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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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3answers
20k views

Frequency spectrum of a set of images in Matlab

I wish the compute the frequency distribution of a set of images. I basically am keen on finding out how the the frequency varies in the images. I know I can use the 'fft2' function to compute the 2-...
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1answer
3k views

frequency domain phase shift

While going through FFT I came to know that, shifting of phase in frequency domain results on shifting of time in time domain. According to this, in wireless communications, from the transmitter side ...
4
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3answers
2k views

Basics of frequency spectrum for onset detection using FFT

I'm pretty new on signal processing and I've been reading a lot the last days in order to understand a little about it. But still there are things that I'm not sure if I got well. Sorry if I don't use ...
7
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1answer
3k views

MFCC feature extraction

I have a block of audio and I have performed an FFT on it. Now what I want to do is convert this FFT into a set of MFCCs, however while I know I need to do something with weighting the FFT samples ...
3
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1answer
657 views

Zero-Padding seems to CHANGE FFT of signal!

I am having trouble with zero-padding. As I understand it, in theory, zero-padding in the time-domain should 'sinc-interpolate' the FFT of the signal in the frequency domain. This would mean, that 1)...
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2answers
284 views

Frequency array feed real values FFT

EDIT The first version of my question has not worked as I expected, so that I will try to be a little bit more specific. The final goal I am trying to achieve is the generation of a ten minutes time ...
2
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2answers
41k views

Removing noise from audio using Fourier transform in Matlab

I'm trying to remove noise from an audio file. This audio file contains speech as well as constant pink noise. I know that I have to use the Fourier transform to convert to the frequency domain then ...
9
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4answers
1k views

DFT-like transform using triangle waves instead of sin waves

We know that DFT (discrete Fourier transform) breaks down a signal into multiple frequencies of sine waves. Does there exist a transform that does the same thing, but for triangle waves? For my ...
5
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2answers
3k views

Simulating analog filter using convolution or converting with fft

My task is simple; I want to simulate analog low-pass filtering of an input signal, using Python. Note that the input signal is an array of values, not an analytical function. My first question is if ...
5
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2answers
367 views

What is the uHz-rotator algorithm?

According to this page, the Baudline software tool implements ...the uHz-rotator algorithm that operates in the complex domain to calculate an extremely accurate estimate of the frequency. ...
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0answers
337 views

What are the differences between the various FFT algorithms? [closed]

Wikipedia lists a number of different FFT algorithms: Cooley–Tukey FFT algorithm, Prime-factor FFT algorithm, Bruun's FFT algorithm, Rader's FFT algorithm, and Bluestein's FFT algorithm What are the ...
3
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1answer
9k views

Spatial and temporal frequency of a wave

I've sampled a propagating wave so that I know its amplitude at several equidistant points on a line, and at several points in time. That is, I have this discrete function: ...
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2answers
724 views

2D FFT of Vector

I have a bunch of 8x8 images which have been vectorized and formed into a 64xN matrix, X. I'd like to take the 2D-FFT of each of these images without reshaping each column of X into an 8x8 image and ...
4
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1answer
13k views

What do I do with the result of an FFT?

So I am working on a project using digital signal processing, which I am not familiar with at all, and a lot of this is all new to me. I am using SciPy\NumPy to play around with code to understand the ...
5
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3answers
953 views

Determining the mean of of a sampled signal (improvements to time averaging method)

I am sampling a signal at 1kHz, which is essentially a single-frequency sinusoid with a DC offset (and a small amount of noise). I don't know the precise frequency of the sinusoid (it is at most 10Hz),...
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0answers
161 views

How to prevent Signal drifting when changing frequency

When I recreate a signal it seems to look great. But when I try and increase it's frequency (which is done in a for loop and the variable new_freq=2) it starts to drift. See Image below (The top ...
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1answer
4k views

retrieving original data from phase and magnitude of Fourier transform

I use this snippet of python code to transform data to Fourier phase and magnitude and then retrieving original data. ...
4
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2answers
1k views

Optimizing FFT code for 12 bit processing on microcontroller

I want to port the following FFT code on a microcontoller having a very little floating point capability. I want to perform FFT of the output from 12 bit ADC. Since my values are all 12 bit numbers it ...
4
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1answer
299 views

Discrete Fourier Transform a dummy approach

I'm trying to understand how DFT works exactly. However, when experimenting around, I compared both Matlab generated FFT result with a dummy approach result and I get similar result. However, the ...
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2answers
1k views

Solution for FFT size 1536

I have already asked this question 2 times, but still i am confused because this is not available anywhere on the internet and explained properly. I found some information in this document and i am ...
1
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1answer
5k views

Determining sampling rate in OFDM demodulation

I have seen that in OFDM, the sampling rate is much lesser than twice the bandwidth of signal. why is it so? How is sampling rate calculated in OFDM? Does it depend on the modulation type used inside ...
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1answer
10k views
5
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3answers
7k views

Effect of windowing on FFT magnitude

I have 2 sin waves: Fs = 1500; N = 100; t = (0:N-1)'/Fs; sinwave = sin(2*pi*15*t); sinwaveshifted = sin(2*pi*15*t + 1.3); If I get the magnitude at 15Hz using ...
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2answers
144 views

Do I get less accurate results if I do not use a power of 2 points for my FFT?

I heard that the padding with zeros is mainly for efficiency and speed, other than that, is there a downside to use fft(signal) instead of fft(signal, N) where N is a power of 2 assuming signal length ...
3
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3answers
9k views

Am I understanding the phase angle from FFT correctly?

Long story short, I have a sinusoidal wave with DC offset going in with a given frequency of say 100 Hz and $\phi = 0$ (which I'm guessing is really $\pi/2$ since it is a sine wave not cosine). I get ...
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2answers
209 views

Ways to analyze electrical signals without FFT?

I am collecting a 15Hz sine wave at 1500Hz. Therefore, every 100 points of data collected should contain a single complete sine wave. If I sample for 10 seconds, I will have 15000 points of data. If ...
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3answers
11k views

FFT Zero Padding - Amplitude Change

I'm just learning about Fourier Transforms and as an input I'm using a WAV file (Matlab) and taking one channel of it and performing a quick Fourier transform. I've zero padded the input, as I've been ...
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2answers
986 views

Sense of zeropadding in a time domain

I have the task related to Radon transform which contains a subtask which uses resampling by means of DFT. Let's consider the non-periodical discretized signal (Fig.1) (for example the string of ...
4
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1answer
4k views

Why can the FFT always be mirrored in the middle of the x axis? [duplicate]

Plotting the FFT of a signal with Matlab e.g. by the code: y_fft = abs(fft(y)); f = fs*(0:length(y)-1)/length(y); plot(f,y_fft); where ...
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0answers
262 views

Points of interest in a spectrogram

We have several audio tracks, all of which are different versions of the same track under one type of distortion: the speed of the track is increased by a constant factor K (hence frequences are ...
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3answers
2k views

What is the relationship between frequency vector and the peak frequency in FFT?

I am fairly new to both Matlab and FFT. I would like someone to explain to me the relationship between the frequency of the wave being processed, the number of point (usually called NFFT), the Nyquist ...
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1answer
216 views

FFT function isn't working on imported signals greater than a second

I have an fft function in matlab/octave that works great on an imported mono wav/audio signal that is 1 second long, but when the signal imported is more than 1 second the frequency values become ...
4
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1answer
57 views

Transform Length and event duration

Does the length of a Fourier Transform affect the ability to discern a small event in the data? This is a tricky question to ask, let me clarify. I don't care about when the event happened and I don'...
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0answers
586 views

How to convert a sound file to any other data format which can be manipulated

I know this is very basic question and i am new to this site. But can anybody help me with my question. How to convert a sound file to other data format say using some type of transform (FFT or DFT) ...
3
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1answer
2k views

Extract sharpness from FFT

I've got in total 6 images, from 2 cameras (thus 3 per camera) storing 16 bit data. What I need to feedback to the user is like some sort of sharpness coefficient, preferably live while he is ...
1
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1answer
128 views

fft result order compared to direct implementation of DTFS using text book

To find the complex Fourier coefficients of this sequence, then The $k^{th}$ coeff. $c[k]$ is given by the DTFS formula $$ c[k]=% %TCIMACRO{\dsum \limits_{n=0}^{N-1}}% %BeginExpansion {\displaystyle\...
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1answer
860 views

Fast fourier transform and frequency domain spike in wrong area

I used MATLAB to produce a simple sine wave over time: ...
3
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0answers
137 views

Compensate for standard deviation loss

I am not sure if this question will be a little off-topic on this forum, that I will give it a try anyway, since it implies signal process arguments. By using ...
5
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1answer
853 views

Constant Magnitude Chirp across audio band (Parseval's Theorem?)

I want to excite a loudspeaker with a constant magnitude chirp that sweeps across all frequencies in the audio band. I downloaded a test file from this website to start with. I plot the log magnitude ...
5
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2answers
573 views

Autocorrelation Jitter

I'm doing autocorrelation (via fft) on pure, synthetic generated sinusoids and getting unexpected jitter in the results. The jitter depend on where in the sine the signal buffer is started. I get ...
1
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1answer
61 views

How to get some law parameters from its FFT figure?

Given an hidden law $$I(t)=\frac{1}{2}[A+B.cos(2\pi\nu_0t+\phi_0)]$$ I observe its signal with 256 samples and compute its FFT over this interval, and represent the FFT result from 0 to 1 (normalised ...
2
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1answer
2k views

About voice filtering

I have a waveform containing voice some children screaming some times when there is no noise at all some times there are cars driving by when talking I want to detect the start and ending times of ...
2
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1answer
151 views

Guitar String Replucks

I'm analyzing guitar string plucks and sustains. I'm having good success with auto correlation using FFT's. Now I'd like to detect plucks while the string is still vibrating. Since I already am ...
3
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3answers
1k views

Estimating Low Frequencies (including amplitude and phase)

For the past month I've been chasing the DFT/FFT rabbit. Along the way I've been cut-n-pasting the name of other rabbits to chase later: Autocorrelation, Phase Vocoder, Parabolic Approximation, ...
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2answers
257 views

FFT Data Array Indexing [closed]

I've been Googling "FIR filters" and "FFT convolution" etc. and I ran across something interesting. Array Index vs. Pointers: http://www.bores.com/courses/intro/program/7_array.htm http://www.dspguru....
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1answer
3k views

Finding periodicity of spikes, matlab

In my time series data, I have often spikes with regular periods. Sometimes, I get two or three different periodical spike sequences (two electrical noise sources), some with T1 seconds period, some ...
3
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0answers
773 views

What is the proper way to select the time-bandwidth product when doing multi-taper spectral estimation?

Frequently programs for calculating the spectrum of a signal using the multi-taper method require the user to specify the time-bandwidth product. What considerations must be made when selecting the ...
2
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2answers
3k views

How to predict the spectrum of an aliased/folded signal without looking at the FFT?

This seems like a trivial concept but I just can't get the hold of it. I have a signal with two complex sinusoids having frequencies -25MHz and 17MHz. Currently it is being sampled at 64MHz so I get ...
5
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2answers
4k views

Complex impulse response functions?

I think I must have misunderstood something regarding the relation between impulse and frequency responses. I have calculated the Impulse Response Function, IRF, from the Frequency Response Function,...
3
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0answers
387 views

Discrete Fourier transform in a multidimensional space

I want to measure the frequencies at which a point oscillates in a multidimensional space, let's take the example of a point on a 2d-surface. For now, I naïvely split the signal in two, along the ...