably due to a broken analog filtering circuit after the fall), so it was still outside of the audible spectrum but available to the sound card and hence visible in her software spectrum analyzer. It also sounded like she was seeing some aliasing effect, i.e. that the data signal was probably out of band for the sound card ADC, but "folded" down into the sampler frequency range due to Nyquist folding "magic" (which is maybe what you meant with "down-shifting", now that I think about it?).

She does speak of aliasing.

Yeah more realistically it was like:

> I was looking at FM radio channels on SDR (rtlsdr came out in 2010) and noticed the RDS. So I looked into it.

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