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# Questions tagged [fft]

The fast Fourier transform is an efficient algorithm to compute the discrete Fourier transform (DFT) and its inverse.

398 questions with no upvoted or accepted answers
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452 views

### Fast convolution with striding step

I want to convolve two discrete functions $f$ and $g$ using convolution stride size $a$ to get the result as $s_{a, i}$: $$s_{i,a} = \sum_i g_k f_{ai-k}$$ I know that simple convolution with $a=1$ ...
773 views

### Autocorrelation Function and Power Spectrum

My aim is to determine the power spectrum of a signal of finite duration using Its autocorrelation function Performing FFT directly on the time-domain data The function is defined as follows in the ...
48 views

### DFT method for a sequence defined by recursion

I encountered the following interesting computational problem in the field of computational statistical mechanics. https://www.dropbox.com/s/8yimd9uy5u0jrdj/dftpr.pdf thank you :)
1k views

### Moving average on the frequency domain - (Welch's Method?)

Is there a name for a filter that is using a moving average on the frequency domain? I am searching for such a filter to filter out long "pads" (synths) from music. Do any of you know about such ...
353 views

### Why the radar (moving) target return is not centered at zero frequency

In Radar signal processing, one has to separate clutter(unwanted signals) from moving target signals. The picture given below is an example of Radar Signal Power Spectum(taken from the book Radar ...
538 views

### Online FFT of simulated signal

it is a couple of weeks that I am thinking about the following: I have a tool which is able to simulate the dynamic behavior a steel tower exhibites due to the coupled action of wind and waves. ...
817 views

### Obtaining an expression for the transfer function when using FFT

Say I have two signals, a(t) and b(t) where the former is the input and the latter in the output. These signals are both recorded by sampling at every 0.01s. The Fast Fourier Transform was applied to ...
1k views

### Why does the inverse fourier transform of a lowpass filter have complex components in matlab?

I am quite confused whether the following numerical differences i find are just severe round off errors made by matlab, or something i am doing wrong. The following happened when trying to see what a ...
430 views

### FFT of complex envelope

I have I and Q samples of a time domain signal, sampled at a rate T, both of length L. I create the complex envelope, C = sampleI+j*sampleQ. Then take the FFT of ...
867 views

### bearing frequency detection

I am working on bearing faults detection. I am doing the following steps: signal->mean zero->high pass->hilbert->y->fft->cepstrum of y. my code is: ...
125 views

### discrete fourier transform in Matlab - theoretical confusion

I have a periodic term $V(x) = \sum_K \exp(iKx) V_K$ where $K =2\pi n/a$ where $a$ is the periodicity of the term and $n =0,1,2,3....$ Now I want to find the Fourier coefficient $V_K$ corresponding ...
992 views

### Frequency shift and phase shift (Doppler)

I'm trying to apply a frequency shift to a generated despreading code in order to find and track a broadcast signal CDMA. Expected doppler shift range of the signal from center frequency would be ...
2k views

### Sample accurate real-time pitch shifting algorithm

I am trying to implement a phase vocoder overlap-add pitch shifting algorithm for part of a plugin I'm writing. I have done this kind of thing offline before and I understand the theory, but this is ...
131 views

### Estimating and Improving the accuracy of fourier transform in the low frequency sector

I have a time series file which starts with 1 and then quickly tends to oscillating about 0.001. The time series has a resolution of 0.01s and has 10^6 sequential points starting from t=0. The goal is ...
114 views

### Denoise a randomly occuring signal

I have a program which takes in data from an oscilloscope, and due to reflections in the medium, we will get random signals at random intervals. The two grey windows at the right bottom show the ...
183 views

### Constant phase for each frequency?

FFT of some signal $x$ has two parts: $|x|$, which is the magnitude spectrum and $arg(x)$, which is the phase spectrum. I want each frequency to have the same phase. Is there a way to accomplish this?
372 views

### FFT frequency domain averaging and zero padding

I'm trying to get the average frequency response of an entire piece of audio by splitting it into segments, doing an FFT on each one, and averaging the magnitudes together. Its working great until I ...
396 views

### FFT of irregular shaped data

I have an irregular shaped 3D image that I want to do frequency domain filtering on. Currently I simply zero-pad the data to make it a cube, then do an fft. I was wondering if this was a valid ...
76 views

### Period of non-harmonic Fourier sum

I am wondering if there is a formula for the period of the sum of $N$ cisoids of the form $e^{j\frac{2\pi}{N} i\, \omega_k}$ where the frequencies $\omega_k \in \left[-N/2,N/2\right]$, $k=0,\ldots,N-1$...
160 views

### How to prevent Signal drifting when changing frequency

When I recreate a signal it seems to look great. But when I try and increase it's frequency (which is done in a for loop and the variable new_freq=2) it starts to drift. See Image below (The top ...
620 views

### How to understand FFT results of scipy.fftpack?

I calculated FFT for a speech wav-file using scipy.fftpack. How do I read (understand) the return of FFT? I have read that it supposed to be like so: y[0] is 0Hz loudness, y[1] is 1Hz loundess, ... y[...
101 views

### Phase distortion of frequency warping by using Praat

I am trying to do speaker conversion by frequency warping method. I modify the function Spectrum_shiftFrequencies() in Praat by scaling the FFT spectrum of a voice. When one scaling factor is ...
273 views

### Getting frequencies out of a FFT

I use this FFT and wrote a short program to test it. ...
70 views

### How to analyse periodic data obtained from a vibration experiment?

I have data from a vibration experiment on a structure. This data was obtained by attaching an accelerometer to a FFT analyser, which records the data at a high sampling rate (typically in kHz range) ...
924 views

### Real-time frequency recognition in python

I'm a newcomer to signal processing, so forgive my ignorance. I'm working on a project in Python that will play a .wav file and pick out the dominant frequency as it plays. I've found a few examples ...
202 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 ...
545 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 ...
1k 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 ...
356 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 ...
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 ...
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 ...
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 ...
41 views

### Non-physical peaks from the Fourier transform process

I was doing some reading on transient signals. When transient signals is analysed by DFT, there are some non-physical peaks from the Fourier transform process (on the figure below). I understand this ...
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 $$f(t) = 1 + 2\cos(2\pi t) + 4\cos(4\pi t)$$ ...
179 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 ...
284 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 ...
195 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, ...
304 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!
157 views

### Filter length in a Filtered-OFDM system

In a filtered-OFDM system, lets say we have the following parameters: ...
800 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 ...
121 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 ...
326 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. ...
523 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 ...
730 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 ...
167 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 ...
370 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 ...