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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697 views

Computation of the Inverse DCT (IDCT) Using DCT Or IFFT

Is there a way to compute the inverse discrete cosine transform (type-2) by leveraging either a DCT, FFT, or IFFT algorithm? I have seen ways to compute the DCT using FFTs, and I've seen ways to ...
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1answer
145 views

Emulating a Variable Delay

I am trying to emulate a satellite link where the delay time between the satellite and ground station changes over time due to the satellite's motion. I plan do this by converting to the frequency ...
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1answer
68 views

Extracting positive frequencies of discrete-time signal

Convolution in the time domain is the same as multiplication in the frequency domain. My data is sampled at 200 Hz, which means that the Nyquist frequency is 100 Hz, and all frequency content is <=...
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3answers
477 views

Calculate ifft using only REAL forward FFT

There are already questions on this topic here and here. I am trying to calculate the ifft using a DSP that only has a library function for a forward REAL (assumes conjugate symmetry) fft. For a given ...
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1answer
127 views

How to calibrate a signal with respect to frequency

I have recordings from accelerometers (non-flat response with respect to frequency) and I'd like to convert my voltage time series into acceleration values. Is there a standard method for doing this? ...
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2answers
362 views

Orthogonal signals in frequency and time domains [closed]

I have 3 orthogonal signals obtained by Fourier transformation. But when I do the inverse Fourier transform in Matlab, my signals are no longer orthogonal in the time domain. Here is my question: ...
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1answer
545 views

Seven term Blackman Harris window

I am currently trying to evaluate a high resolution ADC for which I need to window my output data for the FFT. Up until now I have been using the default 4 term Blackman harris window in MATLAB. ...
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2answers
81 views

A question about Gibbs phenomenon example in Richard Lyons book

If anyone has a copy of this book could you please shed some light here: I was trying to reproduce the example shown in Chapter 5, section 5.3.1 Window Design Method's Figure 5-19 which illustrates ...
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1answer
77 views

Perform overlap add with available FFT size smaller than filter coefficients

I would like to process 12 to 20 seconds of incoming audio at a sample rate of 44100. I must process this audio in real time in an STM embedded kit (perhaps also an Android Smartphone). I'm trying to ...
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2answers
308 views

Linear Convolution in DSP and Hann window

My understanding is that when dealing with discrete finite sequences, convolution done thru FFT is circular. To obtain linear convolution, one would pad the input with zeros up some appropriate length....
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1answer
96 views

The complexity of zero padded DFTs [closed]

Assume that I have L points and want to calculate its Fourier transform with N points (N is much larger than L, link N=256, while L=10), I am think if there is way to calculate it with lowest ...
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78 views

FFT High Frequency Humps

I am using Matlab to take the FFT of a series of pulses. Can anyone explain why there are multiple humps at high frequencies rather than a smooth roll-off?
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3answers
127 views

Efficient calculation of exact frequency from a broad frequency range (1 Hz - 1000 Hz)

I have some input signal sampled at 1-10 MS/s. Let's assume that it contains one strong frequency component (with frequency anywhere in the range from 1 Hz to 1000 Hz or even higher, changing over ...
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1answer
107 views

Calculating THD where peaks are not exact harmonics

I have captured an FFT of a 1 kHz signal that I am generating using a WM8904 codec running at 48k samples/second connected via I2S to a ARM microcontroller (Atmel SAME70). It appears to have some ...
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1answer
163 views

Spectral Subtraction python implementation error

I am trying to implement Spectral subtraction in python. I am new to DSP. I used this article as reference and below code is what I can come up with. Of course I did not get the expected output. I ...
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1answer
1k views

Amplitude of frequency bin in FFT doesn't match time-domain amplitude

I have a sinusoidal current that I am sampling at about 357k SPS. The current signal is about 3A pk-pk @ 750hz (top figure). I am wondering why when I take the fft using numpy/scipy's fft function, ...
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3answers
544 views

How do I calculate peak amplitude of the signal components after zero padding and FFT?

I am learning about DFT and trying to apply it to some audio processing. I am new to DSP but experienced in programming and have some background in math and physics. The FFT algorithm I use (lomontFFT)...
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2answers
325 views

Software to extract frequency spectrum data from an audio file

I've been trying to learn how computers recognize musical notes, and I found apps like spectrogram which shows the frequency spectrum of a sound, but what I need is the data which is plotted or the ...
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3answers
457 views

Denoising a signal

I'm starting hydraulic experiments, where I'd have to measure velocity in an unsteady flow with a device called Acoustic Doppler Velocimeter. In DSP terms, I'd have a nonstationary signal in a shape ...
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1answer
71 views

Small shift in frequency in FFT

I'm trying to compare the estimation of the power spectral density (in this case of a fake cosine signal) by two different methods: 1st, MATLAB's fft function, and 2nd, a direct calculation from the ...
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1answer
100 views

methods of crest factor minimization

i am studying the methodes CF minimization. and have a doubt. in each methods there are steps with DFT and IDFT or like this FFT and IFFT. Everyone provides the fact, that they use DFT and IDFT for ...
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1answer
241 views

How can I show DTFT result in MATLAB? [closed]

I wanna show DTFT result and convolution result in f-domain are same. Additionally, I wanna show sampled function and inverse fourier transform result are same too. How can I show this? If I use ...
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1answer
46 views

Why are these results different about 'frequency shifting'?

I think 'spec_ori' & 'spec_shifted' spectrum amplitude and phase are same in below code. Because of 'spectrum shiting property' (What effect does a delay in the time domain have in the frequency ...
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1answer
709 views

Frequency analysis to determine low-pass cut off frequency

I collected some data for a practical application, where the signal represents force data obtained from an impact of a punch against a force plate attached to a quasi-rigid rig (it moves once the ...
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1answer
267 views

Matrix cross correlation in python

I'm currently performing matrix cross correlation in python using : C = scipy.signal.correlate2d(A,A) where A is a 2D matrix, typically a picture. As you can ...
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1answer
349 views

Why is the second half of the FFT negative frequencies

There are lots of questions here about what the negative frequencies in fft mean, but I'm confused on why the second half of the fft calculation is the correct calculation for a negative frequency. ...
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1answer
362 views

Why MATLAB fft cos makes imaginary parts?

The cos fourier transform has no imaginary parts, but in this code it has imaginary parts that little big. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% figure(7); fs=128; t=-4*pi:1/fs:4*pi-1/fs;...
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0answers
227 views

FFT amplitude vs IQ amplitude

I'm very new to digital signal processing, so please let me know if my question doesn't make sense. I calculated the amplitudes (using the formula : $10 \log_{10}(i^2 + q^2)$) for some IQ data ...
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2answers
421 views

Ifft through Matrix multiplication

I am still new to MATLAB, so apologies if I sound lazy to you. I am attempting to model a transformation as a set of matrix operations. I start with a vector, up-sample it by $U$ (up-sampling rate), ...
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1answer
152 views

Understand Inverse DFT (FFT)

I was reading this article to understand the fft https://jackschaedler.github.io/circles-sines-signals/dft_walkthrough.html I understand that the fft takes a block (256 or 512) of the signal, ...
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2answers
114 views

Radix2 fft - zero padding output size

With zeros padding the FFT radix2 has different input size than output. How to deal with that? I see two solutions but not sure which one is better/appropriate. Maybe you know some other and better ...
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1answer
964 views

Numpy fft2 frequency shift

I don't understand how to make frequency shift in fft2 or higher dimensions. Could anyone explain it, please? import numpy as np import matplotlib.pyplot as plt from mpl_toolkits.mplot3d import ...
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3answers
346 views

Deconvolution Using Complex Division in The Frequency Domain

Consider these two signals: a = [1 1 0 0 0 0 0 0] b = [1 0 1 0 0 0 0 0] their convolution is c = a * b = [1 1 1 1 0 0 0 0] ...
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1answer
50 views

Meaning of a formula about transient frequency bins

I'm trying to understand the following formula in this paper: Sadly, the paper does not define most of the symbols it is using, so my guesses so far are: $T_{tr}$ refers to the sampling interval i....
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1answer
49 views

Complex FFT reversing

I've succeeded to compute complex FFT of a data array of interleaved values using the arm_cfft_f32() API of the CMSIS DSP library for Cortex-M4, and got in the same array the FFT results as frequency ...
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1answer
32 views

Non-decaying Harmonics in Vibration Signal

I am analyzing vibration data collected by an accelerometer positioned in a stamp press machine. The fundamental frequency is around 10 Hz. The problem here is that when I take the FFT of the data I ...
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1answer
51 views

FFT for distance-limited sum of pairwise products

I have a sequence of fixed length $N$ input arrays ($x_0$, $x_1$, ..., $x_{n-1}$), and my goal is to compute output arrays as such: $$d_{00}[l] = \sum_{i = 0}^{N - l} x_0[i]x_0[i + l]$$ $$d_{01}[l] = ...
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2answers
58 views

What to do for the right FFT in Matlab (two peaks and incorrect amplitude)?

This is the FFT I'm using. But two peaks (did not expect the one in the beginning) are occurring and the amplitude is not as specified (expecting a value of 2). Any help is much appreciated! ...
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1answer
61 views

Harmonics in Quantal Histogram

I'm studying what I believe to be a system that produces a (noisy) quantal response (i.e. there should be a minimum increment of response). To do this, I've plotted a histogram of some ~6000 amplitude ...
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1answer
148 views

THD: estimation using periodogram or fft?

I want to estimate the THD of a voltage signal (output from a frequency inverter). At the begining, I need to estimate the power of the signal at frequency domain. To doing so, I should use a ...
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2answers
253 views

How to assess perceptual volume from FFT

So far I could estimate signal volume from FFT using amplitude information, then adding all amplitudes together and divide this number by the number of frequencies. But I think this method does not ...
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1answer
818 views

Checking for VHF pulse with SDR and python

So I have a small NooElec Software Defined Radio and I've got it working well with my Raspberry Pi using GQRX as a controller and to visualize the SDR output. My goal though is to use a python script ...
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4answers
1k views

Why does the frequency in the DFT have to be an integer?

I don't know why the result of DFT and FFT in MATLAB like below images.. fs=128; t=0:1/fs:1-1/fs; x=cos(2*pi*3.5*t); X=fft(x); N=length(x); n=0:N-1; f=fs*n/N; plot(f,abs(X)/N); If I set the ...
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75 views

Making longer a time domain signal by adding values on its frequency domain. What am I doing wrong?

Background Here's the thing: using software for Finite Element Acoustic Simulation I got the dataset of frequency response from a room; software works by solving the wave equation in the interval $`[...
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1answer
171 views

How to interpret the signal in frequency domain

I'm having difficulties understanding some basics regarding FFT. I was hoping you could answer a few questions I have. These questions will probably seem silly to you, sorry for that. I'm a complete ...
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0answers
133 views

Correctly scaling Power Spectrum and Correlation Function of Galaxies from Mock Catalogues

I'm currently writing a code to compute the two-point correlation function of a set of galaxies. As I work with a loooot of galaxies, it was suggested to me not to do it in real space, but to compute ...
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3answers
2k views

Correct method for processing FFT in realtime

I am attempting to build a 'live' FFT for continuous analysis of an audio signal. I am relatively new to this, but have attempted the full thing, and then updating the graph at 20Hz refresh rate. My ...
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2answers
52 views

Why the angle of a real signal in frequency is continuous?

This original signal is shown below (time domain): It is a real signal. According to the signal processing theory, the form part of the angle is conjugate to that of the latter part. That is, $\...
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2answers
154 views

Auto-correlation function, an inverse problem

$x[n]$ is a complex function $n=0,1,2,\cdots,L-1 $ we assume $x[n]$ is periodic in its index: $x[n+L]=x[n]$ Its auto-correlation function $C[n]$ is uniquely defined as: $$ C[n]=\sum_{i=0}^{L-1} x[i+...
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81 views

non-negative FFT spectrum, subtlety

definations $x(t)$ is a function of time. Physically, it can be voltage, displacement, magnetization and so on. It can be real, complex, vectors or more fancy numbers. $C(t)$ is the auto correlation ...