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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1answer
196 views

How to determine relative distance between 3 in-phase transmitters by analysis of received signal

I would like to determine with high accuracy the relative distances between three RF transmitters a few meters apart by analysis of phase interference in the received wave, the receiver being at an ...
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1answer
532 views

FFTW - Inverse FFT input data

I am trying to use FFTW function for IFFT: fftw_plan_dft_c2r_1d(size, out_cpx, out, FFTW_ESTIMATE). My goal is to input desired frequency response data and ...
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1answer
991 views

Power Spectrum in dBFs

I'm trying to get the spectrum of my signal in dBFS units. I'm not sure what is the refernce that I should devide by. I computed the power spectrum as follows: 1) FFT normalized to the number of ...
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2answers
161 views

Simulation of a pressure field developed by a song

Greetings! What I am doing: In short, I want to write a program that is fed a song as a wav file, and spits out an animated contour plot indicating the relative pressure/loudness/quantity indicating ...
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1answer
342 views

Kiss fft does not work after giving it more than 32 samples

I am trying to take data from an accelerometer and apply Kiss FFT to the samples. I'm using a Freescale Kinetis FRDM-K22F board. I want to use 64 samples, but when I run the program I get an error ...
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1answer
2k views

Frequency response of a filter - fft vs. freqz

I have designed my filter using firhalfband(N,Fo,A), which gives me the filter coefficients. Then I try to see the frequency response of it. I use these two ...
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1answer
3k views

comparing 2 audio files in c#

I have 2 audio .wav files recorded by the the same person with the same word. Each audio file has a format of 16bit, 44.1kHz and is a mono channel. I would like to know how to compare these 2 audio ...
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2answers
1k views

MATLAB : How to filter a data signal

I have a data signal (column vector) with N values taken over at specific sample rate (dx) at a fixed frequency. After taking the Fourrier transform of this signal and representing it in the Fourier ...
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1answer
197 views

what I'll get if I take FFT of a frequency domain data

What I'll get if I take FFT of a frequency domain data. I have two questions related to this. For ex, x is a time domain data where X = fft(x), now fft(X)= what i'll get ? While reading SCFDMA, data ...
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2answers
372 views

How to demodulate DSB-SC

I need help with demodulating DSB-SC signals. I took the appproach of using the FFT as follows: I have a signal. I FFT it. Then i take real and imaginary part of the FFT and translate some values: <...
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2answers
316 views

Signal signs are changing after IFFT reconstruction

I want to take FFT of my signal and reconstruct it back via IFFT. It seems the real part of odd numbered samples and image part of even numbered samples are multiplied by -1 which can be observed ...
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2answers
43 views

FFT calculation problem

Now I'm trying to study about FFT with the following example. I just implement into excel to see the how does it work, But the results are different at all as the below image. how do I get the ...
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1answer
146 views

Zeroing FFT bins without taking inverse FFT

I know the question has been asked about why it's a bad idea to zero bins in an FFT, but those all seemed to be taking the IFFT afterwards. What if I want to do all my analysis in the frequency domain?...
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2answers
65 views

Why is the fft magnitude spectra of sine not symmetric?

I thought that the magnitude spectra of real signals was symmetric. When I take the absolute value of the fft of a sine wave the result is not symmetric. There seems to be a peak at the first ...
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1answer
344 views

problem finding 16 point DFT using two 8 point FFT (Divide and combine algorithm) MATLAB [closed]

Write MATLAB code that determines and plot the N-point Discrete Fourier Transform of x[n] defined by the following equations: x[n]=0.5*pi*n n=0:16 Compute and plot 16-point DFT using two 8-...
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2answers
117 views

How make FFT when buffer size is not 2^L

I still try to understand FFT :) I know I can do something like that: float bufferSize = 500.0f; int correction = log2(bufferSize); bufferSize= pow(2, correction); ...
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2answers
87 views

Sampling theory

Imagine I have a 1 kHz sine wave with very low noise. (Assume a signal generator output with 1v peak to peak clean signal). I am using an ADC to sample this signal. Take the following cases: Sample ...
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1answer
254 views

Smoothing or bigger FFT bins?

I have an FFT of a radio signal, to look at "what's happening" in the frequency domain. I'm interested in signals that are "wide enough", eg 1MHz, and thus would like to smooth the result to filter ...
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1answer
1k views

normalizing STFT output and calculating energy of signal

I have two questions regarding power spectra and energy of a signal: For power spectra calculation I used STFT, and I saw that the value of STFT should be normalized before applying logarithmic ...
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1answer
398 views

ifft doesn't recreate original signal

I want to convert given signal into frequency domain and do some calculation and then convert that signal into time domain. Here is the code which I am trying out ...
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1answer
211 views

Spectral leakage issue cannot solved using windowing

I am designing a frequency lowpass filter using FFT&IFFT. My test data is a time series with a large DC component (the pink curve). Before the FFT, the time series was detrended (curve fitting) to ...
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1answer
846 views

Calculate Damping using FFT

Given the following signal I would like to calculate the rate of damping. I presume that I need to find out where the peaks are using some kind of smoothing on the sample data. After performing an ...
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1answer
423 views

FFT window corrupts signal

this is a newbie question. My goal is to create audio filter that cleans human speech from noise. I started with learning all about fft filters. At first I've created simple fft filter with ...
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1answer
4k views

fft of a complex signal

I am new to dsp and just have a vague understanding of fft process. i know fft is used to convert time to freq domain.... i need to generate a sequence of frequencies such that F=[F1,F2,.....Fn-1] ...
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1answer
186 views

Computation time 1) fft, 2) 2-d convolution, 3) two 1-d convolution

Consider filtering square n*n images by square,separable m*m filters. What are the general computation time for the following approaches 1) FFT 2) by 2-D convolution 3) by two 1-D convolutions. (...
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0answers
13 views

Autocorrelation of vector of images

I have a set of images (think a ball getting bigger and smaller). I want to assess the periodicity in this string of images (each data point in my signal is an image). How can I go about doing this in ...
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1answer
52 views

Quite confused with Fourier Analysis results

So I'm meant to show how the DFT can find the frequencies, and respective amplitudes, associated to some data. And I have this data set from the curve $$ f(t) = 1 + 2\cos(2\pi t) + 4\cos(4\pi t) $$ ...
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2answers
74 views

Attenuate frequency bin in FFT array resulting in artifacts [duplicate]

Am trying to remove noise for certain frequency bands by attenuating freqeuncy bins in the FFT array. However this results in artifacts to be formed, where there are clicking sounds in between each ...
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1answer
158 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
100 views

In a periodogram, why is the modulus squared of the DFT taken rather than the DFT itself? [duplicate]

Why can't you just take the DFT itself? And how accurate is this method in spectral density estimation? Are there cases in which it is very accurate?
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2answers
197 views

Phase difference b/w two sinusoidal signals using FFT

I am trying to understand how to find phase difference b/w two real sinusoidal signals and then align the phases of two sinusoidal signals using this difference. In the first step, however, i have to ...
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1answer
169 views

Finding resonance peaks from sine sweep vibration test

I'm trying to perform FFT analysis on data from an engine "rpm sweep" test to find resonance peaks on the DUT. There are three tri-axis accelerometers along with time, engine speed, engine torque, ...
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2answers
866 views

STFT computation

I'm trying to compute the STFT by my self. Thus, I wrote the following code: ...
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2answers
252 views

FFT uncertainity in frequency

Hello What would the uncertainty be in frequency vs amplitude graph for the frequency? the peak amplitude is the dominant frequency, what is the uncertainty of that frequency? Thank you!
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1answer
143 views

Filter length in a Filtered-OFDM system

In a filtered-OFDM system, lets say we have the following parameters: ...
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1answer
396 views

relationship of column of spectrogram to original signal

We can get the spectrograms of a signal which is a 2D matrix. How do I get back from the columns of the spectrogram to the time stamps in original signal? For example, what time stamp does the columns ...
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1answer
743 views

MATLAB's $\tt bandpower$ function - How does it work?

So the function bandpower(x,fx,freqrange) calculates the average power within the selected frequency range. So it basically takes a part of the periodogram, but ...
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1answer
116 views

Making a mesh out of the parameters from a STFT

I do not possess much knowledge about signal processing, so i hope to do a find an answer. I am reading a wav-file and then apply a fft on it. In theory, i think i have a 3 parameters i can use. Time ...
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1answer
295 views

Can't get FFT of Sample & Hold sinusoidal wave in Simulink

Simulating of Sample-and-Hold fails: "Buffer" block generates the error "All sample times must be discrete. No continuous or constant sample times are allowed" Any suggestions ? Thanks in advance. ...
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1answer
488 views

Digital signal and frequency analysis

I am generating a digital signal with software. I have defined the vector $$[0,1,1,1,0,1,0,1,0,1,0,0,0,0,0,0,0,0,0,0] $$ and my sampling rate is 8 million per second. The frequency spectrum is ...
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2answers
485 views

Bin spacing in FFTs

I have a signal that I sample at some frequency (currently 100Hz), and want to extract low frequency information (between 0.8 and 3 Hz) I have three constraints on this: I would prefer to compute ...
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1answer
652 views

Explain how to apply Hanning window to Fourier amplitude spectrum

This example code was used to answer a question about applying a window on the Fourier amplitude spectrum. How to apply Hamming Window? I have a question about the section of code related to the ...
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1answer
64 views

Programming a peaking filter (and how do I FFT?) [closed]

I want to get into audio signal processing. I know how to generate a tone, but I have no idea how filters work. What do I do to a set of samples to boost/cut a frequency with adjustable bandwidth? ...
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1answer
161 views

Use of FFT to compute frequency response [duplicate]

Is there an algo that uses fft to compute the frequency response of an FIR? Currently I follow the textbook method of evaluating the $z$ transform at $e^{-j\omega}$ for $\omega$ running from $0$ to $\...
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1answer
164 views

what is the preffered data and FFT length for blackman harris window for improving Accuracy due to ENBW Consideration of window?

for Rectangular window if data length is 1K then FFT Length Should be 1K FFT only because ENBW of rectangualr window is 1.0 but for Minimum 4-Sanple BlackmannHarris window is 2.0. so for improving ...
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1answer
365 views

fft2 of interferometric image - how to interpret the result in term of dominant frequency

I have an interferometric image which shows fringes at a certain frequency, basically a dominantly periodic signal overlaying the information that I want to extracthere are the fringes that I want to ...
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1answer
918 views

FFT on XY data points [closed]

Are there any algorithms out there that can do a FFT of 2D or XY data (not sure of the exact terminology here), I don't mean XY data like in a graph I mean XY/2D data points like a circle or a smiley ...
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1answer
950 views

FFT of signals with different sampling frequency

I am trying to combine two sets of data together, each of them having different sampling rates. For example if I sampled a 100 Hz sin wave at 1KSPS, and another 150Hz sin wave at 1.5KSPS, would it be ...
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2answers
100 views

glitches when performing EQ on an OLA FFT

i'm trying to filter a signal with a FFT. I window each block of signal (i use a 4096 points signal) and then remove some high frequencies by just multiplying the magnitude response. the result is : ...
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1answer
61 views

How is the decay of a signal exemplified in a Fourier Transform?

Is there any way to tell if a signal is decaying from its fourier transform?