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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
674 views

$1/n$ octave complex smoothing

An excellent answer to this post explains how to do $1/n$ octave energy smoothing and mentions complex smoothing can be done as well, but it's tricky business because of phase wrapping. How is phase ...
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0answers
558 views

How do I find the peak frequency of a wav file in python?

I am fairly new to python and signal processing and I was given a task to record audio for 'x' seconds and then find the peak frequency in the audio file. So far I have successfully implemented the ...
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0answers
41 views

Removing DC Offset (via removing mean) of input and output signals affects their coherence

I have input $x$ and output $y$ signals (fs=10 Hz, length N=10800). I have removed DC offset via the following equation: $x_{normalized} = 100 \frac{(x-\bar x)} {\bar x }$ where $\bar x$ is the mean ...
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3answers
1k 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 ...
5
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1answer
100 views

Use of Tukey window and its effect on a graph

I am trying to minimise the noise in a spectrum of a variable. I am trying to use the suggestion of my advisor, who told me to divide the time history in equal parts, to do the FFT of this time ...
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1answer
37 views

For SNR calculation, should I use FFT|X+iY| or FFT|R|?

I use a demodulator that outputs X and Y quadratures, with the R and Phase which can be calculated. For SNR measurements, when plotting the FFT to find the SNR a signal at a certain frequency, ...
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0answers
28 views

Periodic Data Modeling

I am new to DSP and trying to learn its basics. I have a periodic data set (Y) and am trying to determine a numerical model which fits to the data set. I intend to find an equation which - Can ...
11
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4answers
1k views

DFT - Removing window effect in spectral domain with convolution

I was thinking about the DFT windowing subject and a thought came to my mind. A DFT will yield the spectrum of a signal convoluted with spectrum of the window used, therefore having a main lobes and ...
1
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2answers
764 views

How to Match 2 Signals which Are a Shifted and Scaled Version of Each Other

I have 2 signals S1 and S2 that contain the same information, but S2 is shifted and scaled compared to S1 by an unknown amount (but small; eg shift would be of the order of 1-10 samples). What is the ...
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4answers
68 views

How to detect overlapping peaks in FFT?

I am working with a modular spectral processing STFT toolkit I have written in C. I often need to carry spectral peak detection and analisys (frequency estimation, peak displacement, etc) but I am ...
2
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2answers
199 views

Phase difference between signals sampled at different frequencies

I want to know that if it is possible to measure the relative phase difference between a signal that has been sampled at two different locations with different sampling frequencies? Also can that ...
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1answer
45 views

Zero padding in frequency domain, any tricks to speedup?

there are many situations when one has to carry some operations on spectra, like splitting a spectrum Z wide in two Z/2 spectra, or vice versa merging two Z sized spectra in a 2Z sized one. The only ...
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1answer
30 views

Different representations of frequency space of 2D image FFT

I'm learning images processing using FFT. In my test example provided below the input pixel values are clamped 0-1 (0-255), but I do eventually want to process floating point heightfield pixel values....
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0answers
28 views

Understanding outputs of image Fourier Transform in Numpy

I am trying to understand the meaning of FFT outputs with respect to images. Take a look at the following example: ...
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2answers
57 views

Spectral filtering

I am working with a STFT spectral processing toolkit I wrote in C, now focusing on a module for spectral filtering. In its first implementation, the module simply multiplied an user drawn spectral ...
5
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3answers
14k views

Choosing the length of a signal when calculating the FFT

I'm trying to transforming an already know signal from time domain to frequency domain using fft function in MATLAB. And I got different values of amplitude when I ...
1
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1answer
54 views

How to achieve a smoothed frequency phase response with no outliers

I have two sampled signals, which I then perform wavelet analysis on, by synthesising the Filterbank, perform the dft on each kernel, and the dft on the sampled signals, and multiply together. ...
0
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1answer
55 views

How to manually implement convolution with FFTs?

I'm trying to manually implement a convolution using FFTs and it isn't working as expected. I know I'm probably missing some subtlety with padding, shifting, or conjugation, (all of which I've tried ...
1
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1answer
2k views

Shift in Time Domain After DFT Based Convolution

To my understanding, multiplying a signal in the frequency-domain is equal to a convolution in the time-domain. I wrote a small python program, but i always end up with a shift in the time domain. ...
0
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4answers
84 views

Phase difference measurement of a signal sampled with two different sampling frequencies

I am working on phase interferometry for locating a transmitter. The direction of arrival of an incident wave can be estimated from the phase difference caused by the antenna separation as shown ...
1
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1answer
101 views

Matlab: Different 1-D FFT results with and without zero-padded

Recently I am implementing FFT with C code. So I use Matlab for algorithm verification. As Radix-2 FFT was chosen, I've learned that zero-padding technique could help to reach 2^N limitation without ...
5
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2answers
1k views

STFT overlap add method - What am i doing wrong?

i am trying to implement a short time fourier transform of an audio signal to do some filtering (subtract another stft signal using spectral subtraction). I am using a hamming window function with 50% ...
1
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1answer
120 views

Scaling of the PWELCH function in MATLAB

I am trying to compute the power spectral density of a random signal using the PWELCH function in MATLAB. Since I think have not understood properly how Pwelch scales the PSD, I wrote a sample ...
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0answers
10 views

Splitting audio into multiple audio based on frequency range in Librosa

How can I separate an audio signal into multiple audio signals based on frequency range? I used librosa.display.specshow to vi
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2answers
56 views

FFT Phase interpretation of input signal with non-integer number of cycles in FFT window

It is known that if we take the FFT of a signal at such a frequency which has non-integer number of cycles in the sample window we get different value of phase even if the original signal was at phase ...
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1answer
99 views

Matlab FFT not producing symmetric spectrum

I am plotting a FFT of a sampled RC pulse but my resulting spectrum isn't symmetric - it's offset. ...
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0answers
58 views

How to design FFT for 2000 points?

How should I design FFT with fixed samples - always 2000, sampling frequency is also 2000, memory is external, there is no need to get sorted array. As far I know it may go like factoring 2000 into $2^...
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1answer
22 views

how to interpret the 2D FFT

I know how to compute the 1D FFT (and interpret values from 0 to Nyq). When computing the 2D FFT, do we compute the FFT of row[1] then the FFT of row[2] then the FFT of row[3] up to the last row. ...
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1answer
78 views

Which (Fourier transform?) processing method to use for time variant audio processing? (newbie)

Edit: I've since been able to understand the how the relvant parts of Fourier transform work and how they relate to my problem / what I want to do. And have come to the conclusion that Fourier ...
2
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1answer
59 views

DFT from Fourier transform

I always studied the DFT starting from his formula, but for some reasons I need to do comparison between the FT and the DFT. I found the pdf in this link very useful http://www.robots.ox.ac.uk/~sjrob/...
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3answers
3k views

Impulse Response to Frequency Response in Octave

Having an impulse response of an audio system recorded as a wav file, how to calculate the frequency response of the system with octave?
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2answers
1k views

Shifting in DFT (FFT)

Input image Listing-1 i = imread('Untitled.png'); i = rgb2gray(i); F = fft2(i); %%%F = fftshift(F); F = abs(F); F = log(F+1); F = mat2gray(F); imshow(F); ...
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3answers
711 views

Distribution of energy in frequency bands

On my wav-file (music), I do a short-time Fourier transform (STFT). I have spectrum, which spans from $0\textrm{ Hz}$ to $f_s/2$. I took out the range between $1$ and $1000\textrm{ Hz}$, which I ...
3
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0answers
36 views

Audio sharpening

I'm working on a project where the purpose is to enhance FFT peaks in a signal (musical instruments), by other words, reduce non-harmonic components and enhance the identified FFT peaks. So far I'm ...
5
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3answers
123 views

why should I scale the fft using 1/N?

I am writing a report, and my advisor asked me to explain why I scale the fft by a factor 1/N (where N is the length of the array). I used to use the scaling convention of multipling the fft by the ...
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1answer
33 views

LOW SIGNAL NOISE RATIO FFT

I am processing an audio signal with CS4244 codec at 24 bits and 48kHz. Every 2048 samples the FFT is calculated and its amplitude. Without input signal I get FFT values ​​close to 30dB. With maximum ...
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2answers
543 views

Curved noise floor and ugly unexplained peaks after FFT

It has been so long since I studied this at University, so I'll better ask here if I'm doing something really wrong here. I'm using a USRP frontend for L1 band surveillance. I want a 20MHz bandwidth, ...
2
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1answer
66 views

Serial filtering using FFT

lets say I have 3 audio filters for a realtime DSP application. For simplictity each has length 256 (as well as the input signal). The filters should work in series. Starting with filter IR h1(n) and ...
0
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1answer
562 views

Python tool for time-frequency analysis

I am trying to perform time-frequency analyses using the PyWavelets (pywt) toolkit for python. My ultimate goal is to perform time-frequency analyses for EEG signals but I am starting with something ...
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1answer
884 views

1D FFT algorithm elements explanation?

Can someone explain to me what each element of the 1D FFT algorithm are (i.e. N,x,n,j,k) ? From here, http://zone.ni.com/reference/en-XX/help/371361H-01/lvanls/fft/#details (Can't post pictures on ...
2
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2answers
134 views

Higher-order Kronecker product

I am trying to generate a 2D DFT matrix in matlab, which I need for 2D compressed sensing (CS) problems. Lets say $N_1=8$, $N_2=16$, then according to the requirement of CS, to generate a 2D DFT ...
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1answer
34 views

Coherent averaging of polyphase components of a signal

I want to subsample a multi-band signal to introduce controlled aliasing. My frequency components are uniformly distributed across the spectrum (comb structure) and occupy a relatively narrow ...
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0answers
31 views

SciPy - Distinguish between two group of periodic samples [closed]

First of all, I'm sorry for my bad English writing, plus, I'm new to DSP. Background We have got motion data of 20 cows. 10 cows are healthy and 10 cows are slightly lame. Lameness is an abnormal ...
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2answers
1k views

FFT of ecg signals

Could anybody tell me why should zi made FFT for ecg signals? For me it seems more simple to use time-domain, but I saw some ecgs in frequency domain and I don.t know how to interpret the results. ...
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1answer
38 views

FFT on a random signal

I have applied FFT on a random noisy signal (say for ex, on some stock data). Below figures shows input and output. Now, my question is, how to extract useful peaks (frequencies) from the output? Can ...
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2answers
470 views

Interpolate DFT Coefficient of a Frequency That Is Not in the DFT Bin

I'm using the Jacobsen interpolation to get a more precise frequency of my signal. To get the corresponding DFT coefficient I'm doing: $$X_{f} = \sum_{n=0}^{N}{x_{n} e^{-2\pi ifn}}$$ Where $x_{n}$ ...
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1answer
246 views

Applying a Filter Kernel Defined by a Continuous Equation in the Frequency Domain

I have a filter kernel $K(\omega)$ completely defined in the frequency domain as a continuous function of angular frequency $\omega$. I know $K(\omega)$ defined as a continuous mathematical equation. ...
4
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2answers
367 views

Matching LPC frequency response to raw sample data frequency response

Background: The intention is feature/formant extraction from speech input. I have used an FFT on raw samples in the past and have done simple peak picking from a weighted average of frequencies over ...
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5answers
71 views

Interpreting the amplitude of signals in fourier transform

I totally understand the concept of fourier transform, but one thing thats bothering me is the amplitude that we plot in the frequency domain. What does that amplitude of each frequency signifies? Is ...
0
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1answer
4k views

Finding phase angle via FFT

I am using the FFT to calculate the amplitude and phase of AC cos signal. Regarding amplitude it gives me an accurate result but I get wrong result for a phase: 6 degrees instead of 36 degrees. My ...