4 ms·
Can't they just limit the activation frequency? Seems easy enough
by Froyoh 8y ago
Can't they just limit the activation frequency? Seems easy enough
- tasty_freeze 8y agoI don't know anything about it, but based on the name, I'll venture a guess. The audio system has an A/D converter which samples audio at a specific rate -- say 48 KHz. Aliasing occurs when the input to the D/A convert is above 1/2 the sample rate. A 24001 Hz signal is indistinguishable from a 23999 Hz signal. A 25000 Hz signal is indistinguishible from a 23000 Hz signal, etc. To eliminate these types of problems, there will be an analog lowpass filter before the sampling circuit. There is a gradual rolloff of signal sensitivity. Aliasing still occurs, but the energy of the aliased signals is significantly reduced. My guess is you take a voice command, even if it constrained to be say 200 Hz to 2KHz, then invert the spectrum and shift it to the 46-48 KHz range. When this high frequency is played back, due to aliasing, the software after the A/D converter sees it as a 0-2KHz signal, though greatly attenuated. To overcome that, the source audio can be tremendously loud. Humans can't hear it, so it remains stealthy.
- cozzyd 8y agoThat's clever but that's so many dB down with any sane anti-aliasing filter that it would require quite the sound source. Based on flipping through the pages of the paper (https://arxiv.org/pdf/1708.09537.pdf https://arxiv.org/pdf/1708.09537.pdf), it looks like they're taking advantage of the non-linearity in the response at high frequencies to effectively demodulate a lower-frequency signal that was mixed up to ~22 KHz. Which, if that's what they're doing, is totally awesome!