Questions tagged [fourier-transform]
The Fourier transform is a mathematical operation that decomposes a function into its constituent frequencies, known as a frequency spectrum.
1,291
questions
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1answer
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Approximating Lorentzian Fourier transform with FFT
I have a Lorentzian frequency distribution
$F(w) = \frac{1+iz}{1+z^2}$
Where
$z = \frac{w-\Omega}{R}$
With $\Omega$ being the peak frequency and R the decay constant. I know that analytically ...
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4answers
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“The Fourier transform cannot measure two phases at the same frequency.” Why not?
I have read that the Fourier transform cannot distinguish components with the same frequency but different phase. For example, in Mathoverflow, or xrayphysics, where I got the title of my question ...
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2answers
65 views
Is the FFT magnitudes of multiple audio sources additive?
Is the FFT magnitude of two audio signals when played together the sum of their individual FFT magnitudes across frequency bins? Note I'm talking only about the magnitude, ignoring phase.
For ...
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1answer
50 views
Continuous-time vs Discrete-time Fourier transform
case 1) to calculate the Fourier transform of discrete-time signal(sampled signal) we use Discrete-time Fourier transform.
but my question is:
case 2) if I consider that discrete-time signal as ...
2
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1answer
60 views
Non-Linear, Non-Stationary spectral analysis methods! When and where?
I have been reading about non-linear non-stationary signal analysis methods and it seems to do this type of analysis the go-to method is the Empirical Mode Decomposition (EMD), then Hilbert Transform (...
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2answers
53 views
Phase information of a signal when creating a spectrogram
Sorry for the uninformed question but I need some help understanding this. I'm trying to understand what phase information is and it would be very helpful if someone could correct my understanding of ...
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2answers
940 views
How do I explain a complex exponential intuitively?
What is a complex exponential, explained intuitively?
How do I explain to an adolescent a complex exponential function?
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1answer
59 views
fft function in R vs spec function from 'seewave' package? They don't give a similar frequency spectra
I understand that spec will give me the frequency and the corresponding amplitude of that component, whereas fft will compute the DFT of the signal and throw the complex numbers for each component ...
2
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3answers
47 views
Characterization of transfer functions with no local peaks
Assume that you are giving an arbitrary amplitude frequency response $A(\omega)=|H(j\omega)|$
Is there a characterization that ensures that $A$ is monotone? i.e, $A$ has a global maximum at $\omega=0$...
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0answers
21 views
Removing striped noise from an image
I was just wondering if anyone could explain to me the approach one would take to removing striped noise from the fourier domain of an image. I was reading an article about MRI image from 1 just ...
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1answer
28 views
Graph Fourier transform: the adjoint notation for the eigenbasis matrix
I already asked this question here but there is no response. I'd like to ask this question in signal processing domain.
It is well-known that for a real symmetric matrix $L$ (here, graph
...
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2answers
47 views
Relation between $X(f)$ and $S_x(f)$
We know that for a signal $x(t)$, it is related to $R_x(\tau)$ as, $$R_x(\tau) = \int_{-\infty}^{\infty}x(t)x(t-\tau)dt$$.
$\\$We also know that $$R_x(\tau) \rightleftharpoons S_x(f)$$
$\\$How do we ...
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1answer
97 views
FFT and Power Spectrum Normalization
On many websites, including MathWorks, it was suggested to normalize the fft spectrum (MATLAB or numpy) by dividing it by the total number of samples ($N$). For a sinusoidal signal, for example:
$$x(...
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1answer
57 views
Convolution Theorem: Hamming Window on a Time Series and Fourier Domain
If we have a set of time series data, y, consisting of 100 data points. One can apply a N (odd) Hamming window as a weighted moving average to decrease the noise. Say, if we choose 7 point Hamming ...
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2answers
74 views
Does convolution violate duality of a Fourier Transform
Duality says that
if
$$
x(t) \iff X(\omega)
$$
then
$$
X(t) \iff 2\pi \cdot x(-\omega)
$$
Given that
$$
f(t)\circledast g(t) \iff F(\omega)G(\omega)
$$
By duality, shouldn't that imply that
$$
f(t)...
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0answers
19 views
What is the significance of y-axis in Lomb-Scargle Periodogram?
What does the y-axis of Lomb-Scargle periodogram represent?
Periodogram obtained by Fourier transform contains information about the original data's amplitude, but the so-called "Lomb-Scargle power" ...
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1answer
45 views
What is the effect on Hilbert transform of signal after multiplication by sinusoid?
Question :
Find Hilbert transform of $[u(t-a)-u(t-b)]\cos2\pi f_{0}t\\\\$ such that $\\0<a<b$
my attempt:
we know Hilbert transform of
$[u(t-a)-u(t-b)]\xrightarrow{\mathcal H} \dfrac{1}{\pi}\...
0
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1answer
82 views
Frequency Domain Interpolation: Convolution with Sinc Function
I am reading a paper, and I came across the following definition of sinc interpolation.
Warning. I don't have a strong background in signal processing. Also, I have no clue what that bar on $\bar{F}$ ...
0
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1answer
79 views
How to group similar patterns together that are shifted along the x axis?
I have a set of time series plots that are shifted along the x-axis. I am looking for an algorithm using which I can group them together. My end goal is to average them.
While reviewing the ...
1
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1answer
51 views
complex numbers and fourier transform
Is it possible to define a scaling property for fourier transform when the scale factor is complex?
Usually the scaling factor is real. What happen when a scaling factor is complex?
3
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1answer
37 views
Fourier transform and anti-trasform--identity missing
I have a very silly doubt:
If we define the power spectral density:
S(f)=$\frac{1}{2\pi}\int exp(-i\tau2\pi f)r(\tau)d\tau$ (1)
where $r(\tau)$ is the correlation coefficient.
If we do the Fourier ...
1
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1answer
28 views
Is fft2 in MATLAB unitary? Some differences happen
I meet a problem when implementing fft2 in MATLAB.
The question is I try to simulate the realistic measurements $Y = |FCXF^H|^2$ - the intensity of Fourier domain of object $X$, where $F$ denotes ...
0
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1answer
39 views
Proof that DFT does not require more than N points
I'm trying to show how the discrete Fourier transform (DFT) arises from the equation for the continuous-time Fourier Transform. I've run into an interesting caveat which I can't seem to find an ...
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2answers
63 views
PSD of modulated signal
I know this question has been previously asked just saw it and tried to do myself but reaching an expression which is different from expression of PSD which I remember for modulated signal
Actual ...
0
votes
1answer
37 views
DFT of an audio signal stored as a multi-dimensional array
I'm using the Python scipy library to get the data of a particular .wav file into array format. Now, I'd like to find the Discrete Fourier Transform of that signal. The formula for the DFT is,
$X_k = ...
0
votes
1answer
61 views
One sided frequency spectrum (Matlab vs. Origin)
There are a lot of queries on fft frequency all over the web. I guess the following point not discussed anywhere explicitly. Hope someone can provide an insight here.
If we have and even number of ...
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1answer
50 views
A question about Fourier transformation
Hello, this is my first time actually asking in stackexchange. I am a computer engineering student and currently i am doing a linear system course (i don't really know how this is equivalent in ...
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0answers
19 views
Calculate phase lag between two signals with perturbed frequencies
This type of question has been asked quite a few times on this forum and others now, but I still haven't found a satisfactory answer to my problem.
Given an input signal: $$x_1(t)=\cos\big(2\pi ft\...
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0answers
29 views
Apodization in the Fourier (frequency) domain on discrete experimental data
Let us assume we had time domain signal as a raw data R (Window 1) and we wish to perform the deconvolution process on R using another set of raw data G (window 2). This is accomplished dividing FT of ...
2
votes
1answer
158 views
Shannon-Nyquist theorem reconstruct 1Hz sine wave from 2 samples
Lets say I want to set the minimal sampling rate to reconstruct a 1Hz sine wave, according to the Nyquist-Shannon theorem that states that the maximum recoverable frequency is Fs/2 i.e. we must sample ...
0
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1answer
68 views
Can the Hanning Window be represented in the time domain
I know the Hanning window, $w_{Hann}(n) = 1 - \cos \left(2\pi \frac{n}{N} \right)$, is typically applied to discrete Fourier transforms. $N$ being the total number of points in a transient signal that ...
1
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1answer
103 views
Fourier transform of a damped cosine wave with a linear frequency chirp
I want to take the Fourier transform of the following transient signal,
$$f(t) = e^{-t/\tau} \cos((\omega_0 + m t)t)$$, where $m$ is some gradient parameter in units of $\rm{Hz}/s$.
I thought this ...
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0answers
47 views
How to find fundamental frequency of two signals?
I am facing difficulty with finding fundamental frequency of signals
I mean by fundamental frequency=(1/Time period)
Correct me if I am wrong
consider two continuous time signals with
Time period ...
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1answer
36 views
How do I obtain the fourier series coefficients for a signal obtained by multiplication of two signals of different frequency?
What i assume here is that LCM of time periods of the two taken signals exist that is signals periods are not like pi/2 and 1 but are rather like 1 and 2 (just an example)
I am given fourier series ...
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1answer
38 views
Is it possible to extract peak locations in the time domain using help from fourier/wavelet analysis?
The signal I'm studying has fundamental frequencies of 20 and 60 cycles per minute (shown in the Periodogram graph). It is straight forward to extract the peaks in the time domain belonging to the 20 ...
1
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1answer
46 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 ...
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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 ...
2
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1answer
32 views
Reconstructed output mismatch for LTI system
I have a system with measured input (u) and output (y).
I assume that this is an linear time-invariant (LTI) system and I want ...
0
votes
2answers
52 views
Image Brightness on 2D Fourier Transformation
What effect does the brightness have on the frequency spectrum of 2D Fourier Transformation?
Example. Suppose we have a gray image and calculate 2D Fourier Transformation. Then we increase the ...
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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 ...
0
votes
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 ...
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2answers
72 views
Unit of Energy Spectral Density
The continuous-time Fourier Transform (CTFT) of a signal $x(t)$ (with unit $unit$) is:
$$X(\omega)=\int_{-\infty}^{\infty} x(t)e^{-i\omega t}dt$$
which should be in $unit\cdot sec$ or $\frac{unit}{...
2
votes
2answers
62 views
Phase response of $H(f)=e^{-j2{\pi}ft_0}$
Given is the impulse response: $$h(t)=\delta(t-t_0)$$.
I calculated $$H(f)=e^{-j2 \pi ft_0}=|H(f)|\cdot e^{j\varphi(f)}$$.
Now, the magnitude response of $H(f)$ is:
$$\begin{align}
|H(f)| &=\...
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0answers
53 views
Python FFT outptut
I have a (real) array of data and am trying to analyze its frequency components. I've been using NumPy's FFT routines, but I realized there is something I don't quite understand: why does the output ...
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0answers
94 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 ...
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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.
...
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0answers
31 views
Correlation/anticorrelation as a function of frequency
I am tracking the values of two fluctuating quantities as a function of time and am trying to analyze possible correlations between the two as a function of frequency. The application is experimental ...
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votes
1answer
38 views
Effect of Nyquist frequency on Fourier transformed data
Upper plot is the original data's plot, and the bottom plot is Fourier transformed data. For the bottom plot, x-axis is the frequency and y-axis is the amplitude. I don't understand the weird behavior ...
1
vote
1answer
54 views
fftshift in MATLAB with even number of data points in double sided spectrum
I have a question with reference to this Table.
With even N, the frequency axis extremes should be $\pm$Fs/2, where Fs is the sampling frequency. However in the array we have only one value ...
0
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1answer
79 views
Frequency Axis of Discrete Fourier Transform (DFT) with Odd Number of Data Points
I am trying to understand the logic behind making a frequency axis in DFT. I am using for time based light absorbance. When we have even number of data points (N= even integer), collected over a ...
