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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44 views

Changing time base after applying raised cosine filter

I am generating a random sequence binary 0 and 1 to have OOK signal. ...
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39 views

Near Real Time Algorithm For Approximating Fundamental Frequency

I'd like to analyse the notes being played on a cello C2(65.4Hz) to C6(1046.5Hz) recorded on a PC and thus a likely sampling rate of 44.1kHz. If I can get that to behave then I'd like to be able to ...
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FFT on a specific part of an image [python]

I'm currently working on a project where I have to analyse an interferogram. Instead of doing an FFT on the whole image I'd like to focus only on a small part of it (one fringe) is it possible to ...
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1answer
33 views

Unexpected result with custom FFT implementation

To fully understand the Fourier Transform I have been working on a rather standard implementation of the FFT algorithm. I tried generating a 440 Hz sin wave and passing it through the algorithm, ...
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49 views

"DFT". Understanding the formula $e^{-i2\pi k}$ $k$ is a real number

I am studying about fast Fourier transform. Assuming that $x_0$, $x_1, \ldots, x_{n-1}$ are complex numbers, the DFT is defined as follows. $$f_j = \sum\limits_{k=0}^{n-1} x_k e^{-\frac{2\pi i}{n}jk},\...
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How Can I correct amplitude and smoothness of FFT?

I am trying to create 10 individual signals with on-off keying modulation format, add them up to have a final signal, and then take a Fourier transform of that signal. I tried to zero padd the final ...
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1answer
52 views

Why does the 2D-DFT of a sinus gradient not show energy along the diagonal straigh lines and only vertical/horizontal from the diagonal point?

I have been experimenting a little bit with simple examples of the 2D DFT to get a better sense for it's interpretation. For this purpose I have been using sinus gratings with the following code: <...
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1answer
21 views

Scaling of FFT2 magnitude in image-processing

I got the following code: ...
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29 views

Do CQT, FFT, and NNLS provide different "perspectives" on the original signal?

From a bird's-eye perspective, do CQT, FFT, and NNLS algorithms provide different "perspectives" on the original signal? Does each preference some features of the original signal? I'm using ...
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2answers
112 views

Can a FFT be decomposed into multiple parallel channels?

Suppose I have a data stream that is sampled at a certain frequency, i.e. 1 GHz. I now want to perform a FFT on that data stream but since the hardware I have is limited to a maximum clock frequency ...
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power spectral density scaling factor frequency bin width [duplicate]

How to choose scaling factor for calculating PSD? Because I saw abs(fft)^2 scaled by fs/N (frequency binwidth) and also ...
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1answer
29 views

Plotting the real or absolute of correlation?

I had two vectors of data and I computed their correlation using corr(f,g) = ifft( fft(f) .* conj(fft(g)) ) / cov(f,g). I cannot figure out whether to use the real ...
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How to segment single-frame fft by frequency

I'm really hoping this isn't a duplicate, because the keywords I know to use did not lead me to what I need. I am trying to implement the phase vocoder described in "Phase-Vocoder Techniques for ...
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3answers
151 views

Understanding FFT output in Matlab

I'm very new to Fourier transform and I want to plot the Fourier transform of an image in Matlab. I've seen many examples and in all of them, the fft() of the image is a black picture with some white ...
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20 views

Spectrogram normalizaion to train a Fully Convolutional Model for Audio dereverberation

I'm trying to implement a deep learning based dereverberation algorithm. The network I am using is inspired by the U-net. The idea is to supply the reverberated spectrogram as input and 'clean' it ...
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36 views

What's wrong with a filter for subband OFDM?

I am confusing about why it is necessary to pack the QAM symbols onto the frequency bins, such that the allocated range of frequency bins is located in the middle of the subband (as in lines 26-29). ...
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1answer
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How Are MATLAB's `cpsd()` and SciPy's `csd()` Related?

I am trying to determine an estimate of the H1 transfer function between two acceleration signals in Python. To do this, I am calculating Pyx using ...
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Pressure signal

I have got an raw file which has pressure and time and temperature format and after coding I have got the plots but my main task is to analyse the data and understanding the influence of Bessel and ...
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2answers
66 views

Interpolation by zero padding FFT

I'm currently studying the book Vibration-Based Condition Monitoring (second edition) by Robert Bond Randall. I'm trying to implement in Matlab an algorithm to "increase" the sample rate for ...
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3answers
71 views

Sinc Interpolation between output samples of FFT

Let's say I'm designing a spectrum analyzer. While doing this I take the FFT of the real time data with the FFT size of 2048. Is there a way to increase the resolution in the frequency domain after ...
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1answer
78 views

Applying Convolution in Frequency Domain by Element Wise Multiplication on Time Domain

I'm studying the power spectrum. Right now, I am making a program to try to make sure that the "Fourier transform of the multiplication of some data and window function" and the "...
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1answer
39 views

Autocorrelation via real FFT and real IFFT

I am working on a fast autocorrelation function using the FFT (similar to scipy.signal.fftconvolve). The algorithm is pretty straight forward but I feel like I'm ...
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1answer
32 views

Averaging of a phase flipping signal

Suppose I want to average a signal $s(t)$ which consists of several spectral components without any DC offset. I sample $M$ points in the time domain. I am interested in the power spectrum which I get ...
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1answer
51 views

Measure Frequency-Response of Sensor using a Signalsource and Reference-Sensor

Cheers, I want to measure the frequency response of an acceleration sensor. I have: A signal source (device similar to a loudspeaker that shakes the acceleration sensor), of which I do NOT know the ...
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34 views

FFT on Pulse Trains with Varying Periodicity

I am looking at FFT on events over the course of a year (~365 days). My data is the number of events in a day. I assign occurrence as a pulse with the amplitude reflecting the number of events. I ...
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1answer
70 views

Scipy FFT and Numpy FFT disagree on pulse train spectrum?

I am doing an FFT on a series of pulses. The series is one pulse of amplitude 1 every 7 days over a total of 367 days. The code below is what I run: ...
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1answer
39 views

Spectral Interpolation vs Linear Interpolation

What is the main edge of using a spectral method (Spectral Intp/Trigonometric Intp) for upsampling or downsampling a signal in comparison to using a linear (Trilinear Intp) method to do the same? I ...
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65 views

Removing noise from an audio file only in time domain?

Hello I am doing some work for a course and I need to take an audio file and remove the noise from it. So far I've removed the noisy signal which is a 1000Hz pure tone using the frequency domain. I ...
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62 views

Relation of zero-padding and frequency resolution

Consider simple rectangular pulse and FFT of it in Python: ...
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22 views

what relationships between time sequence and nyquist bin or Dc bin?

I worked on fft recently, and I found kiss_fft code have some comments I do not understand. The sum of tdc.r and tdc.i is the sum of the input time sequence. yielding DC of input time sequence The ...
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2answers
32 views

lopsided Hanning Window function

I am processing sensor signals and have been looking at using window functions to smooth out the signal before I conduct a fft on it. A normal symmetrical hanning function seems to disorientate the ...
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1answer
31 views

Can we apply CZT after FFT?

I want to improve the frequency resolution of the FFT algorithm. Can CZT (Chirp z transform) be better to use after FFT. Because i read in some articles that CZT has better frequency resolution than ...
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Frequency Interpolation of DFT to create Zero-Padded IFFT

As a response to this question I have proposed interpolating new samples in a DFT (meaning the frequency samples of an existing DFT result) sufficient to be the new samples that if we were to take the ...
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1answer
27 views

Creating the frequency-spectrum with VC-curves for Building Vibrations measurement

I'm very new to signal processing and can't get ahead: I'm using an Accelerometer to measure the acceleration of i.e. the building vibrations or elements/machinery etc. For creating the spectrum, I ...
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2answers
92 views

Adding multiple time-domain signals with different FFT sizes

I would like to add multiple sequences with different FFT/IFFT sizes. For instance, we have 3 vector signals, $\vec{x_1}$, $\vec{x_2}$, and $\vec{x_3}$, having FFT/IFFT sizes of 512, 1024, and 2048, ...
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1answer
57 views

Should i normalize FFT signal with z-score?

I am working with EEG data (time domain) in a machine learning task, where each input signal must be mapped to a class/frequency. I am using FFT in order to get data in frequency domain and make ...
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2answers
71 views

zero-centered and causal zero padding

I have followed the link below to simulate two different zero-padding methods (zero-centered and causal) https://ccrma.stanford.edu/~jos/mdft/Zero_Padding.html Sample code ...
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2answers
42 views

Parameterization of a Idempotent Filter

I'm interested in if there has been research into how to parameterize filters so they are idempotent. I'm looking for the property shown below: $$ h \circledast h = h $$ I searched around and found &...
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14 views

How can we divide our long-time domain signal into equal segments and then apply Wavelet transform?

I have a time-domain signal and the samples size is 80000. I want to divide these samples into equal sizes of segments and want to apply wavelet transform to them. How I can do this step. please guide ...
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1answer
55 views

misunderstanding of spectral leakage

I want to understand spectral leakage. I understand that whenever we feed $N$ time-samples of a periodic, continuous, signal into a FFT algorithm we are multiplying in time-domain the true periodic, ...
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43 views

How to find the difference between maximum and minimum of a time signal, having only Fourier series cofficients

I currently know the Fourier coefficients of a signal $c_n$, from the exponential Fourier series of this form: $$f(t) = \sum^{+\infty}_{n=-\infty} c_ne^{in\omega t}$$ Using these Fourier coefficients, ...
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3answers
70 views

Doppler shift in signals

I have started doing some reading into the effects of the doppler shift on signals. In particular, radar signals from ships moving relative to a receiver. This might seem weird but I understand why ...
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1answer
34 views

Obtain PSD data from a noisy signal

I have a noisy source (like a wheel rotating constantly) and I would be measuring signals when the wheel is running. I looked into how I could convert this signal into a PSD and came across Welch ...
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2answers
78 views

FFT on a large number of samples

I have written a C code for a radix FFT transform over 1024 samples. But my main goal is taking audio files, converting them to raw pcm sample values, and then doing FFT on the whole audio file. My ...
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1answer
33 views

How to increase the low frequency resolution of spectrum of music signal

I know normal FFT with a fixed window size will introduce leakage and rectangular window function will give me the best main lobe width. For example, with 44100Hz sample rate, 2048 window size, a 64Hz ...
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1answer
95 views

Computing a convolution using FFT and more than minimum number of samples

Assume vector s is a set of time samples of length 1000. Assume vector h is a set of samples of length 50. If I want to compute the convolution of those vectors, the result will be 1000+50-1 = 1049 ...
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1answer
54 views

How to plot the phase response of Hilbert transform

The time domain signal of Hilbert transform is: $$ h(t)=1/(\pi t) $$ Its frequency response is: $$ H(j\omega)=-j~\text{sign}(\omega) $$ So if I plot the phase according to the equation I obtain: ...
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1answer
80 views

Applying a 2D Convolution Using 2D FFT

So I was following the article Victor Podlozhnyuk (nVidia) - FFT Based 2D Convolution (Page 7). I have expanded the kernel to the correct way they have done it. However when it comes to the part on ...
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1answer
30 views

What is the usual "best practise" for the base case of a recursive FFT implementation?

I find quite a lot of information about radix-2, radix-4, split-radix, mixed-radix FFT. There are some mentions here and there that choosing a "base case" for the recursion that is larger ...
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2answers
56 views

Why is the error of the frequency estimation dependent on SNR?

Last time, I asked how to estimate the error of the frequency estimation (1) (I want to determine the error as in $f=0.92 \pm 0.013 \; Hz$). I'm analyzing a sample of a continuous signal that was ...

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