# 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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### FFT algorithm gives wrong signs on my output numbers

I was told from another stack exchange (stackoverflow and computer science stack exchange) to post here - so hopefully now i can get some help from people who understand the algorithm, but if this is ...
7 views

### In MRI, why use fftshift after ifft2(fftshift(k_space_data))?

In DSP courses, it is always emphasized that both fftshift and ifftshift can only operate data in frequency-domain. But in MRI reconstruction, fftshift is applied to a spatial-domain data. If do not ...
26 views

### Faded line in spectrogram for a chirp

I've generated this spectrogram using wavelet convolution. The chirp is generated like so: ...
54 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: ...
36 views

### Converting freqz code from Octave to R? [closed]

I'm converting some code from Octave to R. However the code below is giving a problem. The Octave code is: ...
26 views

### Can Interharmonics Be FFT Artifacts?

I know non-sinusoidal waveforms can be decomposed into a sum of higher integer harmonics, but are there any mechanisms that cause interharmonics? I collected some noisy data from a source that was ...
52 views

### Phase Spectrum in Fourier Transform

Probably a simple question to answer. I have been trying to use the fft function in MATLAB and have been succesful to get the amplitude spectrum of a known signal. But when working out the phase ...
29 views

### Asymmetry In The Mirroring Of An FFT Of A Sinusoidal Signal

I am trying to implement a Cooley-Tukey FFT algorithm in its iterative version. Specifically, I am implementing this Wikipedia pseudocode (please look at the section Data reordering, bit reversal, and ...
28 views

### How to downsample a fourier transformed signal?

I have a signal of length 100000 timestamps sampled at a frequency of 25kHz. First I apply a high pass filtering at (300Hz) and then do the Fast Fourier Transformation. Then the absolute values are ...
39 views

### What is the purpose of dividing an audio signal into segments and analysis each segment?

I read bunch of materials for extracting feature from audio signal and they all tell me to break signal into segments, why don't we analyze all the audio signal? I don't know what are the advantages ...
39 views

### Output shape of numpy's rfft function

For numpy's rfft function, the length of the output is about half of N, the length of the input sequence. What is the reason behind this? What are the frequency bins?
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### Impulse response from frequency response in Matlab

I am trying to design equalization filter and therefore I want to define my own amplitude and phase response and then to obtain the impulse response of the filter. I thought that the output of the ...
281 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 ...
651 views

### How to calculate LF/HF power ratio from heart rate RR-intervals

I've been searching everywhere, but I'm having trouble finding a step-by-step guide for someone with little knowledge of signal processing. I have a file with a list of RR-intervals in milliseconds. ...
31 views

### Convolution for audio, how to control amplitude? [duplicate]

I am implementation a convolution reverb method for audios array in Python using scipy.signal.convolve. I choose the fft mode, which essentially a product of the ...
94 views

### Using FFT for implementing a filter bank

I am trying to use Fast Fourier Transform (FFT) for decomposing an audio signal into 8 sub-bands according to this link but the problem is the frequency response of the result contains only the first ...
32 views

### Why is it that the input to an IFFT is first reordered in an OFDM Modulator

I am back with my questions! I recently came across a paper titled "Low Latency IFFT Design for OFDM Systems Supporting Full-Duplex FDD" where they claim a new method of IFFT input mapping ...
32 views

### Receiver function, frecuency domain deconvolution not giving logic results

I'm working on some code for receiver function method in seismology. For anyone one not into the topic, it's just a deconvolution of two time series (seismograms). This can be done in the time domain ...
30 views

### Does this FFT algorithm give correct IFFT when the input does not contain symmetry?

This FFT function is from Numerical Recipies in C and I am using it in my java projects. The source code is given below: ...
29 views

### How to classify audio files based on their ZCR, energy, and enthropy of energy features?

Let's say I wanna classify the audio files of race car exhaust and normal car exhaust. I attain few features such as ZCR, energy and enthropy of energy from the training set and I wanna build the ...
41 views

### Mulitplication in Frequency Domain as a Similarity Measure of Audio Clips

I have an audio clip and I copy a 256-point part satarting at 10th second. Then I create a 256-point frame on the original clip. At every itreation, I calculate the DFT of the frame, multiply it with ...
790 views

### What is the difference in between the different STFT implementations? (Overlap-shift with zero-padding, overlap-add, overlap-save, etc)

I'm trying to understand STFT overlapping, why segments are concatenated and what are the consequences of this. My implementation (from other questions and repositories found) of the STFT in Python ...
20 views

### Direct path and reflections of ultrasound transmission

I am using the following system: Ultrasonic chirp signals with central frequency 100kHz are being transmitted using ultrasonic sensor. After the transmission through the medium(air/solid/liquid/...
32 views

### Does FFT on two samples from the same audio but with a very small sampling offset return the same results?

Using a single audio source, suppose I take two samples of the same length (say, 10s), with the same sample rate (say, 44100Hz), one just 1/3 sample length (7 microseconds) away from the other. Does ...
613 views

### Understanding the FFT phase spectrum with a simple example

I'm trying to compute the DFT using scipy's functions. I don't understand why the phase spectrum of a simple sine wave with 2 Hz frequency doesn't show $\pm\pi/2$ at the $\pm 2Hz$ frequencies. Instead,...
74 views

### What is the difference between the V-OFDM and OSDM

I am trying to understand the difference(s) between V-OFDM (Vector Orthogonal Frequency Division Multiplexing) and OSDM (Orthogonal Signal Division Multiplexing) which are used as a multicarrier ...
55 views

### Difficulty regarding grasping concept of fft butterfly?

The term "butterfly " is often used in association with fft texts But what exactly is a butterfly ?a matrix/group of i/o's is known as butterfly? Such as 2 input 2 output butterflies in ...
10k views

### Tips for improving pitch detection

I'm working on a simple web app that allows the user to tune his/her guitar. I'm a real beginner in signal processing, so please don't judge me too harshly if my question is inappropriate. So, I ...
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### calculate fft convolution for an LTI system given its input (in time domain) and its poles and zeros

I have considered taking the fft (MatLab) of the input (x) and create frequency bins for the first half (problems with an odd number of samples, tried solution zero paddings) and evaluate the transfer ...
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### Gnuradio scaling on Ubuntu issue

I have problem with scaling on gnuradio. When i use more than one WX widget - which shows parameters of signal - i can't see last widget. How can I deal with this problem - just to scale scopes, and ...
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### PSD in MATLAB, problem with the coefficient $\frac{1}{2\pi}$

Using the FFT to mimic the Fourier Transform I have this question. The definition that I use for the PSD $S(f)$ is: S(f)=\frac{1}{2\pi}\int_{-\infty}^{\infty}\exp\left(-i\tau2\pi f\right)r(\tau)d\...
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### Confusion regarding plot of linear convolution vs fast convolution via FFT

I am reading Proakis Digital signal processing using Matlab, 3rd edition I am performing example 5.23 but i am not getting same plot/results as book although i am using almost same code First i am ...
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### Why use PSD to analyze a signal instead of its Fourier Transform?

Let's imagine I have a perfectly known signal $s(t)$ and I want to analyze its frequency components. I noticed that mostly everyone would use the PSD of $s(t)$ to do that, instead of simply use the ...
64 views

### For SNR calculation, should I use FFT|X+iY| or FFT|R|?

I use a demodulator that outputs X and Y quadratures, with the R and Phase which can be calculated. For SNR measurements, when plotting the FFT to find the SNR a signal at a certain frequency, ...
23 views

### Fast convolution applicable;only circular or also linear/standard convolution?

Fast convolution is the convolution performed using fft Does fast convolution only works for circular convolution or also works for linear/standard convolution?
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What are the differences between radix-2 FFT and radix-4 FFT, Are the only differences following two: In case of radix-2 $N$ is a number that is a power of 2 and in case of radix-4 $N$ is a number ...
32 views

### Conjugate symmetry of the DFT of real-valued sequences

I have read about Fourier transformation that real signals are "mirrored" in the real and negative halves of the Fourier transform because of the nature of the Fourier transform. For ...
28 views

### What is the “Energy” equivalent of the Magnitude Spectrum

I have a semantic issue regarding the discrete spectrum $X[k]$ of a time domain signal with limited length $x[n]$, so we're talking about the FFT. Now, when I what to compare the bandwidth and ...
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### Red noise estimation in power spectra. How to model the red noise component of data and derive corresponding confidence levels.?

I have generated a power spectrum using fft (python) and now I want to estimate the confidence levels with respect to red noise. Something like this: https://static-01.hindawi.com/articles/jcli/volume-...
108 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. ...
43 views

### confusion understanding FFT butterfly diagram?where are 12 , two input butterflies?

https://www.alwayslearn.com/DFT%20and%20FFT%20Tutorial/DFTandFFT_FFT_Butterfly_8_Input.html I was reading above link and then i came across a confusing sentence, i highlighted it in attached ...
104 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 ...
78 views

### Auto Tune Shifting

So i have an algorithm that gets the fundamental frequency of windows, and can shift the pitch of each window as well (by a factor of x). For example. if x is 2 and the f0 is 440, the resulting f0 ...
14 views

### What is the SNDR in ADC output

i have a sinus value at 80MHZ amplitude 0.5 with DC at 0.5 which was converted to digital sinus of values from 0-255. I have peformed FFT and found exactly a DC tone at 127 and 127 tone at 80MHz. both ...
27 views

### Wavelets / Identifying bursty signals

I thought my network was experiencing outages. I collected bandwidth (Mbps) measurements every 30 min. A cumulative probability distribution shows the probability that bandwidth will exceed any given ...
147 views

### How to detect overlapping peaks in FFT?

I am working with a modular spectral processing STFT toolkit I have written in C. I often need to carry spectral peak detection and analisys (frequency estimation, peak displacement, etc) but I am ...
58 views

### Cant find Doppler shift with LFM signal

Im stuck on this one. I hope someone helps me! Problem: I need to find Dopler shift with array of reflected chirps. I have bursts of chirps which then i sampled, and calculated fft. Im suposed to ...
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### Zero padding in frequency domain, any tricks to speedup?

there are many situations when one has to carry some operations on spectra, like splitting a spectrum Z wide in two Z/2 spectra, or vice versa merging two Z sized spectra in a 2Z sized one. The only ...