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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78
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3answers
23k views

Why is it a bad idea to filter by zeroing out FFT bins?

It's very easy to filter a signal by performing an FFT on it, zeroing out some of the bins, and then performing an IFFT. For instance: ...
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2answers
48 views

Phase delay between two signal using FFT

I am working in calculating the phase delay between two signals. The signals are sent through the same transmitter and received in two different receivers. They have many frequencies, but with a ...
0
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2answers
211 views

Threshold for peak detection of noisy signal in frequency domain

I have a noisy signal (Gaussian Noise) with a known SNR and known Noise variance. This signal contains a number of Frequency components. My ultimate objective is to detect the components of the signal....
0
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1answer
82 views

Silence removal from the audio

Suppose i've a mp3 file. I performed FFT of that mp3 file. Then is it good to state the following: 1. Silence occurs by that particular DFT point whose magnitude is lesser than some threshold. 2. ...
0
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1answer
87 views

number of samples and Amplitude accuracy,what's the solution to this problem

I'm colleceting data samples from a device (temporal signal) in order to analyze it with FFT function in MATLAB R2015b,first i get N=4063 data points with (delta_t=0.000015s) so my frequency ...
0
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1answer
28 views

Does this make sense for cross-correlation in frequency domain?

If I have a signal A at 2 kHz and signal B mixed at 2 kHz + 3kHz, and then i take the FFT of them both, and then cross-correlate them many times, the 3kHz peak should disappear? Since this peak is not ...
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4answers
85 views

How to detect overlapping peaks in FFT?

I am working with a modular spectral processing STFT toolkit I have written in C. I often need to carry spectral peak detection and analisys (frequency estimation, peak displacement, etc) but I am ...
0
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1answer
63 views

Zero padding in frequency domain, any tricks to speedup?

there are many situations when one has to carry some operations on spectra, like splitting a spectrum Z wide in two Z/2 spectra, or vice versa merging two Z sized spectra in a 2Z sized one. The only ...
0
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1answer
34 views

How to verify if my cross-correlation algorithm works?

So I have a signal generator which can produce two separate signals. I use a spectrum analyzer to read these signals, and can interface with Python. I wrote a Python code which takes in these two ...
0
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1answer
50 views

How can I manipulate the time signal to insert zeros in between frequency domain bins?

I generate Multi tones in the frequency domain with constant amplitude and phase, for a fixed distance between tones. As the figure below, where r is the change in frequency spacing between different ...
2
votes
1answer
4k views

Calculate 1D Power Spectrum from 2D Images

Imagine satellite images, these are irregular sampled in X and Y direction and the shapes are of course are oddly off. We now want to estimate a 1D power spectrum from the whole image to estimate the ...
0
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2answers
70 views

Spectral filtering

I am working with a STFT spectral processing toolkit I wrote in C, now focusing on a module for spectral filtering. In its first implementation, the module simply multiplied an user drawn spectral ...
0
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0answers
30 views

Detecting and Recognizing a pattern in the 2D-FFT

More theory before I ask a practical question based on the task I have. I've discovered that performing a 2D-FFT (python or matlab implementations) on some test imagery I have results in a unique ...
0
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1answer
73 views

How to manually implement convolution with FFTs?

I'm trying to manually implement a convolution using FFTs and it isn't working as expected. I know I'm probably missing some subtlety with padding, shifting, or conjugation, (all of which I've tried ...
0
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1answer
110 views

Discrete Cosine Transform (DCT) Coefficient Distribution

I have two images : Original Image Binarize Image I have applied Discrete Cosine Transform to the two images by dividing the 256x256 image into 8x8 blocks. After, I want to compare their DCT ...
0
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1answer
625 views

Discrete Fourier Transform frequency bin size

I want to ask a simple maybe stupid question, why in DFT the frequency bin size is limited to $n/2+1$ http://support.ircam.fr/docs/AudioSculpt/3.0/co/FFT%20Size.html. What I see from Wikipedia and ...
0
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1answer
25 views

Harmonics below oscillation frequency in the brain

Apologies if this question is trivial, I'm a neuroscientist and would hugely appreciate your help to correctly interpret my analysis. I am recording EEG signals and am manipulating the brain using ...
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4answers
152 views

Detecting a three-tone signal in real-time

I'm new to digital signal processing, and I've been looking into FFT, filtering, Matlab etc. What I'd like is to build a program which will detect when a certain BEEP is transmitted through a trunked ...
0
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1answer
40 views

MATLAB: Applying Inverse Fast Fourier Transform to an array of real numbers

I'm trying to understand the difference between applying the Discrete Cosine Transform (DCT) and the Inverse Discrete Fourier Transform (IDFT) to the log Mel-filterbank energies as explained in the ...
0
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1answer
32 views

A Delay Between Two Filtered Chaotic Signals

it is a common practice to use a shift of cross-correlation peak to evaluate a group time delay of two signals (chaotic signals are included). Can one synchronously and equally filter these chaotic ...
0
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1answer
35 views

Torque signal fft

I have the following torque signal picked up with a 10.240Hz sampling rate from a testbench. I am studying its fft which I create on Octave with the following code: ...
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2answers
241 views

why is my FFT magnitude of real time recording so volatile?

My goal is to do spectrum analysis of a gear motor. I used Python with the Pyaudio package and made a crude spectrum analyzer that displays the FFT as the sound is being recorded. I recorded 10 ...
1
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1answer
206 views

Best way to resample an OFDM signal without interpolating + filtering?

I am working on a custom transceiver with a sampling rate of 3.84 MHz (256 OFDM subcarriers at 15 kHz, although 56 subcarriers are null). I would like to resample the signal to 5 MHz, in order to use ...
0
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0answers
36 views

Getting back the original x axis after IFFT-FFT in Python

I have an analitically generated spectrum $I(\omega)$, x axis represents angular frequency, y represents intensity. The spectrum is centered around some frequency value, which is often called central ...
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1answer
30 views

Can the (point-wise) multiplication of two discrete band-limited functions be inherently aliased?

I'm not fully used to the language of DSP, so forgive me if I confuse some of the notation, or if the below is trivial. I'm interested in the case where we want to do the point-wise multiplication ...
0
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1answer
59 views

Why are FFT numbers so high?

My input numbers are close to 10, here is a sample of some: ...
2
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2answers
116 views

how I can use the FFT data to compute the convolution?

First,I have a vector A of length N,through process ...
0
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1answer
31 views

how to interpret the 2D FFT

I know how to compute the 1D FFT (and interpret values from 0 to Nyq). When computing the 2D FFT, do we compute the FFT of row[1] then the FFT of row[2] then the FFT of row[3] up to the last row. ...
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2answers
110 views

Multi Resolution STFT for audio

I am looking at trying to achieve a multi resolution STFT analysis on a log sine sweep audio measurement. I have an impulse, and I would like to perform two different windowed length FFTs on the ...
0
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3answers
507 views

What is the proper way to implement a real time spectrogram?

I am trying to implement a real time spectrogram and I am not sure that I am implementing it the right way. First I decided to use Matlab, I was taking a short recording of lets say 0.1 seconds and ...
0
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1answer
39 views

Frequency domain multiplication

I am confused why zero-padding in the frequency domain does not result in linear convolution in the time-domain using the relationship: $$corr(a,b) = ifft(fft(a) fft(b))$$ See my process below for ...
0
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2answers
62 views

Difference between bandpass filter and using fast fourier transform to extract power of frequency bands

In signal processing, I have heard of two terms: Using band-pass filter to extract some frequency bands. Using Discrete/Fast Fourier Transform (DFT/FFT) to extract some frequency bands. Are these ...
5
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3answers
139 views

why should I scale the fft using 1/N?

I am writing a report, and my advisor asked me to explain why I scale the fft by a factor 1/N (where N is the length of the array). I used to use the scaling convention of multipling the fft by the ...
3
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1answer
74 views

DFT from Fourier transform

I always studied the DFT starting from his formula, but for some reasons I need to do comparison between the FT and the DFT. I found the pdf in this link very useful http://www.robots.ox.ac.uk/~sjrob/...
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0answers
31 views

performing cwt and inverse cwt using fft

I'm currently working on my project related to implementing an speech enhancement algorithm on fpga which uses continuous wavelet with morel wavelet, how can i use efficient fft algorithm to calculate ...
0
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2answers
23 views

Extracting different frequency bands (alpha, beta, gamma) from brain activity data

I am new to signal processing and despite googling about my question, I still couldn't find the solution. I have an Electrocorticography (ECoG) dataset. This consists of recording brain activity from ...
0
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1answer
17 views

Output shape of numpy's rfft function

For numpy's rfft function, the length of the output is about half of N, the length of the input sequence. What is the reason behind this? What are the frequency bins?
0
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2answers
2k views

Getting the amplitude and phase of harmonics in C

I have a periodic series of 360 values. How can I use the fftw c libraries to get the amplitude and phase of the first, second, third and fourth harmonic? If I do this for N=360 ...
0
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0answers
13 views

Why does spectral accuracy of laplacian decrease with sampling size?

We know that for any real-valued function $f(x,y,z)$ whose Fourier transform is $\mathcal F[f]$, its laplacian can be computed from a spectral interpolant as follows. $$ \Delta f(x,y,z) \simeq \sum_{...
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0answers
10 views

Image Transformation Functions to Identify if the Text in the Document Originally There or Just Synthetically Overlaid

I have a copy of scanned ancient dirty document and synthetically generated ancient dirty documents. I want to use Discrete Cosine Transform to characterize noise types existing in the document and ...
3
votes
2answers
414 views

Why is my DFT/FFT always 90° out of phase?

I'm doing an FFT using Python and Numpy on one machine, and C# on another. I'm using some dummy data that mimics how I'll eventually be gathering data from sensors in the C#/UWP application. The two ...
0
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1answer
673 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 ...
0
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0answers
14 views

What is the correct input format to perform IFFT on a spectrum?

Say I have response of a band pass $\left(f_1-f_2\right)$ or a lowpass filter $\left(0-f\right)$ and if I want to estimate filter length then I may take the inverse transform of the frequency response ...
0
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2answers
25 views

Discrete-time system: getting the true frequency from normalized frequency

Lеt's say that i have a plot where i can see the spectrum of some discrete signal and let's say that it's x axis is normalized (by normalized i mean $\omega/\pi$ on the $x$ axis). Now, i know that in ...
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0answers
25 views

Acceleometer Noise and FFT

Background: I'm fairly new to signal processing and sensors, etc. I'm working on my thesis, which involves the use of an MEMS acceleration sensor as a vibration measuring device (for rotating ...
0
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1answer
35 views

Given a static time domain signal, why does the magnitude oscillate in the frequency domain after FFT?

I'm trying to: Generate a sine wave in the time domain (locally within my program at 500 Hz). Analyze it with FFT Print the results in the frequency domain. The expected results are that since the ...
0
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0answers
54 views

Phase shifting in frequency domain

I am aware this topic has already been addressed in at least one question here, but despite that I could not find the solution to my problem. Also bear patience with me since I am not a mathematics ...
3
votes
3answers
1k views

How Do You Save / Convert an Image to Be in the Frequency Domain?

Quick Note: I don't believe this is a subjective question... however if it is I will gladly modify to be more objective In ImageJ, if I take the FFT of an image, how can I save this new frequency ...
0
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1answer
38 views

Fast Fourier Transform for varying period (Order analysis)

I am trying to do a fast Fourier transformation on accelerometer data from a shaft rotating at varying speed in Python. What I have done so far: 1: The original plot was in the time domain, and I ...
1
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1answer
60 views

FFT on a random signal

I have applied FFT on a random noisy signal (say for ex, on some stock data). Below figures shows input and output. Now, my question is, how to extract useful peaks (frequencies) from the output? Can ...

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