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Comment by hgomersall | Hacker News Reader

If you know you have a single frequency close to an actual frequency of interest, you can use the fact you know you're in an aliased band to get a precise frequency estimate.

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I guess thats sort of like a weird PLL thing? But I'd imagine you'd have to have prior knowledge of which string you're tuning otherwise the analysis is going to alias against every harmonic.

Every non-linear mixing of signals gives you sum and difference frequencies. It's less a weird PLL thing and more a weird trig function thing.

Its not even a non linear thing. Its a sampling thing. Even ideal sampling exhibits aliasing.

https://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampli...

Yes, and. You can look at aliasing as a special case of heterodyning. Sampling is a nonlinear mixing.

Presumably there is an antialiasing low pass filter somewhere before JS gets to the data. I have a similar sample rate and it certainly didn't work at all for me.

If the accelerometer samples at 50Hz, how could there be an antialiasing filter?

What would that filter look like?

Anything physical which dampens higher frequency oscillations would act as an antialiasing filter.

What sort of size do you think something that would damp 25Hz vibrations in something that weighs a gram or two would need to be?

They have analogue AA filters just before the sampler.

aka a stroboscopic measurement,

but I don't think it will work well for this case.

It's just higher nyquist zones.

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