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Questions tagged [fft]

The fast Fourier transform is an efficient algorithm to compute the discrete Fourier transform (DFT) and its inverse.

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45 views

Why are these results different about 'frequency shifting'?

I think 'spec_ori' & 'spec_shifted' spectrum amplitude and phase are same in below code. Because of 'spectrum shiting property' (What effect does a delay in the time domain have in the frequency ...
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1answer
450 views

Frequency analysis to determine low-pass cut off frequency

I collected some data for a practical application, where the signal represents force data obtained from an impact of a punch against a force plate attached to a quasi-rigid rig (it moves once the ...
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1answer
156 views

Matrix cross correlation in python

I'm currently performing matrix cross correlation in python using : C = scipy.signal.correlate2d(A,A) where A is a 2D matrix, typically a picture. As you can ...
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122 views

How to know whether numerical roundoff error or not in MATLAB?

This is result of real and imaginary parts of cos fft. I think e-13 number is numerical error but, I don't know how to know they are real numerical error. If I have to try change primitive types, ...
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37 views

Confused about obtaining impulse response from overlapped FFT'd signal

I have two audio signals which I have performed an FFT to plot the magnitude vs frequency response in realtime. To do this I have divided the incoming signal into small blocks of size 2048, ...
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1answer
305 views

Why is the second half of the FFT negative frequencies

There are lots of questions here about what the negative frequencies in fft mean, but I'm confused on why the second half of the fft calculation is the correct calculation for a negative frequency. ...
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1answer
267 views

Why MATLAB fft cos makes imaginary parts?

The cos fourier transform has no imaginary parts, but in this code it has imaginary parts that little big. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% figure(7); fs=128; t=-4*pi:1/fs:4*pi-1/fs;...
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148 views

FFT amplitude vs IQ amplitude

I'm very new to digital signal processing, so please let me know if my question doesn't make sense. I calculated the amplitudes (using the formula : $10 \log_{10}(i^2 + q^2)$) for some IQ data ...
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2answers
302 views

Ifft through Matrix multiplication

I am still new to MATLAB, so apologies if I sound lazy to you. I am attempting to model a transformation as a set of matrix operations. I start with a vector, up-sample it by $U$ (up-sampling rate), ...
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1answer
140 views

Understand Inverse DFT (FFT)

I was reading this article to understand the fft https://jackschaedler.github.io/circles-sines-signals/dft_walkthrough.html I understand that the fft takes a block (256 or 512) of the signal, ...
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2answers
94 views

Radix2 fft - zero padding output size

With zeros padding the FFT radix2 has different input size than output. How to deal with that? I see two solutions but not sure which one is better/appropriate. Maybe you know some other and better ...
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1answer
646 views

Numpy fft2 frequency shift

I don't understand how to make frequency shift in fft2 or higher dimensions. Could anyone explain it, please? import numpy as np import matplotlib.pyplot as plt from mpl_toolkits.mplot3d import ...
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3answers
208 views

Deconvolution Using Complex Division in The Frequency Domain

Consider these two signals: a = [1 1 0 0 0 0 0 0] b = [1 0 1 0 0 0 0 0] their convolution is c = a * b = [1 1 1 1 0 0 0 0] ...
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45 views

Meaning of a formula about transient frequency bins

I'm trying to understand the following formula in this paper: Sadly, the paper does not define most of the symbols it is using, so my guesses so far are: $T_{tr}$ refers to the sampling interval i....
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1answer
40 views

Complex FFT reversing

I've succeeded to compute complex FFT of a data array of interleaved values using the arm_cfft_f32() API of the CMSIS DSP library for Cortex-M4, and got in the same array the FFT results as frequency ...
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1answer
23 views

Non-decaying Harmonics in Vibration Signal

I am analyzing vibration data collected by an accelerometer positioned in a stamp press machine. The fundamental frequency is around 10 Hz. The problem here is that when I take the FFT of the data I ...
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1answer
40 views

FFT for distance-limited sum of pairwise products

I have a sequence of fixed length $N$ input arrays ($x_0$, $x_1$, ..., $x_{n-1}$), and my goal is to compute output arrays as such: $$d_{00}[l] = \sum_{i = 0}^{N - l} x_0[i]x_0[i + l]$$ $$d_{01}[l] = ...
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2answers
54 views

What to do for the right FFT in Matlab (two peaks and incorrect amplitude)?

This is the FFT I'm using. But two peaks (did not expect the one in the beginning) are occurring and the amplitude is not as specified (expecting a value of 2). Any help is much appreciated! ...
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1answer
47 views

Harmonics in Quantal Histogram

I'm studying what I believe to be a system that produces a (noisy) quantal response (i.e. there should be a minimum increment of response). To do this, I've plotted a histogram of some ~6000 amplitude ...
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1answer
106 views

THD: estimation using periodogram or fft?

I want to estimate the THD of a voltage signal (output from a frequency inverter). At the begining, I need to estimate the power of the signal at frequency domain. To doing so, I should use a ...
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2answers
167 views

How to assess perceptual volume from FFT

So far I could estimate signal volume from FFT using amplitude information, then adding all amplitudes together and divide this number by the number of frequencies. But I think this method does not ...
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1answer
539 views

Checking for VHF pulse with SDR and python

So I have a small NooElec Software Defined Radio and I've got it working well with my Raspberry Pi using GQRX as a controller and to visualize the SDR output. My goal though is to use a python script ...
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701 views

Why does the frequency in the DFT have to be an integer?

I don't know why the result of DFT and FFT in MATLAB like below images.. fs=128; t=0:1/fs:1-1/fs; x=cos(2*pi*3.5*t); X=fft(x); N=length(x); n=0:N-1; f=fs*n/N; plot(f,abs(X)/N); If I set the ...
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69 views

Making longer a time domain signal by adding values on its frequency domain. What am I doing wrong?

Background Here's the thing: using software for Finite Element Acoustic Simulation I got the dataset of frequency response from a room; software works by solving the wave equation in the interval $`[...
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149 views

Split Radix Cooley Tukey Code implementation

I've implemented the Radix-2 cooley tukey algorithm from wikipedia in java & c and I've also managed to do a Radix-4 Variant of this algorithm. I'd like to implement a split radix variant of the ...
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97 views

Simple tests for normalisation of power spectra

I am trying to compute power spectra of multidimensional (2d, 3d) arrays. In particular, I want to compute radially averaged power spectra $P(k) = P(|\mathbf{k}|)$ using FFTW, where $P(\mathbf{k}) = |\...
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107 views

Normalize FFT signal (DC part)

I'm struggling for few hours now with FFT normalization. I'm working in Matlab to prove my concept before moving to embedded application. I have 4 sine wave signals with: ...
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1answer
126 views

How to interpret the signal in frequency domain

I'm having difficulties understanding some basics regarding FFT. I was hoping you could answer a few questions I have. These questions will probably seem silly to you, sorry for that. I'm a complete ...
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97 views

Correctly scaling Power Spectrum and Correlation Function of Galaxies from Mock Catalogues

I'm currently writing a code to compute the two-point correlation function of a set of galaxies. As I work with a loooot of galaxies, it was suggested to me not to do it in real space, but to compute ...
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3answers
1k views

Correct method for processing FFT in realtime

I am attempting to build a 'live' FFT for continuous analysis of an audio signal. I am relatively new to this, but have attempted the full thing, and then updating the graph at 20Hz refresh rate. My ...
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2answers
48 views

Why the angle of a real signal in frequency is continuous?

This original signal is shown below (time domain): It is a real signal. According to the signal processing theory, the form part of the angle is conjugate to that of the latter part. That is, $\...
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136 views

Auto-correlation function, an inverse problem

$x[n]$ is a complex function $n=0,1,2,\cdots,L-1 $ we assume $x[n]$ is periodic in its index: $x[n+L]=x[n]$ Its auto-correlation function $C[n]$ is uniquely defined as: $$ C[n]=\sum_{i=0}^{L-1} x[i+...
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68 views

non-negative FFT spectrum, subtlety

definations $x(t)$ is a function of time. Physically, it can be voltage, displacement, magnetization and so on. It can be real, complex, vectors or more fancy numbers. $C(t)$ is the auto correlation ...
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62 views

How to properly filter an image in the frequency domain using matlab

my problem is the next: I´m starting to understand the world of image processing and I´ve been kind of bugged with the idea of filtering images in frequency domain. I have made a test programm in ...
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2answers
68 views

How to detect a long signal inside an even longer one?

I'm trying to detect a long pattern inside a long recorded audio. In order to do so I am implementing a matched filter in time by performing a FIR filtering using the coefficients of my pattern ...
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1answer
331 views

extracting ultra accurate phase and frequency with FFT with real world data

I was doing some experiments in Matlab, and write this code to experiment with it. I want to know if it's possible to extract accurate within -+0.01° phase info from samples of an ADC, the matlab FFT ...
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125 views

Split Radix Version of the Cooley-Tukey FFT Algorithm

I've implemented both Radix-2 and Radix-4 of the cooley-tukey fft algorithm based on the description and the pseudocode on wikipedia.Now the next thing I'd like to do is the split radix version of the ...
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52 views

which is the relationship between domains (x and k) in FFT?

I need to plot an escalar field f(x,y) in an interval xv= (xmin,xmax) in Nx steps and ...
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1answer
84 views

Precompute parts of an FFT on streamed data inputs

I've implemented Radix-2 & Radix-4, DIT & DIF variants of the cooley-tukey FFT algorithm and they perform well on test data. Now for a project, my algorithms will be applied to a real-time ...
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0answers
71 views

How does this Odd stacking DFT work?

I have just read on page 3 of this paper: https://www.onsemi.com/site/pdf/Filtering_Hearing_Ai.pdf That an "odd stacking" DFT can be created, essentially sacrificing the DC and Nyquist bands to ...
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1answer
76 views

Remove Frequency Range

I got a signal and I would like to remove a specific frequency range. I am a frequency noob, so I will explain it by images: I have a frequency filter (butter bandpass): I have a signal and it's ...
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62 views

how to plot log intensity of a 2D image wrt frequency?

I'm reading an article about frequency filtering and I came across this figure which represents log intensity of an image wrt frequency. I want to code this function with ...
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2answers
104 views

How to implement discretized CWT with FFT?

I tried to make circular convolution (correlation actually because it is easier to understand for me) look like the Continuous Wavelet Transform (CWT) but got stuck because the conjugation was on the ...
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1answer
97 views

Measuring Amplitude of sound wave FFT

Please, pardon my ignorance. This may be a simple question, but I am still unable to explain what is happening. I am using a turtlebot to listen to sound emitted from a stationary sound source. My ...
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0answers
166 views

Edge effects in cross-correlation

This is a follow-up of this question. The problem explained in the linked question has been in part solved by the mean of a windowing function. But my cross-corrleation still exhibits strong edge-...
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4answers
662 views

DFT (FFT) of a Real Even Function Doesn't Yield Real Only DFT Signal

There are a lot of similar questions already, but I can't find one answering this problem. I used the following MATLAB code to perform the FFT on a Lorentzian input signal ...
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0answers
177 views

Phase Noise in OFDM simulation

I am trying to simulate phase noise in OFDM and in search of a mathematically tractable model. So far, I have managed to get phase noise by multiplicating $e^{j\phi(n)}$ in frequency domain where $\...
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1answer
55 views

Improvement of SNR by sampling different number of cycles using FFT of a fixed length singal

My question relates to Fourier transform of signals with different lengths in time but the same fixed length of the digitized signal with different sampling rates. Specifically, how does the number ...
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136 views

Compare twoo Arraylists that contains peaks (fingerprint) of an audio files using python

My main idea is to compare the similarity of twoo audio files,so i already did detected the peaks elements (as shown bellow) of my files -Soud A & Soud B- and put those peaks on an ArrayList -...
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3answers
141 views

Decreasing Sample Rate in DFT (FFT) for Audio Analysis

I'm working with audio data analysis through FFT algorithm. My example audio is a sine wave at 440 Hz and 44100 Hz sampling rate. FFT methods in programming, like ...