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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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88 views

### Theoretical question on Fast Fourier

I have a question regarding Discrete Fourier Transform and Discrete Time Fourier Transform. Suppose that we have a sequence $x(n)= \{1,2,3,3,2,1\}$. So is it possible to reconstruct the Discrete ...
150 views

### How to do a spectral fade out?

I'd like to do a fade out of a sound in a few seconds, but in a "spectral way", with a special curve : (here x-axis is time, y-axis is frequency from a few hz to 20khz). All the part "to be removed"...
17k views

### Cross correlation with FFT and fftshift

When I calculate the cross correlation between two vectors with the following code ...
4k views

### Estimate frequency and peak value of a signal's fundamental

How can I find out frequency and peak value of a signal's fundamental? Please do consider the following: The peak value of some harmonics might be higher than that one of the fundamental The ...
230 views

### How to prove that Dilution of a time domain signal results in duplication in Frequncy domain?

While reading a description on FFT from this book, I found the following statement: ...diluting the time domain with zeros corresponds to a duplication of the frequency spectrum. How can I prove ...
2k views

### Performing classification based on FFT results

It is perhaps important to start with the fact that I am a complete beginner in DSP. I have got a number of audio recordings (of sampling rate 22 kHz) - of bird songs - which I have been trying to ...
151 views

### why if we pad any signal with enough zeros we can get the same result

if we pad any signal with enough zeros we can get the same result as linear convolution whilst using the fft function we compute circular convolution. Why?
114 views

### Denoise a randomly occuring signal

I have a program which takes in data from an oscilloscope, and due to reflections in the medium, we will get random signals at random intervals. The two grey windows at the right bottom show the ...
448 views

### FFT of a periodic function [duplicate]

Suppose you have N samples of a sine wave of frequency $f_0$ sampled at $f_s$. $$x[n] = A\sin[2\pi(f_0 /f_s)n]$$ How do you get a delta function in the frequency domain (i.e. all of the energy is ...
311 views

### M-point FFT Amplitude of N-samples of a sinusoid

Suppose you have N samples of a sinusoid of frequency $f_0$. $$x[n] = A\sin[2\pi (f_0/f_s)n ]$$ Suppose you take an M-point FFT (where M may be greater than N) of your N samples of x. What will ...
1k views

### How does one know how many times to downsample in Harmonic Product Spectrum?

I have been working on a Harmonic Product Spectrum Algorithm. Now, all the literature I've read about the subject tells me to downsample an N amount of times. How does one determine what this N value ...
77 views

### If I want to apply a filter to an Image should I use the IIR or FIR filters?

So I have discrete pixel values in the image and each row or column would make a signal. I would like to filter some of the rows or columns to remove high frequency components from them. Should I use ...
175 views

### How do you choose a size for a windowing function in FFT?

How does one choose the proper windowing function to apply in an FFT? What are the benefits of larger sizes and smaller ones? Which would be the best for pitch estimation? Also which window function ...
271 views

352 views

### Can you quarter the processing time for real, symmetric FFTs?

The DFT of a real signal is Hermite-symmetric, so you can roughly halve the computation time/memory by not bothering to calculate half the values of the spectrum (and complex conjugating the existing ...
2k views

### Why use a gaussian low pass filter, when we can convolve with gaussian kernel

Sorry if this question is too simplistic, but I am new to image processing and after successfully writing two programs, a convolution program and a low pass filtering program, I noticed that for ...
1k views

### DFT and then IDFT does not provide the same signal

I'm new at Fourier Transforms. I'm using AForge.Math FFT/DFT algorithm (C#). Said that I have this situation: My signal: $$1.4 + sin(2\pi t) + 0.4sin(5·2\pi t)$$ Sampling: 100 samples each second....
480 views

### Averaging frequency spectra gives incorrect RMS

I am averaging several FFT results together in an attempt to get the average frequency spectrum of a track of audio. I'm attempting to use Parseval's Relation to get the RMS of the audio from the ...