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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How to generate time-series from a given one-sided PSD?

Generating time series from a given PSD has been discussed in this and some other forums already (which I've referred some of them below) but almost all of them are mostly descriptive and none of them ...
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Understanding FFT bins interpretability

I am familiar with the continuous Fourier transform yet the more I try to understand FFT, the more I'm confused. If we've got a discrete N sample signal, FFT is a faster way to calculate DFT. If ...
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FFT Artifacts and their cause beyond frequency resolution

I am trying to work through an issue with an FFT for audio : I have no spectral leakage while playing back audio, but the moment I change or zero out a value in any of the FFT bins, artifacts are ...
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Why do some signals have "DC component" whilst others don't?

I am analyzing a series of oscillating data of items that gradually become less "floppy"/oscillatory and I have observed that those items that are less floppy/oscillatory tend to have, what ...
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How do I convert the Amplitude from Power/Amplitude spectral density?

I recently started working on PSD of seismic signals. PSD signal can be expressed in 2 ways. One in ($PSD=g^2/Hz$) and other in $PSD=((meter/sec.^2)^2/Hz)$ and $ASD = \sqrt(PSD)$. More details have ...
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Help Understanding FFT Plot?

I want to analyze FFT and Harmonics of my signal. As a proof of concept I tested FFT against standard signal. I am unable to understand why the frequency output is going to -1. Shouldn't it be +1 at <...
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Improving the intuition for the 2d fourier transform

As far as I understand, the 2d fourier transform is calculated as following: ...
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Can we recover $|X(k)|$, given $|x(n)|$?

Given a complex vector $x[n]$, we can find the magnitudes of the spectrum by computing: $X_m[k] = |X[k]| = |FFT[x]|$ This involves performing a complex FFT and computing the absolute value of the ...
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How to Calculate Frequency "X values" from a FFT/DFT Transformation

First of all I am totally new to the DSP field and I have no background in it whatsoever, but my work in biology has led me to data that would greatly benefit from DSP. Any answers devoid of specific ...
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7 votes
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Can I "reverse" spectral leakage by solving a linear system?

When I compute a spectrogram of (say) a piece of music, there is a lot of frequency "smearing." Often we can reasonably expect that the "true" generating process is much sparser in ...
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Can I reduce the complexity of multiplication with FFT if the input vector is repeating?

I have a Fourier matrix $F$ with size $N \times N$, such that $y = F \times x$, if I have the vector $x$ contains four identical parts, for example $x = [x_1, x_2,x_3,x_4]’$ and $x_1 = x_2 = x_3 = ...
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Doubt regarding scaling of Welch Overlap Segment Averaging with Lomb Scargle

I am doing something similar to this question Comparison of results own implementation and python signal.welch. In my case there is no Welch method associated with the scipy lomb scargle spectrum so I ...
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Excessive low frequency component of electroaccoustic system impulse response

I'm using GNU Octave to achive speaker to my laptop microphone path impulse response. For this I use NLMS system identification algorithm. I've analyzed impulse response of system with fft to being ...
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kSpace data manipulation

I have access to GE MRI scanner data and have been trying to convert the raw kspace data to image format using ifftshit(ifft2(.)). However, the reconstructed images don't match the expected outcome: ...
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Why aren't I getting my expected FM channel when multiplying these two frequencies?

Hope y'all are doing well. Recently I've picked up a LimeSDR and interfacing it with GNURadio and been having a blast with it. I got my first FM receiever demo working using examples from LimeSDR FM ...
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Phase of an FFT after zeropadding

How does zero padding effect phase of an FFT? I thought that symmetric zero padding in the center of an image or signal produces no phase change in the FFT (See Here), but I am getting different ...
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Low pass filter behavior in frequency domain

I have a relatively simple task, which is to implement a low pass filter for a sum of exponentially dampen complex sinusoids in random Gaussian noise. To showcase my problem I have generated data ...
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Can I calculate the DFT of a specific wavelet by its FT formula?

I'm going to use a Morlet wavelet to filter my signal. The first step is to calculate the DFT (or FFT) of the signal and the wavelet kernel respectively. I guess that as the FT of a Morlet wavelet can ...
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Multiplication of Fourier repeated matrix with a higher dimensional matrix

I have a square Fourier matrix $F_{2\times 2}$ with size of $2 \times 2$. It’s repeated twice following this way: $F_r = F \bigotimes I_{2 \times 2}$, where $\bigotimes$ is the Kronecker tensor ...
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Reconstructing a signal from a Nyquist plot

I have a system which is like a blackbox which has just one input which could be a sinusoidal wave which is a sum of a range of frequencies, now the problem is that I dont have the time-domain output ...
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FFT to work out optimum number of samples to average

I have a magnetometer, a LIS3MDL to be precise, and I am taking readings from it every second. As expected there is variation in the readings. For example, if I take five readings I get: 1164, 1190, ...
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creating a power vs frequency graph on for different harmonics Matlab

I am trying to create a Power vs Frequency graph for different harmonics(x_5(t), x_20(t), and x_100(t)) of the half triangular wave. I want to see how the difference in power between the original and ...
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How to create overlapping frames of input signal for stft using HDL coder for FPGA implementation?

I want to implement stft on FPGA. For this purpose I use HDL coder and the problem is I have no idea how to create overlapping frames of data for further FFT. In my example I want to use 256 frame ...
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Which type of noise can we see from the given image signal and its abs of 2D FFT?

1. Background: I get an 128*128 2D one-channel noisy image as follows: I get the shifted absolute values of 2D FFT result by the following Python code: ...
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How can I interpret the output of indexing an fft in python?

I am looking to determine whether a given audio file contains a certain frequency, similar to this question. However, as a beginner, I need help understanding the output. If I am understanding the ...
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PSD to FFT for audio [duplicate]

I'm trying to convert a noisy input to fft and extract the signal using a local average. By the way, after the transformation of fft, it was found that the values of adjacent times of the same ...
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Estimate SNR using Voice Activity Detection

I´m working on a script written in Python for estimate SNR of speech signals using Voice Activity Detection (VAD). The problem is that the results are inconsistent, for example, a signal that has a ...
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9 votes
2 answers
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FFT-based fast convolution vs IIR filtering

I'm doing an multichannel audio equalizer system on a chip, which is parametric EQ, usually implemented in cascaded biquad IIR filters. My problem is that I use a lot of IIR filters due to multiple ...
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Zero Padding in Implementing FFT from scratch

I'm trying to implement an FFT algorithm from scratch. I'm using the recursive algorithm where if N is a power of 2, then I have M = N/2. The algorithm is divided into even and odd parts and I have ...
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2 votes
1 answer
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Fractional Frequency Shifting a Discrete Signal in the Frequency Domain

In our current hardware-based signal processing pipeline, we have a time-domain signal $x[n]$ that we want to frequency shift by $f_0$. To do this, we multiply the signal by a complex exponential in ...
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How to calculate power spectrum from mechanical vibration signal?

I am not 100% sure if this question is fit here but it seemed the best place to post. So my task is to perform power spectrum analysis on a signal measured with a sensor outputting movement speed of ...
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FFT of a gaussian signal in Python

I've been trying to get the FFT of a gaussian in Python. When I use the following parameters, the FFT goes hand in hand with the theoretical FT of the gaussian, but if I increase $\sigma$ they rapidly ...
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Average of discrete periodic signal

I am working on a project which, at some point, relies on the computation of the average of discrete signals, which typically look like this: The project generates hundreds of such signals, and: the ...
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2 answers
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Can anyone explain how dft works as a filter bank?

When we take the fft of input signal, the fft formulas say us to down convert the 2pik/N frequency content of input signal and sum one period interval. This gives us a just one complex number,not an ...
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Sampling, filters, windowing, FFT. From theory to help on this coding list

My plan is to analyse the spectrum of samples from a microphone. I wonder how correct this suggestion is. The below description may then fail on several points. I am in need of somebody with a red ...
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2 votes
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How to improve my FFT output?

I am to reproduce the figure below but with a different blue line (source), which shows the ideal normalized frequency spectrum (grey) of a spectral ripple with 0.7 ripple-per-octave and an electric ...
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4 answers
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Given a signal that is not bandlimited, how do you properly take the FFT?

I assume that the Nyquist theorem doesn't apply, at least not in the standard sense, for a non-bandlimited signal. In my case, I sample the signal (in the time domain) above the Nyquist rate and then ...
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Adding Fractional STO to an OFDM Signal using Frequency Domain Zero Padding

I have an OFDM system. I would like to introduce a fractional delay of (0.25 samples). Therefore: I increase the sampling rate of the OFDM signal by a factor of 4 (via frequency domain zero padding (...
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Does Zero padding cause noise in the high frequency region?

I have an OFDM system. I would like to introduce a fractional delay of (0.2 samples). Therefore, I increase the sampling rate of the OFDM signal by a factor of 10 (via frequency domain zero padding), ...
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How to properly filter offline data by zeroing the unwanted frequencies after FFT?

I am new to the field of DSP. I have a measured signal, and I wanted to apply a low pass filter on the data. I just simply applied FFT, wrote zeroes at the unwanted frequencies then applied inverse ...
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1 vote
1 answer
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Lowering Spectral Resolution of FFT

I find myself in the position of having to lower the FFT resolution. Basically I have a signal of length M and I would like to make an FFT with N<M frequency bins. I cannot simply make several FFT'...
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2 votes
1 answer
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Is it possible to implement a block-wise Hilbert transformer using FFT

I'm implementing a discrete Hilbert transformer and I know that an ideal Hilbert transformer is anti-causal and has infinite length so we can only make approximation. There are some FIR and IIR ...
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1 answer
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understanding a spectrum of a modulated signal

I plot a spectrum of a modulated signal. It is a GMSK modulated signal. My goal to find the best parameter for a realisation it. My first spectrum with oversampling ration M = 4 and BT (the product of ...
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1 vote
1 answer
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In what cases can you get aliasing below the Nyquist frequency?

I took the one-sided FFT of a signal and plotted up until the Nyquist frequency. Then, I took the real part of this FFT multiplied by $i\omega$ following a calculation that I'm trying to do of a ...
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Sparse signal FFT

Say I had a time domain signal $x[k]$ wich is sparse: $\log(N)^2$ nonzero samples and the fourier transform has only a very (very!) small number of high frequency components. Are there any techniques ...
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Power Spectrum for feature extraction

I want to extract frequency domain features for my machine learning model, and to do so I have calculated the power spectrum of the EMG signal. One of my first questions is that my frequency is just ...
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Should I be using FFT

I am using python (scipy.fft) to live analize EEG data. The app resonds instantl based on the signls coming from EEG (Alpha Beta...)The problem is that when I run the FFT on only 100 samples or so (...
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mp3 encoding in the frequency domain

Let's start with an arduino signal, which can be periodic over time. When this signal is converted from analog to digital it "turns" into a series of bits. At this point, is this signal in ...
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Are there any order analysis functions in Python?

I would like to perform the following order related tasks in Python: Convert tachometer pulse signal to RPM signal (tachorpm) Convert RPM signal to even-angle signal (orderwaveform) Time synchronous ...
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FFT Data - How to get frequencies?

I have some python code running an FFT on EEG data, I now want to be able to read frequencies, for example something like x = fftdata(10hz - 50hz). This is my code ...
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