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Questions tagged [dft]

Discrete Fourier transform (DFT) is a specific kind of discrete transform, used in Fourier analysis.

Given a sequence of N samples f(n), indexed by n = 0..N-1, the discrete Fourier transform (DFT) is defined as F(k), where k=0..N-1:

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Source: engineeringproductivitytools

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Unexpected peaks of FFT of positive-only sparse signal [OCTAVE or MATLAB]

Suppose a signal corresponds to day values over a year (365 days). It is composed of all zeros except for a few sparse values which corresponds to isolated peaks all separated by the same interval (30 days). I obtain the frequency spectrum with the…
Nic
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Calculate energy of time domain data

I am new to digital signal processing. I have the following sensor sample data Time(milliseconds) data ------------------ ------------------- 0 0.30865225195884705 60 0.14355185627937317 100…
danishjo
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KISS FFT output with or without windowing

I am currently trying to implement fft into avr32 micro controllers for signal processing purposes using kiss fft. And having a strange problem with my output. Basically, I am passing ADC samples (testing with function generator) into fft (real…
Jin
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How to compute Discrete Fourier Transform?

I've been trying to find some places to help me better understand DFT and how to compute it but to no avail. So I need help understanding DFT and it's computation of complex numbers. Basically, I'm just looking for examples on how to compute DFT…
jamesvphan
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How to calculate frequency of a give wave and time

I have data for Velocity vs time. The time steps are not uniform, but the Velocity data is a wave. How do I calculate the principal frequency of the velocity using FFT of Python? Most of the examples I am seeing online are for uniform time…
Pradeep Kumar Jha
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fourier transformation with missing values

i have a time discrete signal that may contain many missing values. and i want to do a fourier transformation on it. what can i do to handle them properly? following diagram may show the case signalpresence x x x x x x x x x x …
vlad_tepesch
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Discrete fourier transform on time series in R

I would like to use discrete Fourier transform to identify dynamic of sales and then cluster similar patterns. However, I'm new in using R and after searching for a solution, I found a prodecure fft(), but not exactly sure if I get the same result…
user3463225
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Inverse fourier transformation in OpenCV

I am new in OpenCV and image processing algorithms. I need to do inverse discrete fourier transformation in OpenCV in C++, but I don't know how. I searched over internet and I didn't find answer. I am doing fourier transformation in my program with…
rwrx_
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DFT to spatial domain in OpenCV is not working

I have created dft of an image and after some adjustment with filters i want to convert it back to the real image but every time when i do that it gives me wrong result ..seems like its not converting it back. ForierTransform and…
wolvorinePk
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Inverse filtering on OpenCV - accessing DFT values and multiplying DFT matrices

I am trying to perform an inverse and a pseudo-inverse filtering in the frequency domain. However I am having trouble accessing DFT coefficients and multiplying DFT matrices afterwards, since I got complex numbers and, therefore, actually two…
Moacir Ponti
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Round trip through cv::dft() and cv::DFT_INVERSE leads to doubling magnitude of 1d samples

I'm playing with some toy code, to try to verify that I understand how discrete fourier transforms work in OpenCV. I've found a rather perplexing case, and I believe the reason is that the flags I'm calling cv::dft() with, are incorrect. I start…
kd8azz
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Create 2D hanning, hamming, blackman, gaussian window in NumPy

I am interested in creating 2D hanning, hamming, Blackman, etc windows in NumPy. I know that off-the-shelf functions exist in NumPy for 1D versions of it such as np.blackman(51), np.hamming(51), np.kaiser(51), np.hanning(51), etc. How to create 2D…
Mohit Lamba
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FFTW single-precision library outputs incorrect DFT when using SIMD optimizations

I'm testing the FFTW 3.3.8 C library for 1D Discrete Fourier Transform (DFT) calculations. The results I get are often incorrect when I use the float (single-precision) version of the libraries, configured with the --enable-generic-simd128 or…
Puk
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Finding 2d impulse peaks in MATLAB

What is the best method for finding impulse peaks (dirac delta) in a 2d matrix. More specifically, I would like to find the harmonic frequencies of a given image and so I need to find impulse peaks in the image absolute value DFT. I thought of…
smichak
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Issue with lowpass image filter

I need to apply a simple low-pass filter on an image, however this needs to be done in the frequency domain. However the final result is a very noisy image, suggesting that my process is incorrect somewhere. My track of thought has been 1) Center…
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