# 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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### How to Remove the Periodic Oscillations from a Signal

The task that I have is to remove the annual and semiannual oscillation from a set of temperature measurements, taken over several years, by means of least squares method. I found the method ...
1answer
220 views

### Poisson Noise Removal from an Image

What type of filters can be used to remove Poisson noise in an image? I tried applying Gaussian filter to few points in the spectrum.
0answers
56 views

### How to find frequency and corresponding time for an audio signal?

I want to read in a sound file and get the frequency at each time interval of the signal. I thought that the best way to approach this is by applying FFT to small chunks of the signal. Because FFT ...
2answers
80 views

### Overlapping in real time fourier transform?

I have an algorithm and I need to record audio and perform short time Fourier transform to obtain which frequency is the most common. I am using a Hanning window to try and reduce spectral leakage as ...
3answers
378 views

### Upper Bound for the DFT (FFT) Coefficients of a Bounded Signal

I have a discrete signal $x[n]$ of length say 1024, such that : $$-1 \leq x[n] \leq 1, \qquad for \ all \ n,$$ and let $y[k]$ be its DFT. Is there an upper bound for $|y[k]|$ ? (i.e. for the ...
2answers
3k views

### How to Actually Use FFT to Implement Low Pass Filter

have searched high and low for a practical guide to implement a simple 50Hz low pass filter on a signal.I understand the theory but its the practical but that's missing. If I was using MATLAB it would ...
1answer
836 views

### Remove Checkered Noise (Pattern) from an Image

I'm trying to remove what looks like a checkered noise pattern that has been added to an image. Comparing the 2D FFTs The one on the left is the original, while the middle one is the corrupted one. ...
1answer
70 views

### Mathematical description of the ACF using fft2

I computed the acf of an image with the following code: ...
1answer
400 views

### 2D Convolution in MATLAB Causes Artifacts (Boundary Issues)

I`m trying to do a 2D fast convolution in Matlab of large matrices. If I use FFT version based on convolution theorem ( https://en.wikipedia.org/wiki/Convolution_theorem ), there are some artefacts in ...
2answers
127 views

### Is there a rule of thumb for zero padding in image processing?

I see there are a lot of answers on why zero padding is necessary and how it avoids wrapping around the sides of images. However is there a rule of thumb on how much padding will be good for the image ...
0answers
35 views

### Window function correction when using frequency domain likelihood

I was wondering if someone could tell me or point to resources which discuss what kind of of windowing corrections factors one should use when using a frequency domain Whittle likelihood for spectral ...
0answers
41 views

### FFT command line application?

Are there any command line programs for Windows, preferably free and stand-alone, which can report the peak/strongest frequency within a given range of frequencies? I need something like this to ...
1answer
47 views

### Fourier transforms and time shift

There is probably something trivial behind this, but I am missing something. I need to create a stationary random time series data v(t) which is the the sum of another time series u(t) and u(t) with a ...
1answer
163 views

### Threshold for peak detection of noisy signal in frequency domain

I have a noisy signal (Gaussian Noise) with a known SNR and known Noise variance. This signal contains a number of Frequency components. My ultimate objective is to detect the components of the signal....
0answers
19 views

### Why do I need this modulo tweak to bandpass filter an I/Q signal

I have an I/Q signal with $f_c=2.06\text{MHz}$ and $f_s=50\text{KHz}$. I am looking the first 256 samples of the signal, which is a NumPY array of complex numbers. If I do a spectrogram of this ...
0answers
20 views

### Detecting changes in signal due to server delay

I'm currently working on an application that can be used to determine when a signal changes due to a server delay. Essentially, I have an API that is used to output data to a UI. However, for reasons ...
1answer
56 views

### Silence removal from the audio

Suppose i've a mp3 file. I performed FFT of that mp3 file. Then is it good to state the following: 1. Silence occurs by that particular DFT point whose magnitude is lesser than some threshold. 2. ...
1answer
71 views

### number of samples and Amplitude accuracy,what's the solution to this problem

I'm colleceting data samples from a device (temporal signal) in order to analyze it with FFT function in MATLAB R2015b,first i get N=4063 data points with (delta_t=0.000015s) so my frequency ...
2answers
107 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 ...
1answer
154 views

### Comparison of results own implementation and python signal.welch

I am trying to replicate the output of Python's signal.welch function to make an estimate of the PSD from an FFT calculation. I don't want to use the built-in function to understand better what is ...
1answer
870 views

### Autocorrelation: numpy versus FFT

I have a series a of values (0 and 1) coming from a Brownian process with drift for which I am studying the autocorrelation. I used two methods: 1) numpy autocorrelation: ...
1answer
110 views

### Window function for signal with 3-5 sine waves inside

Should I apply some window before FFT if I want to find 5 frequencies in signal? Signal is pure 5 sinusoids + some noise generated during recording on analog cable.
1answer
19k views

### What is the phase and magnitude response of white noise?

I would like to create white noise in the frequency domain, and then transform it to the time domain using python. To understand the problem, I simply generated white noise in the time domain, and ...
3answers
70 views

### fft and inverse fft error size and applying impulse response matlab

I have a signal called noise. I am using the following command to calculate the mean squared error after fft and inverse fft process: ...
1answer
64 views

### Method for determining probe angle by analyzing skewed sine wave

I've got a fun problem and would be curious to get feedback on how some of you would go about solving this. Imagine I have a probe and am scanning the surface of some material. This material surface ...
1answer
456 views

### GCC-PHAT (Generalized cross correlation) always peak at delay=0 on real audio signal

I have studied the GCC-PHAT algorithm to estimate TDOA on audio signals at 2 mics. Here is my MATLAB implementation: ...
1answer
37 views

### Interpretation of FFT result when frequency is an exact multiple of frequency bin

My FFT spectrum analyzer works fine in general except when the frequency is an exact multiple of the frequency bin. In the pic at the left the frequency is 5999Hz and at the right 6000Hz. The lines ...
0answers
42 views

### How do I narrow the frequency from a time domain Gaussian signal?

I'm trying to get a frequency domain Gaussian signal that starts from 180.045 THz and ends in 180.65 THz. This is the matlab code I used to get the FFT of time domain Gaussian signal: ...
1answer
52 views

### FFT of a signal with 0 added between samples

I'm having a rough time doing the following question from a HW. Given a sequence of your choice, what happens to the DFT if we add a 0 between every sample? I.e. if we have x[n] = [A,B,C], we turn it ...
0answers
18 views

### What's the significance of lomb-scargle power?

What is the significance of Lomb-scargle power (y-axis)? I have two data sets. For each plot, above plot is Lomb-scargle periodogram of the lower plot (original data). The first data set has an ...
1answer
29 views

### Nyquist Frequency on semi-unevenly sampled data

I have a data set that has 'kind of' constant sampling rate - it switches between 1 min and 2 min. About 70% of the times, samples are taken every 1 minute, and about 30%, samples are taken every 2 ...
1answer
276 views

### Is Wavelet Transform better than FT if I'm only interested in knowing the frequencies and not interested of when did they happen?

I'm still in the first stages of learning about these different transformations. But one thing that came to my mind while learning about them is the question above. I learned that ...
2answers
160 views

### How to separate mixed signals, 3 identical gears meshing together

I have 3 identical gears that are meshing together, on each shaft that is holding these gears I have an accelerometer recording vibration data. Once I move to the frequency domain via MATLAB's ...
1answer
38 views

### Equalisation of FFT spectra

The image shows the frequency spectra of a signal recorded using a SDR. The SDR is of DirectDownSampling type and has some compensation in the signal chain to compensate for the DC spike which would ...
0answers
96 views

### How to normalize PSD to get the same magnitude as FFT peak

I am trying to use FFT and power spectra density estimation with python (np.fft.fftand scipy.signal.periodogram). And trying to ...
0answers
14 views

### Symmetry in Lomb-Scargle transformation

I'm observing weird symmetry and repeating pattern on my unevenly sampled time series data after Lomb-Scargle transformation. I used astropy lomb-scargle. ...
3answers
310 views

### Does it make sense to use a wavelet that is equal to a sine of one period?

Would I get same results with STFT method (short-time Fourier transform) if I try this?
1answer
39 views

### What happens if I zero out the initial terms of the fft output and take inverse fft? [duplicate]

I am new to signal processing domain. When I run fft on data of length 1000, I get 1000 complex numbers. Now, if I want to extract the low frequency information or signal approximation, I take the ...
1answer
190 views

### Overlap/add time-domain audio frames: How does normalization/scaling work with overlap greater than 50%?

I am attempting to overlap-add time-domain audio frames in a real-time context (with a continuous stream of data coming-in). Each frames consist of 1024 samples and have been previously "Hann" ...
2answers
94 views

### Interpolation and harmonics

A real valued causal sequence $x1[n]$ exists with length of the sequence being $N$. Valid indices of x conform to $0 \le n \le N-1$ The DFT of x[n] is:  X1[k] = \sum_{n=0}^{N-1} x1[n].e^{-j.2.\...
2answers
143 views

### Zero padding and 2D Fourier transforms: how does zero-padding affect phase?

It's pretty clear that zero-padding an image before performing Fourier transform simply enlarges the magnitude image (stretching it to the new, padded size). What I can't understand is how it affects ...
3answers
3k views

### Relationship between FFT amplitude and sample size

Would anyone know if there is any relationship between the FFT amplitude vs. data length? For instance, in Matlab, if signal_30k had 30,000 data points, sampled at ...
0answers
34 views

### Optimizing Spectrogram Parameters

I'm looking for references that explain the trade-offs between more averages with shorter fft sizes or longer fft sizes with fewer averages when generating a spectrogram to perform energy detection of ...
1answer
27 views

### SDR : Signal divided between the edges of the receiving window

I'm using a SDR (specifically the RTL-SDR) to analyse the spectrum and i noticed a phenomena that does not seem logical. The signal I want to capture is centred on 868.3 MHz and the central ...
0answers
64 views

### Additive Synthesis - clean playback of a spectrum frame

First of all - I have only a limited of Fourier transforms and the mathematics involved, so please pardon any potentially bad terminology or huge gaps in conceptual knowledge. I'm experimenting with ...
1answer
95 views

### Why resampling change this FFT output?

I have been studying the following code but still unable to explain its output. I really hope someone could shed some light. Let me begin with the baseline. Suppose I have a 20 seconds sinusoid with ...
1answer
61 views

### What is the proper way to filter out unwanted noise from accelerometers?

I'm a beginner in signal processing and have a hard time to filter out environmental noise from the floor vibration data collected using accelerometers. I have tried using butterworth filter (IIR) ...
0answers
30 views

### Correctly interpret Butterworth filter frequencies

I am using the butter function from the signal package in R to filter my data. The data is Fourier-transformed (complex numbers) - and I want to filter out specific frequencies (I have a 256Hz ...
1answer
58 views

### Why am I not getting the intended coefficients in this 2D Fourier demonstration?

I am trying to demonstrate how the 2D Fourier decomposition of an image works with Matlab and a very, very simple example. I create a 4 x 4 pixel image with cosine basis vectors as such: ...
0answers
81 views

### Plot Frequency Spectrum of Binary Sequence in Matlab

I am new to Matlab and I am trying to implement a section of a published paper, the basic idea of the part that i am implementing is to show the frequency spectrum of camera aperture. The shutter ...