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
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{...
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1answer
858 views

DFT and multiplication/convolution equivalence

Is there a simple or potentially intuitive explanation for, with the DFT, vector multiplication in one domain being equivalent to circular convolution of the transforms of the vectors in the other ...
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1answer
604 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
293 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 ...
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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
361 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: ...
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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 ...
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1answer
1k 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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197 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
308 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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389 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
885 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....
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3answers
791 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?
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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
191 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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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
158 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 ...
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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 ...
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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 ...
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1answer
987 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 ...
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2answers
362 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
290 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 ...
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1answer
367 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. ...
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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 ...
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1answer
1k 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 ...
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1answer
214 views

Is it true that convolving a 3x3 matrix and a Full HD image is slower with FFT?

Is it true that convolving a 3x3 matrix and a Full HD (1920×1080) image is slower with FFT, than with normal for-loops? Because you have to do zeropadding to get to a power of 2? (with the traditional ...
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2answers
2k views

Artifacts in FFT

I recently realized that FFT's aren't perfect. Meaning if I take a signal and then take it's FFT, and then do an inverse FFT, the resulting output isn't exactly the same as the input. Here's an image ...
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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 ...
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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 ...
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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 ...
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1answer
630 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
281 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 ...
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2answers
40k 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 ...
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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 ...
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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 ...
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2answers
364 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
332 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 ...