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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4answers
451 views

Clarification on defining FFT bin sizes

When reconstructing the frequency domain for an FFT, what is the most self-consistent way to do this -- i.e. how is it best to define the bin width, $\Delta f$? For example, previously I thought it a ...
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1answer
29 views

Unexpected frequency components found in Periodogram

I am trying to make sense of Digital Signal Processing with the help of R language. So, the issue came up when I tried to create a periodogram for a simple sine wave. The regular periodogram shows 2 ...
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1answer
48 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 ...
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1answer
37 views

Is the sum of all FFT bins from Audacity equal to RMS amplitude?

Say I open an audio segment with Audacity. Then I perform a size 4096+ FFT with a rectangular window. The program outputs bin magnitude values in dB. If I add them all together like this: sum = 10*...
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0answers
22 views

how to cut off frequencies above 10 Hz with Welch method

This is the first time I have worked with any code so I am rather unfamiliar with all the lingo and processes involved. I am trying to convert a signal to the frequency domain and want to find the ...
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2answers
28 views

The DC bin of an FFT should be the mean of all the other bins in the spectrum, why does the FFT produce 0?

I am experimenting a bit with FFT's. From what I have read the first bin of an FFT is called the DC bin -- that is it is the mean of the other components in the FFT. I have also read that it should be ...
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1answer
26 views

Measuring group delay through a linear phase filter

I'm having a difficult time wrapping my head around a phase/ group delay problem and it corresponding to where you start sampling so I built up a system to test some of these issues. The system passes ...
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1answer
20 views

Should you scale the FFT bins by $1/N$ where $N$ is the number of points in a transient signal?

I am going over the following tutorial. In section 2.1 the author says To calculate the $N$ point FFT the Matlab algorithm 1 can be used. Here, after taking the FFT, its magnitude is calculated ...
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1answer
64 views

MATLAB FFT Amplitude

I am trying to understand FFT, DFT through Matlab and I am fairly inexperienced in all of these. I have a time vector t and a corresponding simple sine wave x = 0.25*sin(t). Now I would like to view ...
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1answer
22 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 ...
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2answers
67 views

Have you seen these kind of DFT artifacts?

we've been working with the Spiral DFT Implementation on an FPGA and managed to get it working. Unfortunately we see these strange artifacts and have no idea where they come from. Our input is a ...
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1answer
32 views

Using fourier coefficients to reconstruct data in matlab

When doing a discrete fourier transform on some data using matlab's fft function, its output is a set of fourier coefficients but I was wondering how do I go about converting these into an and bn so I ...
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0answers
31 views

When plotting the cosines of the phases of some sensors, I got an interesting plot

I have a laser that I modulate at some frequency, typically 1250 Hz. I have a sensor that tracks what happens inside a reactor when it's illuminated. I am doing Fourier analysis on both the laser ...
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0answers
7 views

How to preserve amplitude of original data in Lomb-scargle periodogram

To my knowledge, peaks in Lomb-Scargle periodogram does not preserve the amplitude of the original data. It just shows how much variance each frequency explain about the original data. For example: ...
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0answers
12 views

Autocorrelation of vector of images

I have a set of images (think a ball getting bigger and smaller). I want to assess the periodicity in this string of images (each data point in my signal is an image). How can I go about doing this in ...
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0answers
14 views

cpsd calculation help

I have several channels from where I am reading sensor signals in time domain. I have 9 channels where I am reading time domain signals from each and every channel for 10 seconds. Now if I perform ...
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1answer
16 views

Does FFT magnitude of recorded sound depends on the sound card used?

Based on some reading, here's my understanding of how sound recording works: Microphone outputs a voltage based on sound pressure. The microphone voltage is amplified and the amplified voltage goes ...
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0answers
10 views

How to preserve amplitude of original data in Lomb-Scargle transformation?

I have unevenly sampled data. I tried FFT, and failed. I tried Lomb-Scargle (LS) transformation, and it succeeded. Here's what I understand about FFT vs LS. Correct me if I'm wrong. FFT requires ...
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1answer
57 views

Overlap Add - which length to use?

I am doing realtime audio. Let's say I have a signal $x(n)$ which is passed in blocks of 512 (siobuffer) to the program filter/FFT method. Within this block I want to convolve the signal with the ...
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0answers
19 views

Dual FFT - difficulties extracting phase

I am performing dual FFT analysis on a system. 1) I generate a Log Sine Sweep signal and save it as my reference array 2) I then produce an inverse filter to correct for amplitude and save this as ...
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0answers
14 views

Question about FMCW range-velocity plot

I am working on plotting range-velocity heatmap of the receiving FMCW signal. I actually have already plotted. However, I do not really understand the mapping relationship between Doppler FFT bin and ...
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0answers
28 views

Double Sided FFT Power Spectrum on Matlab

I'm trying to plot the power spectrum of a signal but I cannot understand clearly the units of the amplitude. ...
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1answer
43 views

What should be my sampling frequency?

I have an accelerometer that I am reading the x, y, z values for m/s^2. I've written up my code in Python as follows. ...
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0answers
36 views

Can I find FFT for signals with odd number of element

I am said to divide signal into two parts even and odd numbered {index/value of n} .What if number of elements in x[n] is odd out will I divide into two networks?
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1answer
26 views

Phase measurement with FFT for DMM Keithley 7510

I am currenly working on a measurement driver for the DMM Keithley 7510. I implemented a harmonics measurement using FFT (from MathNet library). First I simply sample my input voltage signal and ...
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0answers
42 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 ...
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0answers
31 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 ...
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0answers
36 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 ...
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1answer
38 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
18 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 ...
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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 ...
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2answers
64 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 ...
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1answer
61 views

Mathematical description of the ACF using fft2

I computed the acf of an image with the following code: ...
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1answer
28 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 ...
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2answers
58 views

Interpolation of estimated frequency between fft bins: what about phase?

I am developing an additive re-synthesier, which involves identifying partials from FFT frames. Partial identification requires amplitude peaks. The simplest way to do this is to just look at the ...
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3answers
53 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: ...
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1answer
31 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 ...
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0answers
37 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: ...
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1answer
44 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 ...
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0answers
35 views

Retrieve covariance matrix from array of fft

I have 10000 power spectral density vectors of 64 frequency bins each. My input is therefore are 10000x64 matrix of real values. Those are obtain from fft of time series of temporal signal I don't ...
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1answer
58 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 ...
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0answers
9 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 ...
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1answer
21 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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1answer
30 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 ...
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0answers
51 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
9 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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3answers
299 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?
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1answer
26 views

How can I graph the amplitude for a given frequency window over time?

I have simple audio files of single plucked notes from a guitar: I want to graph the amplitude of each partials over time, rather than just have this color-coded "dB" intensity graphing I have here. ...
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1answer
35 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 ...
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0answers
25 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 ...