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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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1answer
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 ...
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2answers
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"...
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1answer
17k views

Cross correlation with FFT and fftshift

When I calculate the cross correlation between two vectors with the following code ...
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4answers
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 ...
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1answer
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 ...
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1answer
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 ...
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1answer
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?
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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 ...
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3answers
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 ...
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1answer
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 ...
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2answers
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 ...
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1answer
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 ...
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1answer
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 ...
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1answer
271 views

How to detect specific behavior in time series?

I was not quite sure what the right SE for this was, so I posted this also here on SciComp. Please tell me which one to remove :) Problem statement I have a few hundred unrelated time series, say $...
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0answers
143 views

Causal bandstop filter

I have a vector of zeros and ones with length ~2000. I need to detect and filter out the once, that appear periodically. There may be none to 3 such periodic signal's components. Typical periods are ...
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0answers
646 views

What is the best pitch (frequency detector) for a Voice Activity Detection Algorithim?

I am programming a Voice Activity Detection algorithm. After researching several methods, I implemented a simple method that determines whether or not the person is speaking based off the RMS of the ...
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1answer
21k views

Why should one use windowing functions for FFT?

So I just revised my pitch calculation algorithm using a harmonic product spectrum algorithm. I was just curious about why this explanation of Harmonic Product Spectrum states that you need to ...
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1answer
77 views

Is there a general relation between mean power and period

I posted this question at Math Stack Exchange a few weeks ago without any luck. It occurred to me I might have more luck by posting it here. The basic question is whether there exists a known ...
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1answer
637 views

How to convert a filtered time-domain signal into frequency one? [closed]

I have a signal in matlab like this: I have designed a band-pass filter in 16-32 MHz range: and applied the filter to the signal, the result is like this : My Question: what is the easiest way to ...
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1answer
536 views

Are the DFT/FFT multiplications in the fourier domain complex multiplications?

If you want to compute a convolution with a DFT/FFT, you will have to do a DFT on the image and a DFT on the kernel. And you will need to multiply the spectrum of the image with the spectrum of the ...
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1answer
1k views

Using fft in matlab to retrieve input signal magnitude

I am trying to understand how you use an FFT in matlab to measure/retrieve the magnitude of the input signal. For my understanding I have put this script together, creating a 15Hz sine wave with a ...
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0answers
186 views

Constant phase for each frequency?

FFT of some signal $x$ has two parts: $|x|$, which is the magnitude spectrum and $arg(x)$, which is the phase spectrum. I want each frequency to have the same phase. Is there a way to accomplish this?
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2answers
92 views

DSP Topic for Newcomers

Does any one of you know what topic to teach people related to signal processing. The audience is mainly for a group of software programmer working on a telecommunication product who have limited ...
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0answers
76 views

Get excitation from two signals

I have two signals, one whisper data and one normal voice data of the same sentence. I'm trying to build a voice conversion, but just want to start off with some simple signal manipulations. I ...
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2answers
3k views

Confirmation on how to calculate phase differences at every frequency point between two wideband signals?

I have a problem that I thought was going to be simple, but it has become surprisingly stubborn and I question my method... I have used the method (described below), but I wanted to confirm that ...
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1answer
2k views

simplest Notch filter in the world

What do you think of this notch / band reject filter for a very very narrow band ? ...
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1answer
994 views

Guitar Tuner or Guitar effects Algorithms? [closed]

I need to explore the algorithm of the digital guitar tuner or the digital guitar effects algorithms. Is there any Book or articles that you recommend of? All I know is that relate to DSP. thanks
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1answer
2k views

Preparations to implement FFT on a wav file (code Java, not Library)

I am making preparations to implement FFT on a wav File.wav Only Java. I'm reluctant to use matlab I have a File wav stereo, 16 bits. I need to calculate the FFT, but I have confusing. My File has ...
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1answer
2k views

How to process video with DFT/FFT - getting the frequency of green color changes?

Goal: I have a video stream from a webcam. I need get frequency of the green color changes on the video. Video color space is RGB The signal looks sinusoidal with a frequency range 0.5 - 2.4 Hz I ...
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2answers
695 views

Implementing FFT in Matlab

I am facing a trouble implementing the fft and appreciate your advice. My data structured as follow: The data are retrieved from the share market for one day change in the share price. The time range ...
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1answer
202 views

what I'll get if I take FFT of a frequency domain data

What I'll get if I take FFT of a frequency domain data. I have two questions related to this. For ex, x is a time domain data where X = fft(x), now fft(X)= what i'll get ? While reading SCFDMA, data ...
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4answers
8k views

Algorithms for computing FFT in parallel

I am trying to parallelize the computation of an FFT on terabyte-sized signal files. Right now such an FFT using an open-source library takes many hours, even running through CUDA on the fastest GPU I ...
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1answer
4k views

defining the frequency range with fast fourier transform

In the following example I have used various examples found on the web to understand fft and detecting signals with matlab. ...
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1answer
507 views

Algorithm to find spike

I've been doing FFT's (on sound) for a while and usually use the highest intensity or the ones above average to get the spikes in an FFT result. What would be a good method to get all the 'spikes' in ...
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0answers
881 views

Matlab: Estimating power spectral density of an experimental data?

How do I estimate PSD in matlab ? There are lots of methods to estimate, depending on application. I am using an experimental data to identify faults ( e.g. interturn fault of electric motor), which ...
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1answer
784 views

How to do smoothing without affecting phase

I have signals captured from two channels of a measurement system. There is phase and magnitude differences between these two signals. I want to apply nonlinear smoothing to the magnitude, but the ...
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1answer
388 views

spectral division

Sorry if this is elementary. If I add a small sinusoidal signal to my data and then take FFT of both my original and the modified one and then inverse FFT the division of the two, I should then be ...
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1answer
5k views

Advantages and disadvantages of Hilbert-Huang Transform versus Fast Fourier Transform? [closed]

Which one is better for frequency detection? In what circumstances? Advantages and disadvantages of both? Which would be better for a Voice Activation Detection algorithm? Which would be better for an ...
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2answers
4k views

Time domain interpolation using FFT with zero padding on the end

I've got a situation where I'd like to use an FFT to do interpolation in time on some complex data (I need to go to the frequency domain anyways to window my data). The notional way of doing this ...
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1answer
930 views

Gammatone filter vs 2D FFT for spectograms

I am working on speech segeration and each source I am reading , spectograms are being made by filterbanks which uses gammatone filters . I can't find a proper c++ source code which would make me ...
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1answer
2k views

Understanding FFT bin resolution

For a signal sampled at $f_s$, the frequency resolution (or "bin width") for an $N$ point FFT is $f_s/N$. Does this mean that the $k$th bin will contain energy from sinusoids within $0.5f_s/N$ on ...
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2answers
4k views

Phase of the FFT of a sine function

I'm trying to wrap my mind about what happens in Matlab/Octave when you compute the fft of a sine function. Theoretically, the Fourier Transform of a sine is given by two delta functions multiplied by ...
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3answers
613 views

Does convolution done via the FFT _undo_ computational gains when signal has zeros every other sample?

Suppose we have filter $h[n]$, length $N$. If I use it to do convolution in the time domain, then there will be $N$ multiplications each time during the sliding. (Ignore border conditions for now). ...
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1answer
183 views

Add harmonics to a FFT decomposed signal

i would like to add even harmonics from the data of FFT of a signal. How do i do that ? do i have to search for higher magnitude frequencies then add the $2f$ then $4f$ etc ? I understand how to do ...
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2answers
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 ...
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1answer
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 ...
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1answer
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....
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0answers
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 ...
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1answer
5k views

Using FFT to extract a specific frequency range in Matlab?

I have a signal and from that I want to extract a specific frequency range, e.g. 10Hz to 15Hz. Now I heard that I should use Fast Fourier Transformation. I'm going to do that in Matlab, but I have ...
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2answers
494 views

Image processing free open source libraries

This is the first time that I post a question here so excuse me for asking such a beginner question. what free open source <...