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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4answers
4k views

Intuition for sidelobes in FFT

I was wondering if there's a intuitive way to understand why sidelobes appear when performing an FFT on a signal of fixed length?
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Zero-padding or Interpolation in 3D FFT

I'm trying to perform a FFT of a 3D regular grid and then compute the bin average (in spherical shell bins) of the Fourier transformed grid. The problem is that the resulted vector is very noisy as I'...
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1answer
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+50

Flipping of x axis values when the Fourier Transform is compared to the FFT

EDITED 3-Jun-20 I have a lorentzian lineshape $$ f(z) = \frac{1+iz}{R(1+z^2)} \qquad (1)$$ where $$ z=\frac{-2\pi(f - F0)}{R} \qquad (2)$$ and $ R $ is the decay rate, $ f $ is the frequency, $ F0 $ ...
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PSD Accelerometer and Gyroscope

I used Xsens IMU and collected 1 Hour static data for accelerometer and gyroscope. I used the pwelch analysis in matlab to calculate the onsided PSD. The plots looks the following: How do I ...
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2answers
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Why can we have non-integer frequency bins in FFT?

I am studying DFT/FFT and I'm very confused about one thing. I read online that the frequencies we can sample with DFT must be integer (Why does the frequency in the DFT have to be an integer?). Later ...
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1answer
46 views

Pitch shifting with bins creating large feedback

I am attempting to create a pitch correction algorithm. I started by performing a test. The test goes as such: Get WAV file Split it into bins of size n (512 in my case) Shift each bin by 2 semitones ...
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1answer
60 views

Cross power spectral density (CPSD) comparison between signals

I want to compare various signals with Matlab's CPSD to identify shared frequency components. (If there's a better method, I would like to know!) I have 3 signals, A, B, and C. B is pure noise while ...
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1answer
1k views

Python tool for time-frequency analysis

I am trying to perform time-frequency analyses using the PyWavelets (pywt) toolkit for python. My ultimate goal is to perform time-frequency analyses for EEG signals but I am starting with something ...
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0answers
42 views

Why does the frequency in fft plot of the baseband modulated signal isn't changing at the receiver

I have designed a transceiver system for cellular frequencies(800MHz). On the receiver side, I have converted the band signal to baseband signal through digital mixing. I have added the Inphase and Q ...
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1answer
14 views

How to extract the Freezing Index from Accelerometer data?

I'm working on a project to detect Freezing of Gait episodes for Parkinson's Disease patients, and according to this paper and other ones, extracting the freezing index which is "The power in the “...
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0answers
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How to calculate FMCW Radar Chirp STFT?

I have received signal from FMCW Radar which is of the shape [128x64] = [Number of samples per chirp x Number of chirps per cycle] and i want to calculate the stft i am calculating it with stft ...
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1answer
137 views

Inverse FFT of complex exponential with time delay - window leakage?

I am working in MATLAB with a frequency domain signal that is a sum of complex exponentials, e.g. $X(\omega) = \sum_i e^{-j\omega t_i} $ where $t_i > 0$ $ \forall $ $ i $ I expect the output to ...
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1answer
312 views

Spectral Subtraction python implementation error

I am trying to implement Spectral subtraction in python. I am new to DSP. I used this article as reference and below code is what I can come up with. Of course I did not get the expected output. I ...
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1answer
115 views

Inverse FFT - Synch the Phase

Is there any way to synchronize phase of output of inverse DFT in each buffer? When I send the output of inverse DFT to the ...
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2answers
91 views

Strategy / Method for Implementation of the Fastest 1D Linear Convolution / Correlation

I am after the method and its implementation of the fastest convolution of 1d signals: $$ y \left[ n \right] = \left( x \ast h \right) \left[ n \right] $$ Where $ \ast $ is the linear convolution ...
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4answers
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On the simplification using trigonometric functions

Assume I have a matrix $D$ whose its entries are as below : Where $A$ and $B$ can be written using using the trigonometric functions for (1) as: My question, Is it possible to simplify (1) more? ...
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FFT of a variable-frequency signal

I'm writing a software that read engine rpm during acceleration, using a recorded audio. To do that, I divide the record in small time-steps, and for each one I do an FFT, to know the rpm in that ...
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fft of a signal in the space at a fixed time

I have to do a fft of a signal in the space at a fixed time. I have always done fft of signals in time, so this is strane for me. In particular I have some difficulties in defining the Nyquist ...
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1answer
33 views

Construct IFFT from only the major frequencies of an input signal?

I have data which when plotted looks like the green line in the below image. The data is from the walking-motion of a reinforcement-learning-model in a simulation and therefore the values do not ...
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0answers
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64 point FFT Hamming window interpolated to 720-point sequences

Can someone here explain the meaning of, 64 point FFT Hamming window interpolated to 720-point sequences and its inverse? 720 point is from 0 to $2 \pi$ or (0 to 720 degree). I get the 64 point ...
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2answers
476 views

Efficient calculation of correlation function every $N^{\rm th}$ point

I would like to calculate a long correlation function of length say, 1e6 points. I have a prior knowledge that the correlation peak will be in point k*1000. Is there an efficient way to apply this ...
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1answer
732 views

$1/n$ octave complex smoothing

An excellent answer to this post explains how to do $1/n$ octave energy smoothing and mentions complex smoothing can be done as well, but it's tricky business because of phase wrapping. How is phase ...
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1answer
38 views

WOLA vs Simple Hamming Window on signal before FFT

Im working on a project in which I should FFT a signal coming into my FPGA from ADC. I want my FFT to show the peak values as clear as possible without the side lobes. The solution should be straight ...
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27 views

Relationship of input-outputs in Hartley transformation based on few elements in a vector

This question is related also to that question HERE, but with further details and another issue I need to solve. Assuming we have a vector $X = [x_0,x_1,\ldots,x_N]$, its Hartley transformation is ...
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2answers
45 views

Relationship between input and output sequence in Hartley transformation

As you know that Discrete Hartley transformation is related to the discrete Fourier transformation, $i.e$, assuming we have a vector $X = [x_0,x_1,\ldots,x_N]$, its Hartley transformation is equal to $...
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2answers
445 views

Power spectrum RMS or peak

I’m a newbie and wondering if I calculate the power spectrum from the peak amplitude or from the amplitude rms ? This is my matlab script so far: ...
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0answers
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How to take the inverse FFT of Welch's power spectral density in Python?

I have calculated the power spectral density of a weakly-stationary signal using Welch's method (as implemented by scipy.signal.welch in Python), and am looking to take the inverse Fourier transform ...
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3answers
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Circuit simulator with variable simulation time step. How to obtain FFT afterwards?

I know that I am not the first one with this problem, but I didn't manage to find the proper answer, so I hope you would like to help me. From an electric circuit simulator, I simulated my circuit in ...
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1answer
27 views

Length of Hamming Window to Resolve a Harmonic

I am trying to answer the question: "Given a single-sourced pitch signal with a fundamental frequency of 440Hz and a sampling rate of 44100Hz how long should the Hamming window of the STFT analysis ...
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2answers
50 views

How is the frequency vector being derived?

I am having a hard time understanding how the frequency vector is being derived in this code. I would appreciate any guidance from anyone. ...
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4answers
946 views

How do I calculate peak amplitude of the signal components after zero padding and FFT?

I am learning about DFT and trying to apply it to some audio processing. I am new to DSP but experienced in programming and have some background in math and physics. The FFT algorithm I use (lomontFFT)...
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3answers
93 views

Why should the number of subcarriers of OFDM system be a power of 2?

I notice that almost all papers use the number of subcarriers of 64, 128,256.. so on, which is a number of $2^k$ and $k$ is an integer. Why is the number of subcarriers always like that? Can we use ...
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1answer
21 views

Wavelets / Identifying bursty signals

I thought my network was experiencing outages. I collected bandwidth (Mbps) measurements every 30 min. A cumulative probability distribution shows the probability that bandwidth will exceed any given ...
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2answers
43 views

Fast Fourier Transform showing the same results but with different files?

so I'm pretty sure that I'm plotting the wrong frequency domain against my FFT values because I keep getting basically the same graph shape every single time. I think that I just don't comprehend the ...
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1answer
40 views

Scipy.signal noise with rfft compared to fft

I'm trying to get the fourier transform of a signal with real values, however the results I get with rfft are noiser than those with fft. I wrote the following code: ...
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1answer
34 views

Downsampling - how many extra samples to include

I want to downsample a finite section of an equidistant time-series $$x_i, \qquad i=0\dots n-1$$ by doing an FFT and dropping the upper part of the spectrum. Since the FFT looks at $x_i$ as a cyclic ...
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2answers
2k views

FFT showing peaks at 0Hz?

so I've been attempting to do a FFT on some data and I'm seeing a peak at 0Hz, which I can't really comprehend why since I'm quite new to signal processing. I read in my file using pandas, saved them ...
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1answer
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Recover frequencies from IQ samples

I have a set of IQ data. From those data I'm trying to get the amplitudes and the frequencies of my signal as I then want to plot them vs. time. I am able to obtain the amplitudes by squaring both ...
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1answer
27 views

What's the difference between “Magnitude Spectrum” and “FFT”?

I am particularly talking about the matplotlib's axes.magnitude_spectrum() function that plots the magnitude spectrum. Is this a representation of the discrete Fourier transform?
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1answer
47 views

Blackman-Tukey Autopower equation

I am looking over Blackman-Tukey method for Autopower. It uses the DFT after a window is applied on a autocorrelation. $$ Power Spectrum = \frac1{2\pi} \ \sum_{(k=-(N-1))}^{N-1} w[k] R[k] e^{-i\...
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1answer
64 views

How to achieve a smoothed frequency phase response with no outliers

I have two sampled signals, which I then perform wavelet analysis on, by synthesising the Filterbank, perform the dft on each kernel, and the dft on the sampled signals, and multiply together. ...
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1answer
23 views

RMS from calculation from PSD using Welch's method [duplicate]

I've used Welche's method on a vibration signal to get the averaged FFT which I want to used to calculate the PSD from which I can calculate the RMS. Within the Welche method i've use Hann window ...
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0answers
25 views

How can I generate a time domain square pulse that relies on frequency and not zero centered at MATLAB?

I want to create a square pulse, then use FFT on it. However, this code does not relies on central frequency to generate a pulse, and it is centered at 0 in time and frequency. ...
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1answer
44 views

Why there is a peak at zero frequency?

I have a very simple code for ploting frequency spectrum of a signal in MATLAB. My center frequency is at 5e6 but I see a much higher peak at zero frequency despite the fact that I remove DC component ...
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1answer
33 views

Does Circular Convolution Correspond to Periodically Expanded Linear Convolution?

I have a question whether circular convolution and periodically expanded linear convolution corresponds in following case or why it does not? Think about a signal $x[t]$ and a signal $y[t]$ both of ...
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2answers
95 views

Windowed N-point FFT

In the previous question, Smoothing in frequency domain and impulse reponse of the filter in time-domain I guess I have made a mistake in-terms for applying windowing to FFT. After reading "...
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2answers
197 views

How can I insert message bits into the frequency spectrum of an audio signal using Matlab?

I have been trying to use the DSSS spread spectrum technique for audio steganography in MATLAB. I multiplied the PN sequence with the message bits. Now I want to insert this spread message into a ...
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1answer
175 views

Matlab: Different 1-D FFT results with and without zero-padded

Recently I am implementing FFT with C code. So I use Matlab for algorithm verification. As Radix-2 FFT was chosen, I've learned that zero-padding technique could help to reach 2^N limitation without ...
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2answers
1k views

STFT overlap add method - What am i doing wrong?

i am trying to implement a short time fourier transform of an audio signal to do some filtering (subtract another stft signal using spectral subtraction). I am using a hamming window function with 50% ...

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