
Inaudible ultrasound commands can be used to control Siri, Alexa, and Google Now - rbanffy
https://www.theverge.com/2017/9/7/16265906/ultrasound-hack-siri-alexa-google
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j_s
dupe
[https://news.ycombinator.com/item?id=15191640](https://news.ycombinator.com/item?id=15191640)

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andrewaylett
Ingenious, but it feels like this should be fairly easy to patch against with
a low-pass filter.

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labcomputer
You need a hardware LPF, though.

The trouble is that the sampler in the ADC down-converts harmonics of the
passband into the passband. To make this concrete: you might sample at f0 to
capture all frequencies from 0 to f0/2, but you also get all the energy in
frequency bands f0 * 3/2 -- f0 * 5/2, f0 * 7/2 -- f0 * 9/2, etc. Unless you
pre-filter that before it gets to the sampler, that is.

By the way, a variation of this trick was used by HP and Tektronix in 1960's
oscilloscopes to display multi-GHz signals.

Depending on the power of the ultrasonic source, the hardware LPF might need
to have a quite fast roll-off, which might imply a large number of stages,
which is hard to design and expensive to manufacture.

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AstralStorm
Specifically one good size capacitor would do against it. or a slightly more
complex tiny filter.

Unless said "ultrasound" is just high audible data, in which case I'm not
impressed. (Similar to "ultrasound" audio Doppler used in some cellphone
apps.)

Very loud ultrasound carries badly through air and produces nasty effects in
dogs, cats and some people.

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singularity2001
hey siri, open www.mynastyhackpage.edu

