# 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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### Created Matlab echo signal: Is it an actual “echo?” Or am I just manipulating the signal to make it seem like an echo?

This is just an outline of what I did (assume that all my dimensions fit): ...
402 views

### Amplifying part of a signal

If I want to amplify a certain frequency in a digital signal, can I just take the FFT, increase the real part of the value for the frequency I want (leaving the imaginary part alone), and then take ...
826 views

### I have a mix signals (2 signals with different dc offset). How to recover another dc value which has been buried by the stronger dc value?

Let say I have a time-varying signal which is a mixture of two signals with different dc offsets. If I apply the fft on that signal, I will only get the dc value (at f=0hz) with stronger energy. How ...
795 views

### FFT analysis of longer data sets

I have a problem with a FFT analysis in MATLAB, which is probably related to my limited understanding of the fundamentals of Fourier analysis. I use MATLAB’s built-in fft analysis function. The ...
10k views

### MATLAB - remove the frequency at zero in FFT

I am getting a high frequency at 0Hz. How can I remove it? I am not supposed to get it. My code is ...
598 views

### Polarity inversion in frequency domain (polar coordinates)

I don't fully grasp how polarity inversion is performed in the polar coordinates of frequency domain. The frequency components of the signal do not change, so the amplitude part is going to stay ...
214 views

### Noise analysis with periodogram. problem with different resolutions

I don't understand what happened in my simulation. I would like to perform a trade off between 2 ways to estimate power in frequency bands. I want to detect spurious frequencies. First method: I ...
417 views

### Looking for an algorithm that computes weighted average of frequencies intensity from narrow band in spectrograph

I am searching for the algorithm that takes (audio) signal as an input and outputs weighted intensity of the frequencies of some narrow band (axis X) in time (axis Y). Frequency in the middle of that ...
3k views

### Setup frequency array properly

I've just stepped into the FFT's world since the last weeks, therefore I still don't have an clear overall idea of how to setup the frequency array. In case of even...
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### At which point in the signal processing chain to use Hann Window?

I have a question about signal processing. Experienced with embedded design and code, but DSP is in new territory. Thus far I am using the CMSIS DSP library functions, which are working well. I am ...
154 views

### filtering out specific waves in scattering pattern

I have the following (experimental) image, with its correponding raw data in .csv format here. The pattern represents the scattering of a laser field off a point defect located in the center. One can ...
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### What is the rate of change of amplitude? Frequency?

I know that frequency is not the same as the rate of change of amplitude, but why? Surely a high frequency is when your amplitude changes rapidly. Does an FFT perform something similar to ...
127 views

### What are some ways to detect the fundamental frequency in a signal using without using FFT?

I am trying to estimate the fundamental frequency in a signal to corroborate the result which I have got by FPGA implementation of an FFT algorithm. Hence, could someone give me some idea how to do so?...
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my field of engineering is condition monitoring. I use vibration analysis to determine the current state of health of a machine and then recommend any course of action required to ensure it does not ...
114 views

### deciphering the frequency response from the input data

Newbie to Signal processing, sorry if the question is basic. I have a data set consisting of 601 values. The values being time Vs voltage . The voltage values have transients in them. My aim is to ...
178 views

### FFT filter to remove cd scratch

I have a .wav file which came off of a cd with a scratch, so there is a periodic "clicking" of about 6 Hz. I was playing around with trying to squash the clicking by taking FFT and filtering the ...
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### Understanding IIR , Low Pass, High Pass, Chebyshev, Elliptic Filter? [closed]

I see these terms everyday in every lecture, I do read stuff from wikipedia, follow lecture notes, study solved exercises. All in all I do have a rough idea about these, I know a little from ...
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### Inverse Chirp Z Transform

I am working to understand and use the Chirp Z-Transform. I want to use the algorithm for simple signal processing on data sets that are not a power of two. I need to be able to inverse transform as ...
638 views

### Inverse FFT data format with FFTW

I perform FFT of two real sequences using "two for the price of one" - array $x[n]$ and $y[n]$ is packed as a complex number - $z[n] = x[n] + j y[n]$ - using FFT2 and FFTW. I am able to decode the ...
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### FFT Plot Axis Confirmation

I have a 4096 sequence of FFT coefficient. All of which a real values. My sampling frequency is also 4096 samples per seconds. Now I want to make a plot. I know my Y axis would be my FFT magnitude ...
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### frequency response of a signal in which two signal added in

Here goes the problem I faced: Sig1= A*sin(w*n) Sig2= B*sin(w*n+phi) Length of both signal is L. Then, I put them in ...
54 views

### Dimensions of a spectral representation [closed]

I'm currently trying to perform analysis of a spectrogram, with the use of PCA. I'm confused about what the dimensions of the result should therefore be. Currently, the dimensions are: ...
777 views

### Which come first, PCA, FFT, filtering

Might be a simple question, but I keep second guessing my algorithm, so a confirmation would be good. I first do bandpass filtering, then PCA, then FFT of the top principle components. I also do some ...
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### relationship between block size and detected frequencies in goertzel algorithm

I'm trying to use the goertzel algorithm to detect a pair of frequencies in a signal. The sample rate I'm using is 8000 hz, and the size of the incoming sample buffer is 60 samples. I have a large ...
153 views

### Relating bin numbers to frequency non uniform fft

I understand how to relate bin numbers of FFT output to frequency when dealing with a uniform FFT. How does this process work with a non-uniform FFT?
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### Echo hiding technique

I'm trying to figure out how to use DSP techniques in order to both add artificial echoes to a sound file, but more importantly to be able to detect these echoes. I've been reading about it here: http:...
3k views

### How to get fundamental frequency of a signal using autocorrelation?

I am trying to get the fundamental frequency of a signal that only has a single pitch. I coded out the autocorrelation function using FFT and already got the autocorrelation result. Unfortunately, I ...
920 views

### Android: fundamental frequency

I want to find the fundamental frequency for human voice in an Android Application. I want to use autocorrelation method. My code is this: ...
592 views

### Comb-like peaks in frequency spectrum

I am currently working on a project which involves measuring the sound produced by the onset of a vapour bubble in boiling water. All the experiments went perfectly well and I'm now trying to obtain a ...
766 views

### How to deal with signal not equally spaced in time when performing FFT?

I wonder to know what is the best way to handle not equally spaced in time signal when performing FFT ? I guess it depends on the signal itself. I work with signal with about 1000 - 5000 samples and ...
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### Why FFT does not retrieve original amplitude when increasing signal length

I don't understand why FFT return different maximum amplitude as the signal length increase. I would except that with a large signal length according to the frequency, detected amplitude will be very ...
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### Using FFT to Unit Test a Music Player

I'd like to validate changes to our app music player in our Continuous Integration unit tests (more like integration tests, really). My thoughts were to start with an audio file that contains only a ...
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### Confusion on using FFTW 3.3.4 (C version) for real and even data sets

I am using FFTW3 to transform the autocorrelation function of a particular function. I expect the transform to be real since the autocorrelation function is symmetric with respect to negative and ...
813 views

### Autocorrelation Function and Power Spectrum

My aim is to determine the power spectrum of a signal of finite duration using Its autocorrelation function Performing FFT directly on the time-domain data The function is defined as follows in the ...
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### fft(x) different from fft(ifft(fft(x))?

I normalized the spectrum of a time series and windowed the spectrum, then something strange happened. The code below can run in MATLAB or Octave. ...
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### FFT of Square Wave and Sin Wave

When we have FFT of Square wave of a pure real signal with high frequency(close to f(max), less than half of sampling frequency) we see only 2 symmetric high peaks but when we decrease the frequency ...
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### Is it necessary to taper two ends before fft a time series?

I saw in some codes people always taper the ends of a signal using a cosine taper. Is that necessary and why?
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### Analyzing Spectrum of a signal?

my_signal = 8192*cos(2*pi*6*(0:7)/8) + 1i* 8192*sin(2*pi*6*(0:7)/8); my_signal_fft = fft(my_signal); plot(abs(my_signal_fft)); This is matlab code. What do you ...
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### Pitch Detection: HPS not working

I'm trying to code the HPS (harmonic product spectrum) algorithm and the problem I'm facing is that using my guitar, the fundemental frequency (Here it's 82Hz) just isn't there, so is the 5th Harmonic ...
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### Filter Banks and FFT, are they similar conceptually?

I'm using Filter Banks (Narrow Band Pass) created using the functions in Matlab. However, I wanted to know if there are fundamental limitations like that of FFT which also affect the Filter Banks. Is ...
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### the difference between dft and dfs

in the literature i've found that dfs and dft are one and the same.if they are one and the same why to use two different names for them? if there is really a difference what is it and what is the ...
525 views

### What is 'Normalized frequency in the range [0,1)', à la DTMF & Goertzel algorithm

I have a functioning DTMF program. It takes a larger signal and breaks it up into many small components, then analyzes each segment with an FFT to determine if the magnitude of the response for a ...
123 views

### Compressing A Range Of Decibel Values to One “Representative” Value

Don't have any formal dsp experience - sophomore in college. Working on constructing a 25 band bark spectrogram for psychoacoustic analysis of audio FFT data. To do this, I'm grouping all FFT ...
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### Sin wave spectrum shows multiple frequency

I am very new to DSP, so excuse my ignorance. Using Audacity, I generated a sine wave of: frequency 440Hz amplitude: 0.8 duration: 1 minute Now when i plot the spectrum for this sine wave, I get ...
1k views

### Bit reversal on twiddle factors on inverse FFT

I am currently working on a application which requires an inverse Fourier transform (IFFT) and am using a function library provided by Texas Instruments, in the datasheet they give an example on how ...
1k views

### Design Filter Banks for every 100 Hz

I need to study a wave file for every 100 Hz. However, I dont know how can I design filer for each 100 Hz. Let's say 0-100 Hz, 100-200 Hz, ... , ((fs/2)-100) _ (fs/2) Hz. Is it any MATLAB code for ...
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### Can the Fast Fourier transform be used to compute the magnitude of moiré

I am experimenting with image down-sampling and would like to calculate a numerical magnitude for alias-induced moiré patterns. Is it possible to do this by analyzing the frequency-domain (FFT) of ...
23k views

### Extracting frequencies from FFT

I performed 512 point FFT on a signal. I got another set of 512 Numbers. I understand that those numbers represent amplitude of the various sine and cosine waves having different frequencies. If my ...