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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
6 views

Sparse-NMF using NNDSVD to initialize W and H: negative values in first column/row

When NNDSVD is used for initializing W and H for NMF, the first column of W and first row of H are set to: $W(:,1)= \sqrt{S(1)} \times U(:,1)$ and $H(1,:)= \sqrt{S(1)} \times V(:,1)^\top$ ...
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1answer
40 views

Is it possible to find precise peak frequency using FFT phase shifts?

Been experimenting with FFT on a generated sinusoid and found something strange that doesn't seem to be described anywhere (though I may be missing something of course). A sinusoid that exactly ...
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3answers
255 views

I don't understand harmonics, why do they happen?

I don't understand why harmonics occur in signals. So if I receive a time domain sinusoidal signal at 10 Hz and I perform the FFT of it, I will observe peaks at 10 Hz, 20 Hz, 30 Hz, and so on... with ...
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1answer
34 views

solution on the time domain becomes “periodic” after the inverse fourier transform

I was trying to solve european option pricing problem using Conv method (introduced by Lord in 2008 https://pdfs.semanticscholar.org/0632/460bd50b2151f74ac40028df4cc60e73a884.pdf). The final step of ...
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1answer
42 views

Why does the FFT of a complex variable create negative frequencies?

So I have a demodulator that streams out X and Y values. I use a spectrum analyzer within this demodulator instrument which plots the |FFT(X+iY)| against the frequency domain, which shows up with ...
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0answers
29 views

FFT frequency-magnitude not correct when a positive gain EQ point is applied

I have a measurement system, which applies a log sine sweep as stimulus to a DUT and records the return signal. I then extract the impulse response, and perform an FFT on this to examing phase and ...
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0answers
29 views

FFT first frequency

I have some problems understanding FFT. If I know Fourier analysis frame (1024 samples) and sample rate (48 000 Hz), how can I find the first frequency of harmonic sines and cosines?
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0answers
20 views

Is fundamental frequency information present in speech-shaped white noise?

I am using a so-called speech-shaped stationary white noise, which means so much as a white noise that has been filtered to match the long-term spectrum of a female speaker. The way it was generated ...
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0answers
15 views

how to convert k-scale to frequency scale in FFT?

I have used fft(x) function in Matlab to perform Fast Fourier Transform on a given function and picture is the result. The graph you see is k, X(k). Now I am being asked to rescale frequency axis ...
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0answers
16 views

Some confusion about calculating spectrogram

I am new to DSP and i have a question because it is not the same as theory. I need to get spectrogram of wav file and feed it to CNN. I have used scipy.signal to get spectrogram. I know spectrogram ...
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1answer
36 views

Correctly scaling FFT of different lengths

I have been looking at using two FFTs of different lengths and displaying the output magnitudes on the same graph (a shorter length window for the higher frequencies, and a longer one for lower ...
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0answers
18 views

Transmitting image in certain spectrum without mirroring

I have a problem because I want to transmit string at certain frequency that is simple .png with 0 and 1. I simply take height and width of the image and scale it to the bandwidth and time. Then I ...
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0answers
32 views

How accurate is Python's FFT?

I am getting started with Python's FFT. I tested it on a signal that is a sum of three signals, two of which have an eigenfrequency of the grid, the third one does not (but due to large no. of data ...
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1answer
20 views

Peak to Average Power Ratio of an exponentially increasing signal

I am having an exponentially increasing sinusoidal signal. After sampling, I am dividing the signal into frames of length 128 and doing windowing operation with a hanning window of length 128. Then ...
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1answer
35 views

Why does an anti-symmetric function has zero amplitude at the center of an even length window

I am performing FFT on a real odd function and the resultant transform has zero amplitude in the last bin. Essentially if Y= rfft(X), then Y[-1] is always zero. I stumbled on this answer which says ...
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0answers
31 views

Audio Activity Detection and Classification

I am starting a new project. Actually, my real intention is to learn speech recognition but for warming up I want to improve in audio signal processing. In my project, I aim to record sounds except ...
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0answers
12 views

Multitaper F statistics

I'm having some problems interpreting the F-statistics output from multitaper analysis. To illustrate, the following code-snip in R performs multitaper analysis on the same sine-frequency but with ...
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0answers
17 views

When is a signal non uniform and do I therefore have to use a non-uniform fft? [closed]

I have a signal, which should be periodic at 10 Hz. But sometimes it is 10.2 Hz, sometimes 9.8 Hz. Is there the assumption that the sample points are strictly periodic still a good assumption. I ask ...
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0answers
42 views

Simple and Efficient FFT C or C++ code for HLS implementation

I'm working on my project and it's related to speech processing. I have to implement parts of the project on an Intel FPGA board using Intel HLS Compiler that converts C code to RTL code for FPGA ...
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0answers
19 views

Scaling factor for comparing spectrums obtained via Continuous Fourier Transform and Discrete Fourier Transformation?

Essentially I am trying to calculate the Bremsstrahlung spectrum numerically for magnetized plasma and want to compare the resultant spectra with the standard textbook spectrum formula for ...
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2answers
47 views

STFT window width vs. detectable Hz range

Assuming 44100 samples per second in a short 4-8 second-long audio .wav file, if I want perform FFT to detect the power spectrum, amplitude, and phase shift from 20 Hz to 10000 Hz (humans lose the ...
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0answers
30 views

Scaling x axis for FFTs in Python

I edited the post to be more specific, see below. I have a dataset, on which I need to perform and IFFT, cut the valueable part of it (by multiplying with a gaussian curve), then FFT back. First it's ...
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1answer
31 views

FFT of white noise with different sampling rate

I have a question on how I should interpret the white noise power level (noise floor) obtain from FFT for different hardware sampling rates. I realized if I sample the same noise at different rates (...
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0answers
33 views

Scaling of the PWELCH function in MATLAB

I am trying to compute the power spectral density of a random signal using the PWELCH function in MATLAB. Since I think have not understood properly how Pwelch scales the PSD, I wrote a sample ...
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5answers
2k views

Can someone help explain what this FFT workflow is doing to my signal, and why it works?

I am working with a piece of equipment that does acoustic analysis. It does this by acquiring a time domain signal and performing an FFT on it to extract the amplitude and phase at a single desired ...
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0answers
19 views

Angle bins for FFT DOA estimation

I have difficulties to calculate my angle bins. After doing a fft over N points over the antenna elements I get N bins. For getting the angle of each bin I do the following: f_bin = (2*pi)/N*n (n ...
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1answer
41 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,...
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1answer
60 views

Is it possible to reduce the complexity of radix-2 FFT if the input vector contains identical elements?

my question is about reducing the complexity of radix-2 FFT when the input vector has a specific structure. For an input vector of x with N elements, the complexity is given by O(N log2 N). My input ...
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3answers
33 views

FFT error when number of samples growth - Matlab/octave

I see when I increase number of samples, the imaginary error part of the FFT growth due to the error of sin(n*pi)!=0 where n is the index of the sample in Matlab or octave. Is there anyway to force ...
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0answers
14 views

how to calculate power spectrum of Gaussian noise from the known variance of it?

can someone explain me the following paragraph, i tried to understand it but i am not getting it exactly, Thank you ! Since you know the variance, you can predict the energy of the noise image, ∑x∑y|...
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1answer
17 views

Getting non smooth spectrum after performing fft on numerically obtained acceleration data

I am trying to calculate the spectrum of Bremmstrahlung, which involves calculating the Fourier transformed acceleration. I am solving a non-linear ODE to numerically calculate the acceleration in the ...
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1answer
54 views

Square wave FFT results show frequency that seems too low?

I have created an open source plotting package for a low cost oscilloscope - see link here: GitHub Plotting Software But during my testing I have one result that may be wrong: a noisy 7 kHz square ...
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1answer
73 views

Recovering a signal after filtering

A signal $x[n]$ is to be transmitted over a communication channel. The communication channel is described by an FIR filter of length $M$ such that the received signal is given by $$r[n]=\sum_{k=0}^...
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1answer
51 views

Plotting curved peaks on PSD graph

I am a PhD student with little engineering experience, currently attempting to recreate the signal processing techniques used in this paper: Monitoring Vital Signs and Postures During Sleep Using WiFi ...
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2answers
41 views

Dominant Frequency Peak Decreases with Increasing Window Size

I have a signal that looks like this. I analyse it using fast Fourier transforms to identify the frequency with the largest peak, which is always close to zero. (There are no other clear peaks.) If I ...
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1answer
41 views

Non-physical peaks from the Fourier transform process

I was doing some reading on transient signals. When transient signals is analysed by DFT, there are some non-physical peaks from the Fourier transform process (on the figure below). I understand this ...
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1answer
66 views

Using a Butterworth high-pass filter together with Fast Fourier transform

I have a signal and I'm interested in identifying the dominant frequencies greater than 1/3600 Hz. To do this, I use Fast Fourier Transforms and examine the locations of the peaks in the frequency ...
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2answers
50 views

Is RMS of a sound wave good measure of loudness?

I'm not talking about FFT, just the raw sound wave and the RMS of it. Thanks
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0answers
22 views

Using a Multirate FFT system to analyse audio

I am trying to implement a multirate fft system to try and achieve frequency windowed analysis of audio. Say I have managed to get an impulse response of a system. This impulse may be say one ...
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1answer
44 views

PyAudio recording is taking much longer than planned

I'm trying to record sounds in chunks using PyAudio for 15 seconds, chunk size being 1024 and sampling rate is 1024 *40. I understand the number of chunks would be (time of recording) * (sampling rate)...
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1answer
40 views

Partitioned overlap-add convolution - strange behavior at buffer boundaries

I've implemented a convolution reverb that operates in real-time, one audio buffer at a time (using FFTS for the fft bits). However, there's some strange behavior at the start of every buffer. ...
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1answer
23 views

Audio Spectrogram / FFT - Amplitudes and Missing Frequencies

I have an input wave which I want to transform to a spectrogram. In the example code in a previous question , I get the result, shown below, for a pure sine wave 440Hz. From this video, if I do an FFT ...
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0answers
13 views

Could kernel density map be recovered with known kernel function?

I'm confused whether a KDE map could be recovered with known kernel function. The KDE map (with no noise) generated with fast fourier trasnformation (FFT) could be recovered on very high accuracy (<...
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2answers
76 views

What is the meaning of the length of diagonal lines in FFT

I am relatively new to FFT analysis of images and have a question about them. I like to think I understand the basics but many of the properties shown in a FFT image are beyond me. My main question ...
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1answer
57 views

Methods for aligning FFT bins with an FSK signal

Say I want to demodulate some FSK modulation. My (novice) understanding is a pretty good way to do this is by taking the FFT, with the length chosen such that the bin spacing is equal to the tone ...
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0answers
37 views

Understanding convolution of Chirp z algorithm

I dont´t understand how works the convolution part of the Chirp z. I understand how the DFT is transformed \begin{align*} x(k) = \sum_{n=0}^{N-1} x(n) W_N^{kn} \end{align*} to this expresion: \...
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0answers
40 views

Why are low frequency peaks not sharp in scipy fft?

I am using numpy/scipy to plot graphs of sine waves. frequencies at 15 Hz or higher give nice, sharp peaks, but at lower frequencies, the peaks are smeared and the actual peak frequency can't be seen ...
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1answer
37 views

What determines peaks in FFT?

I ran FFT on three audio files and found that the results for some have more peaks than the other. Could anyone give me any conceptual explanation as to what determines these peaks? Below are plots of ...
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1answer
57 views

Why the spectral coherence is unity for all frequencies between single-frequency time series and itself

In the example below, I am plotting the coherence between time series and itself. The time series do has one frequency.The coherence magnitude was one for all frequencies. I wonder why it is not zero ...
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0answers
26 views

Skew measurement with phase deviation

I am calculating the standard deviation of skew (= phase/2*pi*fin) from the 1000 monte carlo runs of the following code. In the y0, there is no input skew., but in y1 there is a skew present. Its ...