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
78 views

Matlab FFT not producing symmetric spectrum

I am plotting a FFT of a sampled RC pulse but my resulting spectrum isn't symmetric - it's offset. ...
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1answer
118 views

PSD subtraction

I am interested seeing the difference between two power spectral densities (PSD) as a noise reduction exercise The blue line is the psd of my signal, and the the orange line is the psd of the ...
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31 views

How to relate power spectral density with the number of resource blocks in LTE?

I am running a simulation with 100 Resource blocks for LTE OFDM and I plotted a PSD for it. Now I am able to bring down the out of band emissions when I perform an algorithm on it. Based on the ...
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1answer
81 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?
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1answer
61 views

Phase differences between DFT and FFT (in MatLab)

I have implemented a (direct) DFT in MatLab (following script) and compared it to the built-in FFT routine. The magnitude response seems to be identical (excluding some possibly round-off errors), but ...
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33 views

Strange phase for gaussian beam 2D

I have a 2D Gaussian beam : After doing FFT2 and angle I get strange results: This is the code I'm using (Python 3) ...
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1answer
87 views

DTFT frequency range

$$X(e^{j\omega}) = \sum_{n=-\infty}^\infty x[n] e^{-j\omega n} $$ The frequency term $\omega$ in DTFT is normalized as $\omega = \frac{\Omega}{f_\mathrm{s}}$ $\Omega= 2 \pi f$ is the angular ...
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131 views

Restoration problem with numpy fft function and self-made high-pass filter

Hi, I am using numpy.fft functions to do CV assignments in univ. But while implementing the high-pass and low-pass filter, I found something wrong during the process. The difference should converge ...
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1answer
148 views

Purpose of multiplying fft output by Hann window?

I'm working with some acoustic propagation models at the moment. A feature I've noticed in 2 of these models is that the output of the model is often multiplied by a Hann window in the frequency ...
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2answers
182 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 ...
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1answer
159 views

FFT Heart Rate Monitor on Arduino

Part of my project is to build heart rate monitor from Nano and a PulseSensor. I am having trouble working with the Fast Fourier Transform part of the code. This code here is from a library (...
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1answer
40 views

Electret microphone RC filter design contradiction among information sources

I'm building a circuit for an electret microphone and I want to build a bandpass filter around the op-amp. I was using this source until I have found that most sources ( e.g. here and here) indicate ...
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1answer
236 views

Why Does 2D FFT of Gaussian Looks More Sharper than Gaussian Itself?

I am trying to understand why 2D FFT is done on a Gaussian process in a particular code. From my understanding from these posts: https://www.researchgate.net/post/...
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1answer
106 views

Amplitude and Phase of FFT gives messy results - Matlab [closed]

I'm trying to calculate the spectrum magnitude and phase of a signal, but when I plot them I get pretty messy results: The signal is displayed on the right, then I have the magnitude and phase of ...
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20 views

Wavelet Transform Implementation Method

I have been watching the wonderful youtube videos here: https://www.youtube.com/watch?v=4TTpwIZrUAo&list=PLn0OLiymPak2G__qvavn3T8k7R8ssKxVr&index=3 detailing morlet wavelets and fit ...
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0answers
59 views

Zero-padding vs. nonzero-padding in computation of auto-correlation with FFT

Isn't the usual zero-padding in the computation of the auto-correlation function with FFT just one of many possible extrapolations of the original signal? If I have a measured signal which has good ...
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2answers
108 views

What should my reference value be when converting FFT bin amplitudes to dB?

I want to transform my FFT output values into a dB scale, but I'm struggling to determine the function I should run each bin amplitude through. My understanding of the decibel scale is that a value ...
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2answers
91 views

Multiple different length Windows for high resolution realtime FFT

I am looking at trying to achieve a 'realtime' (as quick as possible to acquire, process and present data) FFT application to analyse audio. I have setup my app to acquire a set of samples, apply a ...
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2answers
140 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 ...
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1answer
63 views

Multidimensional FFT with mixed directions

Let's say I have three-dimensional complex array $X[m,n,p]$, $m \in \{0, 1, \ldots, M-1\}$, $n \in \{0, 1, \ldots, N-1\}$, $p \in \{0, 1, \ldots, P-1\}$; i.e. $X \in \mathbb C^{M \times N \times P}$. ...
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294 views

How to Extract frequency bands from Short-time Fourier Transform?

I am reproducing the results reported by the author in a research paper. In which he takes the STFT of EEG signals. The input data has a shape of (1 X500) and I used Scipy library built-in function to ...
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40 views

Zero padding frequency domain data for improving time domain resolution before inverse FFT

I am working in MATLAB with measured data from a VNA. The bandwidth is 1 GHz from 5-6 GHz with 801 samples in the frequency domain. Everywhere I have read that I need to split the data into 2 parts at ...
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43 views

Image from Inverse FFT operation is sparse

I'm trying to train a deep nerual network that takes in input, an signal in the frequency domain, and attempts to learn a mapping to another signal in the frequency domain. Basically, the input to ...
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2answers
55 views

How to select the sampling frequency when the input signal frequency is not known

I am trying to observe the noise produced by UPS present in our lab, under no load (a normal running condition when its power is on). As I don't know its frequency range, I have randomly chosen 4 ...
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1answer
118 views

FFT: do I need to include gaps in data in my FFT domain?

I am currently doing an FFT on a very large amount of vibration data, which I am going to sample with no pattern which I want to PSD eventually My question is in essence can I go from my data ...
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1answer
52 views

Frequency analysis : relation between FFT size and sampling

I am currently trying to perform an experiment given an audio signal. I am therefore sampling the audio frequency range (0 to 22kHz) with a 48k sampling rate. When performing a FFT on my signal, I ...
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1answer
72 views

What's the formula for choosing FFT sample rate and window length to make sure the frequency I want is in the center of a bin?

I don't know why I am having such a hard time with this math, but I am. I am doing phase-sensitive detection of a few sensors, which are triggered by a modulated light source. The modulation frequency ...
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1answer
134 views

How to set STFT-parameters to visualize woodpecker / Sawtooth Signal? ​

I always wanted to work with FFTs and Spectrograms to characterise sounds and their frequencies using python. Lately I recorded a woodpecker at his work. Woodpecker Wav File To know where to look ...
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2answers
252 views

Understanding FFT size in OFDM modulation

I know that the FFT size is the the number of subcarriers OFDM system have. But I also know that the FFT has to be $ 2^N $ big (in order to process N samples from time domain to frequency domain). ...
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1answer
57 views

How to classify the movement type of a target into two categories(direct motion or else) by using a doppler radar?

How to classify the movement type of a target just into two categories(one is direct motion which is almost straight line movement and the other is anything else) by using a doppler radar with a ...
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0answers
56 views

N-dimensional Cooley-Tukey FFT

I am trying to formulate an n-dimension algorithm for the Cooley-Tukey FFT. In two dimensions, it is very clear: $$ Y[k_1 + k_2 n_1] = \sum_{l_2 = 0}^{n_2} \Biggl[ \Biggl( \sum_{l_1 = 0}^{n_1 - 1} X ...
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1answer
121 views

Incorrect dominant frequency from FFT

DATA source: Bearing vibration data FEMTO IEEE PHM 2012 challenge. DATA context: Sample rate: 25.6 kHz Sample period: 0.1 Seconds (2560 points) Motor runs at 1800 rpm Steps taken: Low pass butter ...
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159 views

Spatial smoothing - 2D MUSIC [FMCW MIMO radar processing]

I'm using a 2D Music algorithm for the estimation of Range-Azimuth info in a ULA FMCW MIMO radar. The algorithm procedure is pretty simple as per Belfiori F., Application of 2D MUSIC Algorithm to ...
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3answers
72 views

In case of Complex DFT spectrum, why the x axis range from N/2 to N point mean a negative frequency?

When we transform a complex signal into frequency signal by using a complex DFT, The range from N/2 to N point on the X axis of the spectrum mean a negative frequency... But i cant understand why ...
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59 views

OFDM modulator: Transmission time calculation

I have a problem with a certain task. The frequency of the signal sampling at the OFDM modulator output is 20 MHz. The length of the protection period is 2 microseconds and it is 20% of the ...
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2answers
99 views

Checking Parseval's Theorem for Gaussian Signal by Using Scipy

I'm trying to check Parseval's theorm for Gaussian signal. It's well known that fourier transform of $\exp(-t^2)$ is $\sqrt{\pi}\exp(-\pi^2 k^2)$. So I implement it by using quad and simps. I think ...
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1answer
505 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: ...
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48 views

Method for the Localisation of Impulsive Noise in Speech Signal

I'm sorry if this question has already been answered, but I couldn't find anything. I'm working on a program for noise detection and need to locate the position of impulsive noise. The following image ...
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1answer
102 views

Equivalence of Maximum Likelihood (ML) and Discrete Fourier Transfrom (DFT) Peak Finding for Single Tone Estimation

My understanding is Maximum Likelihood and DFT Peak Finding for a single tone produce the same results assuming the ML is restricted to the same frequencies as the DFT. I was wondering if there was ...
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172 views

retrieving complete 2d autocorrelation function from 2d power spectral density function (numpy.fft.ifft2)

My question is at the boundary between signal processing and python. I have a 2d power spectral density function (PSDF) constructed on Nx/2 + 1 and ...
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1answer
29 views

Clarification on the application of window pre-FFT

Good morning, I have a simple question about the application of a Hanning window before performing an FFT. My application is pretty textbook, i'm decoding a 4-FSK signal. Question: why does the ...
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1answer
37 views

DFT of a series of RC exponentials

Context: I'm trying use matlab to apply a single-pole filter to a time-domain ramp waveform that is generated by a sequence of time-shifted "RC steps" that are added together. The time domain voltage ...
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1answer
96 views

Why is the size of results from FFT half the size of the input, while that is not the case in image processing?

When dealing with real data, Fourier Transform produces symmetric results and FFT algorithms discards half of it. An input with 1024 points will yield only 513 points in the output. However, in image ...
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34 views

PSD Periodogram smaller segments compared to larger segment

I am calculating the power spectrum of a heart rate variability via FFT and a Welch window. I have a sample of 500 points as the input signal. The signal was broken up into two 128 point segments. ...
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1answer
59 views

real time signal analysis of output from an audio device

I'm writing a test program that can detect pops/cracks/noise (unknown frequencies and duration) in the audio output of an audio device. The purpose of this program is to reduce human error and make ...
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1answer
148 views

Why is this recursive DFT algorithm not equivalent to this iterative method?

Edit 1/30 - Taking @Fat32's edits into account, it seems like there is still an issue with the scale of the frequency axis. While version 2 correctly identifies the response at 1 HZ, version 1 seems ...
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42 views

Convolution in the Hartley Domain

The convolution Theorem of the Hartley Transform is given by $$ Y_{Ha}(k) = X_{Ha}(k)H_{Ha}^{e}(k) + X_{Ha}(-k)H_{Ha}^{o}(k) $$ where, $$ H_{Ha}^{e}(k) = \frac{H_{Ha}(k) + H_{Ha}(-k)}{2} $$ $$ H_{...
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103 views

Accelerometer signal processing

I'm new signal processing (this is my first pratical application), so I ask sorry if my question is simple. My challenge is to see the displacement of accelerometer when I have vibration too. I do ...
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1answer
98 views

Does dividing the magnitude spectrum of white noise by sqrt(2) give an RMS magnitude spectrum?

I understand that the RMS Amplitude of a sinusoidal signal is around 0.707 ($\frac1{\sqrt2}$) times the Peak Value, but this is not true for noise. However, an FFT of a noise signal indicates ...
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1answer
227 views

Detrend data with no clear secular trend prior to Fourier analysis?

I am completing Fourier analysis on many different time series of sediment particle flux exiting an experimental flume. Data is collected at a resolution of 1 Hz for durations ranging from ~5,000 to ...