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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18 views

FFT: low magnitude peaks centered exactly 1 Hz apart across whole spectrum

I am running an fft on 60 seconds of data sampled at 1289Hz. The fft is run every second on a single channels fifo buffer which is also updated every second. The algorithm used is numpy rfft with a ...
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14 views

Calculating cross-correlation using Walsh-Hadamard transform

I am trying to implement MLS method of measuring impulse responses. There is an article describing the method: http://www.commsp.ee.ic.ac.uk/~mrt102/projects/mls.... As I understand, to get an impulse ...
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24 views

what is the the resultant, or, “final” frequency of the FFT of a chirped signal

Let's say we have a time-transient with a chirped signal, $$V(t) = \cos\left( 2 \pi f(t) t \right)$$ where $f(t)$ is the frequency of the signal and is time dependant. My question is how would one ...
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113 views

Can I use more than one Window on an FFT?

I was wondering if it is possible to take a signal and run it through an FFT multiple times with a range of different windows, and then combine/average the results to get a more accurate Signal ...
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1answer
37 views

FFT sample ordering with odd N

I have series of $ N =133 $ data sampled symmetrically around the origin in an actual space at $ x(n \tau) = x_n $ with $ n \in \{ -(N-1)/2, \, ..., \, 0, \, ... , \, (N-1)/2 \} $. Now, I'm a bit ...
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22 views

Improving Main Lobe Width of FFT

It is common knowledge that the duration time samples are collected is inversely proportional to the width of the main lobe in the Fourier domain. The simple example is the rectangular pulse in time ...
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2answers
41 views

Windowing of signal before FFT

I am facing an interesting problem: I have a signal in the time domain. In order to avoid discontinuities at the edges of the signal --- which would lead to artifacts when calculating the FFT --- I ...
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2answers
27 views

Is there processing gain for FMCW using heterodyne-style receiver as opposed to matched filter?

Beat signal of a single target will be a sinusoid in the idealized world, so theoretically the signal processing gain of an FMCW pulse correlated with Tx waveform in this way should be analogous to ...
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48 views

Using large DFT for OFDM to avoid the guard interval

In OFDM, we usually insert the guard interval to avoid the inter-symbol interference at the expense of achieved data rate. that guard interval can be either zeros or CP guard interval, which it must ...
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1answer
30 views

FFT with fewer outputs than inputs

Is it possible to write a FFT function, where there is fewer outputs than inputs? What in mean is, we would start computing 16-point FFT, and our result would end up be 12-point. If it is possible ...
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29 views

Time Series processing using fft

I have a set of real data (timestamp and value) with an unstable step between samples (5sec, 30sec etc.). The data is the % of fillage of a vehicle's tank through time. Due to the harsh volatility of ...
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Generating new 2D data using power spectrum density function from spatial frequency domain via ifft?

This is my first post so apologies for any formatting related issues. I also tried posting in stackoverflow but I'm more optimistic that I might get a response here. So I have a dataset which was ...
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1answer
22 views

How to do sliding window multiplication in frequency domain

I want to perform the following computations with the impulse response of length L of a LTI system: Multiply the impulse response with a window w(n) of length M << L. Take the FFT of the result ...
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37 views

Breaking down sin wave to frequency domain causing weirdness in phase

I have tried many many different things and I cannot figure out why my phase is messed up. Here is what I am trying to do. I create a simple signal with a known wavelength, phase, and amplitude like ...
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1answer
21 views

3-point DFT with two multiplications

I'm trying to understand DFT in the context of an integer ring (i.e. the number theoretic transform) and I was wondering how to reduce the number of multiplications by the twiddle factors. Consider 3-...
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How to shift frequency bins in audio pitch shifter

I would like to ask you for help to understand shifting frequency bins in audio pitch shifter. I have two issues that I can’t undersrand: FIRST QUESTION At first how to shift frequency bins when ...
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Image convolution using FFT

I just can't get my head around Fourier transform and convolution in 2D. I am trying to implement image convolution using fast Fourier transform (in julia). So the first thing I need to do is to pad ...
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1answer
36 views

How do I implement a footstep recognition algorithm? One that recognizes the start and end points of each footstep?

I have some time series data captured from a person's footsteps/strides (specifically, a person on rollerblades). It came from an IMU sensor placed on a person's boot. Each data point captures ...
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1answer
23 views

Measuring sine amplitude with ADC

My problem is similar to this one: Measure Sine Wave Amplitude from ADC Signal I have a microcontroller that produces a sine wave by means of its DAC. I can choose the frequency anywhere between about ...
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26 views

Efficient Method for Interpolating between bins in FFTW

I'm working on some oscillator classes right now and perform FFT around 100 times per second. The issue I'm running into is interpolating between the bins so there is not a noticeable change from each ...
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Doubt about sampling rate and spectrum with USRP and gnuradio [duplicate]

I have started working recently with software defined radio (SDR) as well as gnuradio. I have worked with digital signal processing for a while and one feature at one tutorial, which I saw for ...
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1answer
57 views

bit reversed order of FFT matrix for channel estimation

Although the question seems similar to this one HERE, but what I want to ask about is little bit different. Assume we have a signal $X$ whose length is $N$x$1$, we convert that signal into time ...
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Adjusting the plot of Power spectrum

New here tho. This may be a newbie mistake of mine or pretty basic to do so. So I calculated a Power spectrum using FFT from an 'amplitude spectrum' (Altho. I'm still trying to do the Lomb-Scargle ...
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50 views

Can velocity be calculated in first stage FFT?

I understand that, For an FMCW radar, the velocity of a target is proportional to the phase changes at same time instants across the chirps. I have a MxN data cube with N chirps(columns) & M ...
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40 views

Channel estimation using bit reversed order of FFT matrix

Is it possible to use the bit-reverse order of FFT instead of FFT to estimate the channel? Based on my reading, I found that bit reversed order of FFT matrix can diagonalize the channel, but ...
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1D amplitude spectrum with MATLAB

I have written the following code to calculate the amplitude spectrum of a test signal. ...
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1answer
20 views

Minimum phase conversion with Cepstrum method, how to scale the result?

I am trying to convert a zero phase spectrum (magnidue response curve with zero phases) to a minimum phase spectrum, because I need a totally causal impulse response for FFT spectral filtering, and ...
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1answer
37 views

Retrieve DFT coefficients from DCT coefficients

A vector a is transformed via a discrete cosine transform (DCT) to give vector b. For example in Matlab ...
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167 views

Estimation of Amplitude, Frequency and Phase of Linear Combination of Harmonic Signal Beyond the Leakage Resolution of DFT

How can I find a rough ( as accurate as possible) Amplitude of each frequency when there is spectral leakage. Currently, I am dealing with a system that contains special leakage which seems ...
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32 views

power spectrum of passband of ofdm is it right shape or not?

I tried to get power spectrum of ofdm passband signal. Below is the whole matlab code for that with BPSK+ofdm. and I got the power spectrum as below. I wonder that is that right form or not? ...
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How to Quantify a Difference in Spectral Information of two Signals?

As the title states, I am looking for established methods to quantify the difference in two FFTs. I have come across methods such as using cross correlation in the time domain or the coherence ...
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1answer
18 views

Spectral efficiency of with OFDM vs without OFDM?

OFDM has advantage over frequency selective channel with addition of cyclic prefix after IFFT of MPSK or MQAM symbol. but I wonder that the spectral efficiency of with OFDM vs without OFDM.
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STFT(Short-Time Fourier Transform) & DFT(or FFT) in Matlab

I want to try STFT & FFT using Matlab. What I wonder is STFT of signal computes the result that FFT(DFT) of each windowed signal and I can see the change of each frequency value over time. If I ...
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33 views

Best way to do accurate amplitude measurements using FFT

To do accurate amplitude measurements on an digital radar receiver, I am searching for the best method to get the amplitude out of a FFT. The signal is windowed using a Blackman window. So far I'm ...
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1answer
27 views

Is there an a method to fit a wave created from two wave?

I need to get the frequency and amplitude for a wave that consists of multiple function. for example, if I have a sine curve (created from two sine waves), How can I extract the parameters for this ...
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3answers
54 views

Role of windowing in implementing FIR filters using Fast Convolution

In Fast Convolution the filtering is performed by taking FFT of both the signal and impulse response and multiplication in frequency domain using Overlap add/Overlap save method for processing ...
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Analysing DAC Spectra: Transient Noise Analysis

I am working with a new Digital-to-Analog Converter (DAC) design in simulation and I'm trying to analyse the output. The device takes in an ideal 14-bit digital representation of a sine wave and ...
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1answer
60 views

How to avoid leakage in the following FFT

I have written a code for FFT for a signal which time domain plot and FFT is shown bellow. I am not sure about what FFT length to do in order to avoid leakage of energy? I know the number periods has ...
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1answer
372 views

Meaning of a null coefficient at 0 Hz

I am analysing a wave with a timespan $\tau$ and after applying the fft I obtain the following plot of the coefficients I assume that the peak at $t=1$ followed by smaller coefficients means that the ...
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1answer
28 views

Seeing spurious peaks in acquired vibration data; peak frequency changes with sampling rate

I am acquiring vibration data from an accelerometer using a DAQ. Due to some reason I am seeing spurious peaks/impacts in the FFT spectra at specific frequency and its harmonics all the way till the ...
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49 views

Why does numpy FFT only work for samples of 10,20,30 sec?

I'm using numpy FFT functions to find the amplitude, frequency and phase of low frequency signals. focusing between 0-10 hz.Part of my project is to vary the time over which the sample is taken and ...
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1answer
24 views

does ifft of higher order equal upsampling signal

I have a signal with length 2000, and am taking an fft by 2048, after I perform analysis and some operation on signal in frequency domain I tried to convert it back to time domain. I received a signal ...
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2answers
38 views

Filter for DC component in FFT2 of image

I am trying to get the main frequency of the Fourier Transform of the image below, which is the intereference pattern detected by a CMOS sensor. When I apply fft2, I got the DC peak at 0 frequency, ...
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statsmodels STL not finding all frequencies

I am trying to do a signal decomposition with statsmodels.tsa.seasonal's STL to find a seasonality. I have 3000 datapoints spanning over several years. so I know there are monthly, yearly and weekly ...
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1answer
43 views

Using DFT Rotation for Frequency Down-Mixing

Is it possible to perform Frequency Down-Mixing (sometimes referred to as Down-Conversion) by simply rotating the DFT sequence? If so what are the Advantages and Disadvantages of this method?
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35 views

How does Music algorithm differ from simple DFT/FFT for frequency estimation?

I would like to know why would someone choose to use Music algorithm from frequency estimation when one can simply use DFT/FFT which are much simpler. What is the intuitive way for understanding Music ...
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1answer
108 views

What's wrong with my FFT implementation?

I am implementing radix-2 DIT FFT (real only) in C++. For input sizes > 8 I can't seem to match the output of FFTW. Here's my implementation of the FFT: ...
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33 views

Different amplitude for different time resolution with FFT [duplicate]

In the following Matlab code, the amplitude of the spectral domain signal is printed ...
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80 views

Fringe movement analysis to extract displacement

I am using a Michelson interferometer with the interferogram recorded on a CMOS sensor. I have taken a video of the fringes moving when a displacement of 50 $\mu m$ is imposed to one of the mirrors. ...
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63 views

Implementation of Overlap Save Filtering with Decimation

I am trying to implement Matlab code which performs FIR filtering with complex bandpass filter using Overlap Save method and perform decimation as well by using different length forward and inverse ...

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