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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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0answers
249 views

Is there any problem with my FFT method?

I am trying to analysis a system by Fourier Transform. my system has 3 dominant modes. (1,0.05) (0.51,0.01),(0.46,0.01) the pairs are defined as (freq,damping_ratio) I've got those result with ...
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0answers
374 views

FFT frequency domain averaging and zero padding

I'm trying to get the average frequency response of an entire piece of audio by splitting it into segments, doing an FFT on each one, and averaging the magnitudes together. Its working great until I ...
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1answer
6k views

Why does an fftshift in the time domain give a different result compared to the frequency domain?

I am trying to speed up some of my code and for physical reasons I need to do an FFT shift to get the FFT data in the standard DFT fourier representation. In Matlab this is performed in the frequency ...
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2answers
11k views

Algorithm to zero pad data before FFT

I've seen some other questions on here about zero padding (like this one) but I'm still a little confused about my situation. I'm attempting to zero pad my input time series data, so I get an ...
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0answers
90 views

how to plot amplitude and phase of IMDs from their fft values?

i have my output as vector of 60000 complex number representing 10 sequence in frequency domain , each of length 6000. i need to plot the phase and amplitude of intermodulation distortion from this ...
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1answer
931 views

FFT and aliasing

I have got a txt file which contains voltage and time data. When I did fourier transform using fft function in matlab I got complex numbers. I understand that the real part is the magnitude and the ...
2
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1answer
496 views

Estimating the wavelength of a sinusoid

I have a sinusoidal signal embedded in noise, and I would like to estimate it's amplitude and frequency, so I can subtract it out. I realize that an FFT can do the job for me, and that's what I've ...
2
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1answer
129 views

Revert anayltic signal(length(N/2)) to real signal (length(N))

As a beginner in signal processing, i'll try to explain my problem most thoroughly. I'll firstly explain the idea of the transformation the real valued data has undergone. This includes the ideas and ...
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1answer
202 views

Similarity in absolute value of nearby DFT coefficients

Is there any approximate relation in FFT coefficients? For example, consider time domain signal $x_n$. Its $N$ point (assume $N$ to be even) DFT is given as $X_0,X_1,...,X_{N-1}$ Can I say something ...
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1answer
1k views

How to calculate the similarity of two electric waves

I have two electric signals from different sources and I think that are similar. I have the FFT of the signals and I have their spectrums of frequency in separate arrays. How should I proceed to ...
3
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1answer
189 views

Missing harmonics (power-line)

I have some biomedical signals. If I calculate their magnitude spectrums, I see power-line interference at 50 Hz. I was excepting a harmonic at 100 Hz, but it turns out there is none. However, there ...
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2answers
4k views

Low pass filter with DFT

This is my first question at this site, please excuse me if I'm not asking the question properly. I have an acoustic signal of a test and I want to remove the "thin" peaks, because they don't really ...
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1answer
104 views

Isolating Bat Species without prior training base

I am attempting a project and I have been given Bat calls (in .wav) which are not ultra-sound navigation calls and I'm looking to isolate the different species of bats and monitor their activity. ...
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1answer
824 views

Units of FFT frequency

I'm having some trouble understanding FFT plots and measures. According to this, this will give me the magnitude plot of FFT, ...
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1answer
176 views

Is filtering necessary if I'm not downsampling?

My data is sampled at $f_s$, and I'm interested in analyzing certain frequencies $0<f_1<\ldots<f_N<\frac{f_s}{4}$. I then take $T$ samples of data, $\{x_1,\ldots\,x_T\}$ where $T\gg \frac{...
3
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1answer
619 views

Spectral Leakage, Phase and other questions about FFT

I'm currently implementing a voice recognition algorithm using FFT and now some questions came up to me: Spectral Leakage: I know what it is, why it appears and how to solve it using a Windowing ...
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1answer
300 views

What would the behavior be of an FFT without the phase (imaginary part)

If I have a sound recording with a sinusodial frequency with a constant amplitude and I do an FFT on the time domain of this recording to get the frequency back: If I were to ignore the imaginary ...
5
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2answers
2k views

Zeroing bins before IFFT

Endolith's recent question on zeroing FFT bins got me thinking about a possible error in my workflow and I'd like to double check. Consider a signal $x_i[m],\ m\in\{1,\ldots,M\}$, i.e. $M$ samples ...
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2answers
367 views

Given an experimental FFT plot, how to find the best match from a set of generated FFT plots?

What I'm trying to do is, given some experimental data and it's (magnitude normalized to 1) FFT, match the FFT with one of say 1000 other (magnitude normalized to 1) FFT plots. Please note that the ...
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2answers
11k views

When should I calculate PSD instead of plain FFT magnitude spectrum?

I have a thirty-second speech signal that was sampled at 44.1 kHz. Now, I would like to show what frequencies the speech has. However, I'm not sure what would be the best way to do that. It seems ...
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2answers
1k views

Offset effect in FFT magnitude

i will use matlab code just for convenience, i get the same behavior using a C++ code. I have two signals: ...
8
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1answer
6k views

1/n octave smoothing

Given a frequency response obtained with FFT, I would like to apply a 1/n octave smoothing. What filter should I be using and how? Maybe someone could point to a good reference (a paper or book on the ...
4
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1answer
2k views

Matching masked images

Let's say I have two images that are parts of the same bigger image, cut using two masks (for simplicity, let's assume that the masks are just rectangles of the same size). The images are presumed to ...
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0answers
111 views

Complex Cepstral Transform and Audio Watermarking

I am doing research in audio watermarking and have come across the paper that uses the complex transform to embed the watermark in order to improve the robustness and inaudibility of the embedded ...
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0answers
199 views

Analysing 3D image FFT

I'm looking for a way to visually inspect the amplitude of frequency components in a 3D image that I have taken the fftn of in matlab. Does anyone have any idea how to go about this?
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1answer
1k views

is root mean square possible to calculate similarity of FFT result?

I need to compare FFT result from audio files, I have 2 FFT result from 2 audio files.. FFT 1 (sample) ...
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1answer
309 views

3D image sample frequency

I am currently frequency filtering a 3D image in matlab, with x,y,z dimensions of different lengths. The matlab filter takes normalised frequency as an argument. What does this mean for an image and ...
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3answers
1k views

Frequency Components of an FFT

I was reading a couple of these threads and I was wondering: Is it possible for an FFT signal to include frequencies less than $1/N$? Consider a time-series of length $N$, and I perform an FFT on it....
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2answers
3k views

Pattern recognition using 2D fft in matlab

I have a dataset with two columns (width and length, both in milimeter). When I make a scatter plot (please refer to the first figure), I see location of defects in a plane. I want to find if the ...
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0answers
399 views

FFT of irregular shaped data

I have an irregular shaped 3D image that I want to do frequency domain filtering on. Currently I simply zero-pad the data to make it a cube, then do an fft. I was wondering if this was a valid ...
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2answers
904 views

Continuous EQ defined by a curve

I'm trying to figure out how to implement a continuous EQ filter. I have in the past built a parametric EQ and then built a 10 channel EQ using 10 of these parametric filters. This gave nice results....
2
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3answers
815 views

Is possible to compare to audio samples with cross-correlation [duplicate]

Is it possible to compare two audio samples(meaning find word from one in another) realtime with cross-correlation?
2
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2answers
4k views

Cross-correlation peak

How get cross-correlation peak and based on it calculate correlation score for similarity of two audio samples. SO far I've FFT two samples complex conjugate second multiply results IFFT cross-...
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3answers
3k views

How to check the FFT results of a sine wave?

I have been given an audio file (sine wave) 1000Hz as an input to my FFT algorithm. I have got 8192 power spectrum samples in a array. What is the best and easiest way to check whether my output is ...
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2answers
192 views

Finding audio data in recording

I am trying to find a specific block of data that should hold valid audio data that I want to process. For instance, I record at 44.1kHz, but I only transmit a .5s of content. Since the buffer I store ...
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2answers
2k views

Why the carrier frequency does not appear after AM modulation?

Okay, this might be a very basic question, but I'm completely new to the field. I have a $40$ Hz sine wave ($1000$ samples, $fs = 1$ kHz), which I have amplitude modulated to a $4$ Hz sine wave ($...
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0answers
76 views

Period of non-harmonic Fourier sum

I am wondering if there is a formula for the period of the sum of $N$ cisoids of the form $e^{j\frac{2\pi}{N} i\, \omega_k}$ where the frequencies $\omega_k \in \left[-N/2,N/2\right]$, $k=0,\ldots,N-1$...
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2answers
161 views

processing 16bit audio files [closed]

I currently have a 16bit mono file. I would like to read this data to retrieve only the data chunk part and not the entire file with the headers so that I can apply fft on it. I tried finding ...
0
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1answer
3k views

comparing 2 audio files in c#

I have 2 audio .wav files recorded by the the same person with the same word. Each audio file has a format of 16bit, 44.1kHz and is a mono channel. I would like to know how to compare these 2 audio ...
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1answer
6k views

Applying FFT to PCM stream

I would like to use a .WAV file and convert it into a spectogram. Any idea how to do this with Naudio? Basically the flow should be like this, I think: Convert WAV file to PCM stream Convert PCM ...
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1answer
1k views

Using DFT Circular convolution property

I am trying to make proper use of the circular convolution property of DFT. I was taught that the DFT of x[n]*CircularConv*y[n], would be equal the product of the ...
1
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1answer
3k views

Frequency response <-> Impulse response

I am trying to calculate an actual frequency response from a desired frequency response. fftSize = 128; fs = 8000; I have 2 input arrays: f[] and aDesired[], ...
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1answer
2k views

Low pass signal filtering using FFT or simple infinite impulse response filter

I am currently in need for filtering accelerometer data for an Android application. First of all, I used a simple low-pass filter (simple infinite impulse response filter) as follows: ...
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2answers
4k views

Downsampling - upsampling FFTW

I'm having 2 signals which have different sampling rates i.e., 1ms(A) and 4ms(B) and I've tried to upsample/downsample either of the signals based on the code snippet Resampling based on FFT which I ...
2
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1answer
1k views

How do I convert a filter from Spatial to Fourier domain?

I have a small 2D Gaussian image filter that I am currently applying as a window by convolution. I would like to do this in the Fourier domain as a single multiplication. I would like to convert my ...
0
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2answers
384 views

Cooley-Tukey FFT: You don't have to zeropad to a power of 2? [closed]

Someone wrote "The algorithm that Cooley and Tukey presented in their classic paper (Math. Comp. 19 (1965), 297-301. http://dx.doi.org/10.1090/S0025-5718-1965-0178586-1) can be applied to any ...
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2answers
301 views

Does the 'FFTW' have a cost of k*N*log_2(N)?

@Michael C. Grant wrote that "the cost of FFTW isn't an easy formula based on log_2 anymore." But the Wikipedia article says that the FFTW "can compute transforms of real and complex-valued arrays of ...
2
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1answer
374 views

Envelope following using FFT bin power -> Low pass effect on the modulation transfer function

I am deriving the temporal envelope of a signal (in a particular frequency band) by summing some FFT bin powers and taking the average, I do the FFT frame by frame, windowing and some overlap. ...
10
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3answers
9k views

FFT for a specific frequency range.

I would like to convert a signal to frequency domain. The desired frequency range is 0.1 Hz to 1 Hz and the frequency resolution ...
1
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1answer
2k views

FFT: Does the result of $N \log_2(N)$ stand for total operations or (complex) adds and multiplies?

Calculate the amount of operations you need for the Fast Fourier Transform. Take $N=2048 \times 2048$ $N\dfrac{\log(N)}{\log(2)}$ $\dfrac{\log(2048 \times 2048)}{\log(2)}$ is 22. $(2048 \times ...