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POSome signal processing /FFT questions
 

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  1. POSome signal processing /FFT questions
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    <p>I need some help confirming some basic DSP steps. I'm in the process of implementing some smartphone accelerometer sensor signal processing software, but I've not worked in DSP before.</p> <p>My program collects accelerometer data in real time at 32 Hz. The output should be the principal frequencies of the signal. </p> <p>My specific questions are:</p> <ol> <li><p>From the real-time stream, I am collecting a 256-sample window with 50% overlap, as I've read in the literature. That is, I add in 128 samples at a time to fill up a 256-sample window. Is this a correct approach?</p></li> <li><p>The first figure below shows one such 256-sample window. The second figure shows the sample window after I applied a Hann/Hamming <a href="http://en.wikipedia.org/wiki/Window_function" rel="nofollow noreferrer">window function</a>. I've read that applying a window function is a typical approach, so I went ahead and did it. Should I be doing so?</p></li> <li><p>The third window shows the power spectrum (?) from the output of an FFT library. I am really cobbling together bits and pieces I've read. Am I correct in understanding that the spectrum goes up to 1/2 the sampling rate (in this case 16 Hz, since my sampling rate is 32 Hz), and the value of each spectrum point is spectrum[i] = sqrt(real[i]^2 + imaginary[i]^2)? Is this right?</p></li> <li><p>Assuming what I did in question 3 is correct, is my understanding right that the third figure shows principal frequencies of about 3.25 Hz and 8.25 Hz? I know from collecting the data that I was running at about 3 Hz, so the spike at 3.25 Hz seems right. So there must be some noise other other factors causing the (erroneous) spike at 8.25 Hz. Are there any filters or other methods I can use to smooth away this and other spikes? If not, is there a way to determine "real" spikes from erroneous spikes?</p></li> </ol> <p><img src="https://i.stack.imgur.com/sc6CN.jpg" alt="enter image description here"></p>
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    1. COHow did you generate the data? Shaking your phone? Do you get the 8Hz with a different run? without the window function? Your data seems quite clean, so I don't think the window function is necessary, but the data would probably not be as nice in the real world so it might be needed.
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    2. COYou can kind of see the 8Hz in your original data: they are the sharp little peaks, some with quite a large amplitude. Any idea how they appeared? (related to question above about you generated the data.)
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    3. COYeah, I don't think the 8.25 spike is "erroneous" as you've said. How did you get this data?
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