# 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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102 views

### Disagreement between Analytic Convolution and FFT Convolution

I've been working on a small project, and as a part of this I need a function that can compute the structure function of the input data. In the process of writing this part/testing it I found the ...
54 views

### Is there any other method to get the DFT of a signal without actually computing it using normal calculation

Is there any other method to get the DFT of a signal without actually computing it using a normal calculation. Please help me out in this query
109 views

### Time series in spectrogram() and spectrum()

I'm trying to create an order map for order analysis from my accelerometer data. To do that I want to resample my acceleration data and their time stamps in python. I found ...
60 views

### If you are given an inverse DFT, then how do you convert it back to the DFT without actually computing any DFT?

If you are given an inverse DFT, then how do you convert it back to the DFT without actually computing any DFT?
149 views

### How do I perform a time domain phase shift in the frequency domain?

I've seen some information on this topic around, but I don't quite understand it. I have a time domain signal. I understand that if I want to time shift this signal, I can do so by multiplying its ...
371 views

### Evaluating the continuous Fourier transform of a constant, and matching it up with the FFT result

I am following my optics textbook (Optics, by Eugene Hecht), throughout which are given various exact analytical results for the diffraction patterns that result from light passing through differently ...
34 views

### Matlab DFFT phase shift or data problem

Good day, I found a problem with calculating DFFT in Matlab. I have 1200 signals and for each I calculate FFT separately and then plot them together. it is a wave signal (simulation) propagating with ...
28 views

### How can I cut exactly the first harmonic (H1) from a FFT of an image

I have 4D( 2D + slices along z axis + time frames) gray-scale image for the heart beating on different moments. I do like to take Fourier Transform along time axis(for each slice separately), and ...
54 views

### What cause frequency change of the acoustic signal (3 kHz to 10 kHz)?

I am working on Acoustic Resonace testing in which I excite the sample and then record the sound (3 kHz to 10 kHz) by microphones. I do few times repetation of the same measurement in series (one ...
35 views

### Is it possible to estimate what the white noise floor should be, and after cross-correlation?

I am simply sending a signal from a function generator, split it into two signals, and then send it into two inputs of an oscilloscope which can interface with my PC. I take the FFT of these two ...
35 views

### Power spectral density: where does the difference from decibel and regular representation come from?

I am generating a simple DC signal with noise. As expected the fft shows that there is one peak at 0Hz and some noise. I am also trying to get the spectral power density representation of that signal, ...
124 views

### How to compute position by FFT?

I have FFT data of x, y, z - axis, Now I'm trying to find the position through FFT data. Step 1 : For that, applied corresponds to an ordinary frequency i.e rad·s−1, ...
41 views

### pyo: Retrieve amplitudes of audio signal frequenciy ranges

I'm playing around with audio signal processing for the first time and just to start somewhere, I decided to use the Python library pyo. My goal is to analyze an audio input signal from the microphone ...
157 views

### What are the advantages of using fft2 over fft

Assume we have a matrix x of size (8,8), . As known, FFT(x) performs 1D-FFT transformation, column wise. However, FFT2(x), ...
28 views

### Use Cases of FFT in Signal Processing [duplicate]

I am trying to learn about signal processing and fft's applications within it. My understanding is that a classical use case for fft is to remove "noise" (ex. high frequencies that are not supposed to ...
45 views

### Frequency and Order maps

I'm new to python and I'm using it to analyse accelerations from an accelerometer ie. vibration analysis. For that I need to plot 2 types of diagrams: 1) Frequency map; frequency and time on the axis ...
61 views

### Cross power spectral density (CPSD) comparison between signals

I want to compare various signals with Matlab's CPSD to identify shared frequency components. (If there's a better method, I would like to know!) I have 3 signals, A, B, and C. B is pure noise while ...
33 views

### Does Circular Convolution Correspond to Periodically Expanded Linear Convolution?

I have a question whether circular convolution and periodically expanded linear convolution corresponds in following case or why it does not? Think about a signal $x[t]$ and a signal $y[t]$ both of ...
42 views

### Calculating MTF: negative values in the LSF

I want to implement an MTF measurement. The goal is to measure a tilted slit, like this one: However the first half of the LSF is positive but on the second half (white-black edge) it obviously has ...
108 views

### What is Magnitude and Phase actually represents in Fast Fourier Transform?

When we reconstruct the image from its magnitude only using Inverse Fast Fourier Transform, then why the resulting image looks nothing like but the original image? Can someone please explain me in ...
28 views

### Energy Detection Sensitivity Comparison

In the area of broadband spectrum sensing, there are multiple methods used to detect energy. Two common methods are: Energy detection in the time domain (matlab example) Spectrogram (often defined by ...
53 views

### What is the explanation for the inverse FFT of a cross-correlated signal being shifted by some amount in the time domain?

I created two signals in the time domain: signal A which is a square wave, and signal B which is a right-angled triangle wave, as shown in the picture below. I took the FFT of these signals, ...
54 views

### How do I create a polar plot from a multi-channel sound file?

I have a multi-channel sound file from a microphone array. I want to get the polar plot in Matlab in order to get the directivity patter of the sound source. How do I do this? I did the FFT on the ...
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### USRP N210 and FFT

I am getting IQ data from my usrp N210. It has been written in a file as a complex number and I can analyze it with matlab. The IQ data actually resembles the power of my processor. The sampling ...
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### Vibration analysis: harmonics' amplitude

I am studying the vibrations on a FZG gear test rig with two accelerometers on the casing of the gears. These accelerometers sample at a rate of 10kHz. When looking at the main amplitude peaks on the ...
87 views

### Advantage of applying a Window Function in Analysis and Synthesis of STFT?

I'm studying Short-time Fourier Transform and I learned that if you apply a windowing other than rectangular, you need to overlap the frames---for example, 50% overlap for Hamming family---to ...
42 views

### Synthesis of 75% Window Overlap Add method for Short-Time Fourier Transform (STFT)

I'm doing the overlap-add method for STFT and I quite understand the implementation if it's a 50% overlap (where I add the 2nd half of the previous frame to the 1st half of the current frame during ...
51 views

### Reducing one frequency in song

How would I take a song input and output the same song without certain frequency ranges? Based on my research so far, the song should be broken down into chucks, FFT it, reduce the target frequency ...
40 views

### Hand written DFT vs MATLAB FFT has flipped conjugates?

I had to compute the DFT of a signal with 5 points with a hand-written method and using MATLAB's FFT just to see if they are equal. Taking the absolute value of both gives the same spectrum, however ...
40 views

### Using I/Q demodulation/LIA for a fixed-frequency application of several signals - do the phases of the input signals affect the result?

My input is a single signal composed of the sum of four periodic signals, each 4 Hz apart, centered around some resonant f0 typically between 1200 - 1300 Hz; the actual frequencies will change ...
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### Math.Net and alglib returning different FFT outputs by default

I am developing an application in C# with spectrogram drawing functionality. For my fist try, I used MathNet.Numerics, and now I am continuing to develop with alglib. When I changed from one to the ...
83 views

### Window functions and the Fourier transform

I want to clarify the 'correct' way to use windowing and Fourier transforms. My question is somewhat related to this one, but I have a few additional queries. I have some real, non periodic signal ...
36 views

### How to add a particular noise on a signal?

I have a simple ADC model (an input signal == sampled signal ). The adc should be modelled as with following values: 1.5 V (peak to peak).. meaning --> +1.5 V --> 0 dBFS ... how to do this ? The ...
78 views

### Phase delay between two signal using FFT [duplicate]

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 ...
71 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 ...
50 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 ...
40 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 ...
60 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 ...
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 ...
84 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 ...
48 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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### 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 ...
94 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 ...
41 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 ...
65 views

### Why are FFT numbers so high?

My input numbers are close to 10, here is a sample of some: ...
45 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 ...
36 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 ...