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Questions tagged [fft]

FFT is short for fast Fourier transform, any of a set of algorithms for quickly computing the discrete Fourier transform (DFT).

The FFT finds a lot of application in data analysis, particularly time-series and image data, and particularly when the data has a periodic nature, or at least a periodic component. The FFT also finds application in digital filtering. There are many FFT algorithms; they all calculate the Discrete Fourier Transform in O(n log n) operations, while the naive DFT implementation is O(n^2).

Mathematically, the Fourier Transform fits a set of sinusoids to the input data - revealing relative strengths of periodic components of the signal. The fit is optimal in a least-squared error sense. In the case of the Discrete Fourier Transform, the sinusoids are periodically related.

Related topics include DFT, signal processing, convolution, and window functions.

More information on FFT can be found in the Wikipedia article on FFT.

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Bandpass filtering in Python

I have a noisy signal a on which I've performed a Fourier transform (A = fft(a)). The original signal has a frequency of 2 Hz. I'm supposed to remove the noise by multiplying by a Gaussian function, gaussian = exp(-(abs(v) -…
pleasestopsinging
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Normalizing output of opencv idft call

I am using opencv to compute a butterworth filter of an image. The image in questions is a physical parameter, i.e. the pressure, in some units, at every nodal point. It is not just gray scale or color values. I have followed the examples here:…
gsandhu
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rfft or irfft increasing wav file volume in python

I have just started writing a program to manipulate some audio in python. Before I write any filtering functions, I wanted to do a test to compare the input signal to the output signal after having the input signal go through an rfft and irfft. For…
Elon Ullman
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right way of performing inverse FFT in MATLAB?

I've red through the matlabcentral discussion on fftshift and ifftshift but I don't understand completely what's the right thing to do for me. A bit more helpful for one aspect is this answer on stackoverflow. I have an array A with (equally-spaced)…
riddleculous
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Confusion on how the frequency axis when plotting the FFT magnitude is created

This code takes FFT of a signal and plots it on a new frequency axis. f=600; Fs=6000; t=0:1/Fs:0.3; n=0:1:length(t); x=cos(2*pi*(400/Fs)*n)+2*sin(2*pi*(1100/Fs)*n); y=fft(x,512); freqaxis=Fs*(linspace(-0.5,0.5,…
Uygar Uçar
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Matlab, FFT frequency range differences or are they the same?

I'm trying to understand how the FFT in matlab works, particularly, how to define the frequency range to plot it. It happens that I have read from matlab help links and from other discussions here and I think (guess) that I'm confused about it. In…
Gohann
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Further understanding of fftw processing of portaudio signals

I want to analyze a signal I get from my microphone port by using portaudio and fftwpp. For that I followed the explanation provided here. My questions concerning that are now: There it is stated that I should chunk a window out of the incoming…
arc_lupus
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Plot spectrum of a wave as in Audacity

I would like to plot frequency graphs similar to the ones that Audacity can draw: I did not find a software to do that (in command line) so I started to play with python to do it, using the specgram function. As I am not able to redo such a graph…
edwin
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Calculating correlation with FFT between two signals

I am having a problem getting a correct result for a correlation between two sets of values (I know the results are wrong because I've tried calculating the correlation in Octave)(also I'm calculating like the post says). I've been using a library I…
Rok
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Using Accord Audio library with Windows Phone 8.1 App (Not Silverlight)

I'm currently building an application for Windows Phone 8.1 using C#, The aim of the app is to assess audio signals from the device's microphone, initially for frequency, I was hoping to use the Accord library to help with this but have run into…
JBrian30221
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which are the high frequency terms in numpy.fftn?

I am using fftn in numpy to generate a discrete FT of a 20 cell 1D array and of a 20x20x20 3D array and want to suppress the high frequency terms, starting with the highest frequency and extending towards the lower frequency ones. I am more familiar…
James
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Make Math.NET Fourier work

I am trying to find a solution for some weeks now and I really have read all threads related with this topic, so I really hope someone can help me out with this. What I want to do is to use a wave-file as input, fourier-transform it and take these…
webfreak1364
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Why is the kiss_fft's forward and inverse radix-4 calculation different?

I've been spending time understanding and implementing my own mixed radix decimation-in-time fast fourier transform. I've mostly been using kiss_fft and http://www.briangough.com/fftalgorithms.pdf to understand what's happening. From what I've read,…
JodiTheTigger
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Output array after performing fast fast fourier transform of a data set

I'm trying to perform a fourier transform of a data set that I have and subsequently writing its real and imaginary parts separately. This is my code: import sys,string import numpy as np from math import * import fileinput from scipy.fftpack import…
py_beginner
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Gaussian wave generation with a given central frequency

The formula for gaussian wave is 1/[sqrt(2* pi* variance)]*exp{-[(x-xo).^2/(2*variance)]}; I have this question in 3 parts: 1) How to generate a time domain Gaussian signal with a given central frequency. (I tried to control it by changing the…
Amartya
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