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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What Are the Units of a Fourier Transform?

I know that a Fourier transform (FT) outputs a distribution of how much of each frequency is present in a signal, but what would be the units of that distribution? It's not a proportion or a percent ...
0 votes
1 answer
27 views

Is it possible to reconstruct the continuous frequency spectra from DFT/FFT?

Given the output of some FFT (lets call it X) with N bins (lets say 2048) is it possible to recover the values in between the bins in the same way that I can recover the values in between samples of a ...
1 vote
2 answers
75 views

Excel FFT of Two Sine Waves Not Giving Expected Results

I did an FFT in Excel on a 440 Hz time domain sine wave, $y_1(t) = \sin(2\pi\cdot440\cdot t)$, and got the expected output in the frequency domain: a single-spike distribution with 100% of the signal ...
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0 answers
8 views

Pixel Shift when using Fourier Interpolation

I am working on a Super-Resolution network, and want to try to use Fourier interpolation, different from the normally used bicubic interpolation as it relates to MRI imaging. I have tried using this ...
9 votes
3 answers
1k views

Why doesnt DFT Padding cause sinc like features

I'm new to the land of DSP so any incorrect terms please let me know. It seems padding the time domain signal can make the magnitude spectrum look 'nicer', the fact it doesn't gain any more useful ...
0 votes
2 answers
62 views

Peak FFT Frequency Off By One Bin

I used Excel's FFT feature to compute an FFT of a 440Hz sine wave that I also generated with Excel. The peak FFT value should occur at 440 Hz. Instead, it occurred one bin later at 528 Hz, so I'm ...
0 votes
1 answer
39 views

Min. sample duration for FFT of low frequent signals obtained from zeroIF receiver

I am new here and a beginner with SDR & FFT. For antenna pattern measurements, a flying probe antenna is to be used to sample the electromagnetic near-field in front of an antenna. A lightweight ...
1 vote
0 answers
37 views

Discussion on the relationship between the FFT magnitude spectrum and the corresponding actual amplitude

This might be a bit long, but it's a question that has troubled me for a long time, and I hope to get everyone's help. When studying signals and systems, we usually only pay attention to the relative ...
0 votes
2 answers
58 views

Jitter model does not behave as expected after FFT

So I'm currently trying to understand the relationship between jitter and phase noise. To help my understanding I did write a little simulation in matlab. I used this model in the time domain, relying ...
0 votes
1 answer
72 views

What's Nyquist frequency in DFT?

I'm currently studying digital signal processing at university but I can't figure out what the Nyquist frequency means in the DFT coefficients, I know what the Nyquist frequency is in the sampling ...
3 votes
1 answer
515 views

pocketfft delivers wrong values

does anyone understand how to use the pocketfft by martin reinecke? Link: https://gitlab.mpcdf.mpg.de/mtr/pocketfft Basically it's just this snipped of code: ...
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0 answers
42 views

Possible to combine and/or compare a swept Sine and a wide band noise signal?

I am working on a problem where two profiles need to be compared. One is a swept sine in $m/s^2$ (Acceleration) and the other is a sine-on-random signal composed of a swept sine $m/s^2$ and a random ...
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1 answer
463 views

FFT of channel impulse response in python

I have this channel impulse response. $h = [1,\,0.5,\,0.3+0.3j]$ The channel is like this by doing a FFT for h. I resample (up-sample) my OFDM symbol to have smaller path delays and convolve it with ...
0 votes
1 answer
28 views

Can a bad quality sound be measured via THD?

I'm trying to create an app that measures the audio quality, this is ideally to filter out blown out or cracking speaker who has different types of distortions, what would be the best measurement ...
0 votes
1 answer
131 views

How to find frequency , location and phase of two sinusoid signal buried in noise?

There is a data set which has two different sinusoids buried in noise. Data record has following properties: A data records with a total data length of 2048 points. The sampling frequency is 100 Hz. ...
2 votes
1 answer
76 views

FFT Window question - high resolution windows

So I am pretty familiar with FFTs and FFT windows. I know that in general there is a tradeoff between mainlobe width and sidelobe levels. However, I recently stumbled across this discussion, where ...
1 vote
1 answer
45 views

Ignoring Negative Frequencies from FFT on Real Time Series Seems Inconsistent With Time Series

I ran an FFT on real financial monthly time series data. If I plot the FFT frequency domain output on the interval $[0, f_s)$, the dominant frequency pair peaks occur at $f_{A1}$ $\approx$ $0.02 \ ...
5 votes
3 answers
2k views

Problems Using FFT to Compute Impedance in a Model Neuron

I'm a neuroscientist currently investigating the resonance properties of a single neuron model that a colleague and I have constructed. The language we code in is Julia, which I hope is similar enough ...
1 vote
2 answers
109 views

How does background noise affect the fft results for a pure tone

When I am recording a pure tone and conduct a fft, I receive a peak in the spectrum for this certain tone. Let's say the peak is 5 Now I am recording a pure tone with additional white noise. Does this ...
0 votes
1 answer
29 views

Frequency decomposition of forecast error variance

I think my question concerns statistical signal processing. I was referred to this site by a user at Cross Validated. I want to do a frequency-domain decomposition of generalized forecast error ...
1 vote
1 answer
209 views

3D convolution product with Fourier transforms, FFTW and MPI in C

My question is not really from the field of signal processing but I think you are the most suited for answering my question. I am willing to compute the gravitational potential which can be written as ...
0 votes
1 answer
35 views

Binning and Frequency for FFT on Financial Time Series Data

I'm trying to analyze financial market time series data, so are there any particular concerns in using FFT for that kind of data? The data seems to be relatively covariance stationary. My sampling ...
0 votes
2 answers
85 views

How do I isolate a part of this signal in time-domain in Matlab?

I want to isolate a reflection part of a signal which I've transformed from Frequency to Time-Domain (see Picture). So far, I've tried to create a Hamming-Window with the specific window-width for the ...
0 votes
0 answers
40 views

How do you properly cut out negative frequencies from FFT of a real signal if it reduces sample size?

If a real time signal has $N$ samples, then the magnitude of its FFT will also have $N$ samples--half of which will have positive frequencies and half of which will have negative frequencies. The ...
0 votes
1 answer
47 views

Non-uniform FFT

I'm looking for a form of a FFT where the samples in the frequency-domain don't represent uniform spaced frequencies. What I would like to get is a frequency-domain with samples that are unevenly ...
1 vote
1 answer
224 views

Analysing 3D image FFT

I'm looking for a way to visually inspect the amplitude of frequency components in a 3D image that I have taken the fftn of in matlab. Does anyone have any idea how to go about this?
1 vote
1 answer
46 views

Is there any way to make the FFT more efficient for continuously sampling a signal?

So I want to continuously estimate the frequency content of a signal using the FFT such that at every sample timestep, I add a new sample to my current set of samples and remove the oldest sample in ...
0 votes
0 answers
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Program to compare output and input streams to verify volume

I'm trying to wright a program to compare, two sound signals, output (speakers) and input (mic) streams and verify/adjust the volume. I'm using Microsoft Visual Studio C# forms project with the ...
0 votes
1 answer
44 views

Zoom FFT shifting region of interest to be centered at zero Hz

So using this resource: https://flylib.com/books/en/2.729.1/the_zoom_fft.html I think I understand that the zoom FFT allows you to calculate the FFT for a region of interest with a lower order FFT by ...
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How can i add overlap-add into this

Im new to DSP and i wanna know how i can add overlap-add into this to remove artifacts. ...
1 vote
1 answer
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Losing my mind over simple conversion of common Acceleration PSD-Units: g²/Hz to (m/s²)²/Hz

Because it causes confusion within my team I wanted to create a simple overview on how to convert from one PSD Unit to another. I know it is really simple but I've obsessed over it for a few hours. I ...
0 votes
2 answers
691 views

Why some FFT return complex array, some - mirrored real array?

I have 64 samples of a signal, two sines, one at 0.1 max freq, another at 0.5. Wrapped in gaussian: In my JS version of FFT I get an array of numbers, but the size is double (128 elements) and the ...
5 votes
1 answer
280 views

Algorithm to detect tapping-events in an XYZ-signal of an accelerometer

How do I find signal-sections with a re-appearing particular shape in a noisy signal? I want to detect tap-events (user tapped a device) using the 100Hz 3-axis accelerometer data provided by the ...
0 votes
1 answer
42 views

Signal level from its FFT

Suppose I have the FFT of an arbitrary real audio signal z samples long. Is there a way to discover the signal level (intended as the largest absolute value of its samples) from the FFT without ...
1 vote
1 answer
50 views

Anomaly in Magnitude Spectrum plot of an ASK Signal

Following is a SciLab code and Plots for Modulation of ASK Signal: My question is : For these particular values of fm and t, I get two weird small spikes on either side of main lobe, which doesn't ...
1 vote
1 answer
103 views

Frequency spectrum of interpolated sine

When interpolating a sine wave in Python, it seems I get a lot of additional frequencies closely around the fundamental. Why is that the case? I would have expected a more or less similar spectrum, ...
0 votes
0 answers
40 views

Is it possible to compute a 2D-FFT if the input size is not a power of 2? [duplicate]

Let us suppose to have an image with size $28\times28$, and that we want to apply a 2D-FFT without any padding operation. Does exist any algorithm which allows to perform the 2D-DFT calculation with ...
0 votes
0 answers
21 views

Filter coefficients hamming window for low-complexity Welch PSD computation

I am trying to implement the proposed method from this paper 1, to eventually have a more efficient PSD computation for feature extraction on a microcontroller. The method is suitable since I am using ...
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0 answers
22 views

Why a sampling frequency would distort a signal even if it meets nyquist theorem [duplicate]

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1 vote
1 answer
69 views

Predicting the "amplitude" of a damped sine wave from FFT

For a sine wave, $A \sin(\omega_0 t)$, with an amplitude $A$, after FFT and calculating the magnitude spectrum we can get a single peak with a height also equal to $A$. This requires proper scaling. ...
14 votes
4 answers
7k 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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1 answer
44 views

Is the noise at different sampling points related? [closed]

Given the power spectral density of noise, if the values at different sampling points of the same noise are added, what is the power spectral density at this time? Is there any correlation between the ...
1 vote
1 answer
53 views

How am I supposed to acquire/measure vertical and horizontal effective resolution from my images?

I apologize first for some questions that might be dumb. Unfortunately, I am only a bachelor student (writing my bachelor thesis in physics) and have almost zero image processing experience. And I ...
0 votes
2 answers
78 views

Influence of phase discontinuity on power spectrum density

I was wondering about the influence of phase discontinuities on a power spectrum density calculated with f.e. Welch's method. I know you shouldn't have any phase discontinuities in your signal, but I'...
1 vote
1 answer
430 views

Is there a FFT algorithm which is both in-place and constant geometry?

I work in crypto where the NTT (Number Theoretical Transform) which is just a FFT in finite fields is used. I wanted to know if there exists an implementation of the FFT which is both in-place and ...
0 votes
1 answer
181 views

Using the log power spectrum as features for a Classifier

I have a set of time series signals for which I have to develop a anomaly detection algorithm. I am considering using classifiers like SVM to do this. However I am confused about how to properly ...
0 votes
0 answers
75 views

Noise at Corners of MATLAB's ifft2

I'm trying to implement the code in this tutorial in MATLAB using MATLAB's ifft2() function instead of the algorithm used in the shader (looks to be some type of Radix-2 algorithm). It is used to ...
1 vote
0 answers
64 views

Debug the difference between two Frft implementations

I need help to understand the difference between two fractional Fourier transform implementations. On this website two different implementations for the fractional Fourier transform are presented (...
0 votes
1 answer
500 views

Window gain factor and amplitudes in FFT

I have some vibration data (acceleration) on which I need to perform an FFT, integrate it and again do an FFT. I read that for the nature of the data that I have, the input to the FFT must be first ...
0 votes
1 answer
187 views

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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