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I'm analyzing guitar string plucks and sustains. I'm having good success with auto correlation using FFT's. Now I'd like to detect plucks while the string is still vibrating. Since I already am periodically taking FFT's to find the pitch, I thought maybe I can take advantage of the fft results and look for changes that might indicate a repluck. Do I simply add up the bins for some kind of power measurement?

Any ideas?

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    $\begingroup$ Are you saying that you would like to detect when an already vibrating string is plucked again? $\endgroup$ – Phonon Mar 5 '13 at 6:48
  • $\begingroup$ Yes, but more generally, has the string been disturbed, and when is it freely vibrating again, using fft information. How can I use the real and imaginary results of the fft to help me decide? $\endgroup$ – Chuck Carlson Mar 5 '13 at 15:57
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A pluck might produce significantly more broadband noise than a free string. An FFT of such noise would show more relative energy outside of all the FFT result bins that are related to (F0 or overtones of) a single pitch.

Also, a free string has a more predictable decay rate in any FFT magnitudes related to the pitch across successive offset FFT frames windowing the sound data. A sudden change from this decay rate (a stop or a big increase) might infer a disturbance, such as a pluck.

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