Questions tagged [fft]

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

Filter by
Sorted by
Tagged with
0
votes
1answer
828 views

Checking for VHF pulse with SDR and python

So I have a small NooElec Software Defined Radio and I've got it working well with my Raspberry Pi using GQRX as a controller and to visualize the SDR output. My goal though is to use a python script ...
1
vote
4answers
1k views

Why does the frequency in the DFT have to be an integer?

I don't know why the result of DFT and FFT in MATLAB like below images.. fs=128; t=0:1/fs:1-1/fs; x=cos(2*pi*3.5*t); X=fft(x); N=length(x); n=0:N-1; f=fs*n/N; plot(f,abs(X)/N); If I set the ...
0
votes
0answers
76 views

Making longer a time domain signal by adding values on its frequency domain. What am I doing wrong?

Background Here's the thing: using software for Finite Element Acoustic Simulation I got the dataset of frequency response from a room; software works by solving the wave equation in the interval $`[...
0
votes
1answer
172 views

How to interpret the signal in frequency domain

I'm having difficulties understanding some basics regarding FFT. I was hoping you could answer a few questions I have. These questions will probably seem silly to you, sorry for that. I'm a complete ...
0
votes
0answers
135 views

Correctly scaling Power Spectrum and Correlation Function of Galaxies from Mock Catalogues

I'm currently writing a code to compute the two-point correlation function of a set of galaxies. As I work with a loooot of galaxies, it was suggested to me not to do it in real space, but to compute ...
4
votes
3answers
2k views

Correct method for processing FFT in realtime

I am attempting to build a 'live' FFT for continuous analysis of an audio signal. I am relatively new to this, but have attempted the full thing, and then updating the graph at 20Hz refresh rate. My ...
0
votes
2answers
52 views

Why the angle of a real signal in frequency is continuous?

This original signal is shown below (time domain): It is a real signal. According to the signal processing theory, the form part of the angle is conjugate to that of the latter part. That is, $\...
3
votes
2answers
156 views

Auto-correlation function, an inverse problem

$x[n]$ is a complex function $n=0,1,2,\cdots,L-1 $ we assume $x[n]$ is periodic in its index: $x[n+L]=x[n]$ Its auto-correlation function $C[n]$ is uniquely defined as: $$ C[n]=\sum_{i=0}^{L-1} x[i+...
0
votes
0answers
81 views

non-negative FFT spectrum, subtlety

definations $x(t)$ is a function of time. Physically, it can be voltage, displacement, magnetization and so on. It can be real, complex, vectors or more fancy numbers. $C(t)$ is the auto correlation ...
0
votes
2answers
70 views

How to detect a long signal inside an even longer one?

I'm trying to detect a long pattern inside a long recorded audio. In order to do so I am implementing a matched filter in time by performing a FIR filtering using the coefficients of my pattern ...
0
votes
1answer
421 views

extracting ultra accurate phase and frequency with FFT with real world data

I was doing some experiments in Matlab, and write this code to experiment with it. I want to know if it's possible to extract accurate within -+0.01° phase info from samples of an ADC, the matlab FFT ...
1
vote
0answers
53 views

which is the relationship between domains (x and k) in FFT?

I need to plot an escalar field f(x,y) in an interval xv= (xmin,xmax) in Nx steps and ...
0
votes
1answer
119 views

Precompute parts of an FFT on streamed data inputs

I've implemented Radix-2 & Radix-4, DIT & DIF variants of the cooley-tukey FFT algorithm and they perform well on test data. Now for a project, my algorithms will be applied to a real-time ...
0
votes
0answers
92 views

How does this Odd stacking DFT work?

I have just read on page 3 of this paper: https://www.onsemi.com/site/pdf/Filtering_Hearing_Ai.pdf That an "odd stacking" DFT can be created, essentially sacrificing the DC and Nyquist bands to ...
0
votes
1answer
125 views

Remove Frequency Range

I got a signal and I would like to remove a specific frequency range. I am a frequency noob, so I will explain it by images: I have a frequency filter (butter bandpass): I have a signal and it's ...
0
votes
2answers
121 views

How to implement discretized CWT with FFT?

I tried to make circular convolution (correlation actually because it is easier to understand for me) look like the Continuous Wavelet Transform (CWT) but got stuck because the conjugation was on the ...
0
votes
1answer
121 views

Measuring Amplitude of sound wave FFT

Please, pardon my ignorance. This may be a simple question, but I am still unable to explain what is happening. I am using a turtlebot to listen to sound emitted from a stationary sound source. My ...
1
vote
4answers
938 views

DFT (FFT) of a Real Even Function Doesn't Yield Real Only DFT Signal

There are a lot of similar questions already, but I can't find one answering this problem. I used the following MATLAB code to perform the FFT on a Lorentzian input signal ...
0
votes
1answer
67 views

Improvement of SNR by sampling different number of cycles using FFT of a fixed length singal

My question relates to Fourier transform of signals with different lengths in time but the same fixed length of the digitized signal with different sampling rates. Specifically, how does the number ...
2
votes
3answers
248 views

Decreasing Sample Rate in DFT (FFT) for Audio Analysis

I'm working with audio data analysis through FFT algorithm. My example audio is a sine wave at 440 Hz and 44100 Hz sampling rate. FFT methods in programming, like ...
3
votes
3answers
16k views

Apply Low pass Butterworth filter in Python

I want to use a low pass Butterworth filter on my data but on applying the filter I don't get the intended signal. Here is the dummy code: Signal A: ...
0
votes
1answer
180 views

Measuring acoustic phase difference between two receivers

I am trying to measure the acoustic phase difference between two sensors 30cm apart each other. The sampling frequency is 20kHz, and my interest frequency is about 1700 Hz. The below is how I ...
2
votes
1answer
663 views

Scale FFT frequency range for a bars graph

I'm trying to show frequency spectrum for audio data as a bars graph using FFT. If I have data for a sine wave at 440 Hz and with sampling rate 44100 Hz and analyze its first 1024 samples I get an ...
-1
votes
1answer
104 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?
0
votes
1answer
295 views

How to shift FFT spectrum by half a bin?

I am trying to find a way to shift the outputs of the FFT (any algorithm implementation, currently using radix-2) by half a bin with the same bandwidth. To put it another way, say you have a 512pt ...
5
votes
1answer
200 views

Morphing between two FFTs

I'm a musician and I'm making a script that takes a wave file and snaps each of its frequencies from the fourier transforms to the nearest musical harmonic. Thanks to help from another question I ...
1
vote
1answer
275 views

FFT - preferred way of tone detection

I am using FFT to detect certain frequencies (DTMF). I can clearly see the frequencies in the spectrum. Is there a preferred / optimal way to decide if a certain frequency is present or not by looking ...
1
vote
1answer
183 views

Band energy and Parseval theorem

In Python, I consider a dummy signal as follows: fs = 512 sig = np.random.standard_normal((fs,)) sig corresponds to 1s of ...
0
votes
1answer
209 views

Wavelet transform and FFT using to extract feature power bands with EEG signals

I am using 5 channels [ fz , cz , c3 , c4 , pz] to detect drowsiness of driver My First Question is, what is the right input to get feature power band ( Theta , alpha , gamma , beta ) to wavelet ...
0
votes
1answer
272 views

Windowing a signal for FFT

I am actually working on a project involving the cross-correlation of two signals. My cross-correlation code is working fine, but I suspect that I should apply a window to my data set in order to ...
0
votes
1answer
45 views

How to Calculate the number of DFTs taken in a STFT

Given variables of window size (W), overlap (O), and length of the Signal (S) (in my case, length of a song) and I am trying to determine an equation that gives me the number of DFTs that would be ...
3
votes
1answer
181 views

Phase Modulation and Intermodulation Anomaly ? (Spectrum plots inside - **Updated**)

I have a MATLAB script for modelling a two tone signal (a sum of two sinusoids) going through a non linear transfer function (such as an amplifier). The amplitude of a signal is amplified non linearly,...
0
votes
1answer
65 views

psd of an ac voltage signal

In an lab class experiment who's subject of interest was Johnson's noise I measured the ac voltage of a battery. Now I wish to find the psd of the measurement and show that it is constant. 1.Is such ...
0
votes
1answer
187 views

Arbitrary Phase FIR design using FFT

I want to design a digital FIR filter to cancel the phase of an analog Butterworth filter (i.e. the phase is smooth) i.e. the filter has unity gain with just an inverted phase component. The ...
1
vote
2answers
167 views

Adding phase to a signal that has multiple frequencies

An amplifier changes the phase at different amplitudes of asignal. I am using amplifier data in a MATLAB model. I have constructed a signal that is a sum of sinusoids of different frequencies with ...
0
votes
2answers
396 views

pdf of the sum of gaussian distributions using fft Ask [closed]

I am trying to derive the pdf of the sum of independent random variables. At first i would like to do this for a simple case: sum of gaussian random variables. I was surprised to see that i don't get ...
4
votes
1answer
207 views

Zero padding effect on a FFT of gaussian noise

I have a gaussian noise $\nu(t)$ with variance $\sigma^2$. After a FFT I get $X(\omega)$. If now I do the IFFT on the $X(\omega)$ can I say that the result is still a gaussian noise of variance $\...
1
vote
2answers
80 views

Is there such thing as harmony independent timbre characteristic of a sound?

I was wondering if any of you know about any technique to extract a harmony (e.g. harmony in a shape of a chromagram) independent timbre characteristic of a sound frame. What I'm after is a version ...
0
votes
1answer
197 views

Signal squared in time domain

In some publication I have found a formula that for some reason squares signal in time domain. I am analyzing it now. What should be the Fourier transform of a signal that was squared in time domain? ...
0
votes
1answer
88 views

Inverse FFT - synch the phase

Is there any way to synchronise phase of output of inverse DFT in each buffer? When I send the output of inverse DFT to the speaker it sounds nasty. Of course I know the „windowing functions” but it ...
0
votes
1answer
137 views

Sum of Diracs in Frequency Domain

I am attempting to compute the sum of dirac functions in the frequency domain via the following formulas: $$ ReLu(f(x)) = f(x) * \sum_{i} \delta(x-x_i) $$ $$ x_i : f(x_i) > 0 $$ $$ F(ReLu(f(x)))(k)...
1
vote
1answer
118 views

temporal high-pass filtering to approximate phase derivative

I am currently following the algorithm as based on a previously published research article "A Motion Detection Algorithm Using Local Phase Information" by Lazar et al. The basic overview of the ...
0
votes
1answer
87 views

Conjugate symmetry Problem

I'm seeing something very curious in the Matlab code below. X = fft(x); Xt = X; Xt( (end/2+1):end ) = 0; xr = 2 * real( ifft( Xt ) ); Xr = fft(xr); where ...
0
votes
2answers
814 views

How to “rotate the phase”?

I have problem, not sure how to call it but I think it's something called "phase rotating", but not sure. I make fast Fourier transform on simply square wave, and then I make inverse fast Fourier ...
1
vote
1answer
62 views

changing circular shift in hologram reconstruction

I'm trying to write a very simple code for the reconstruction of an in-line hologram of particles in a micro channel (digital holographic microscopy), but I'm getting some results that are hard for me ...
-1
votes
2answers
213 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 ...
0
votes
2answers
54 views

What should I do with FFT result array from microphone input? [duplicate]

I currently want to create a small pitch detector and I have to say that I am very new in this topic of signal processing. At the moment I implemented the FFt algorithm and I got an array of window ...
0
votes
1answer
168 views

Summing magnitudes of individual frequency bins

I am working on signal analyser and have problem with understanding relationship between display resolution and frequency resolution (range). My frequency range is $ 1-20000\quad [ H z ] $ so to ...
1
vote
1answer
250 views

Apply windowing before FFT or after?

I am computing a spectrogram and I've found a code example online which goes like this: ...
0
votes
0answers
85 views

Find cepstrum of a function with dirac function

$$x[n] = d[n] + a\cdot d[n − N_p]\ ,\ \ |a| < 1$$ I need the complex and real cepstrum of this function as well as what would happen for $N=N_p/6$ for the DFT approximation $$x_p[n] = \frac{1}{N} ...