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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how to find the impulse and step response from the transfer function?

H(s)= s/(0.2s^2 + 200s + 1000000) Apply impulse signal as input to the system represented by above model and find the output using pen and paper Draw the calculated response. Apply step signal as ...
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178 views

How to remove power line interference?

I have a couple of datasets that show a peak around the 50-60 Hz range (mostly around 55Hz and in some cases at their harmonics i.e. 100-120Hz.) In some datasets the spike is significant in comparison ...
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38 views

Can I optimize band-pass FFT filter with skipping values instead of zeroing them when decimating?

I currently use very simple band-pass filter: FFT->Keeping needed, zeroing not needed->iFFT->decimation with skipping This works perfectly for my needs (I apply envelope after filter so no ...
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36 views

Mitigating high-frequencies after sampling

I have a dummy signal that I am sampling at $T = 100 \ \mu s$. According to the Nyquist frequency, I can identify frequencies up to $\pi/T = 31416 \ rad/s$, as we can see in the FFT of my signal (blue ...
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Why does my amplitude change upon inverse Fourier Transform when I am only randomizing the phase of the fourier transform using Python numpy?

I am trying to make a surrogate time series of a discrete data series using python, basically I wish to keep the amplitude same and change the frequency I take a Fourier Transform of the data I ...
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EEG Data Analysis

I have recorded EEG data using openBCI which is in the range of -15000 to 15000 micro volts. I want to reduce this to the range of -100 to 100 micro volts. How do I do it?
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4answers
77 views

Linear Phase Filters and FFT

The FFT decomposes a signal into cosine and sine functions, respectively, even and odd components of the signal. Hence, I would expect even symmetric filters to have zero imaginary parts. Suppose a ...
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Window overlapping FFT

I have a sample array of 16,384, I apply FFT on every chunk of 128 samples, and I apply Hanning window on every sample with the chunk size of 128. so I am getting back a 128x128 window spectrogram. I ...
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29 views

Perfect lossless rotation of JPEG image (ITU 81)

JPEG image compression is Fourier based DCT (ITU 81). It divides the image into 8x8 pixel blocks, and processes each using a Discrete Cosine Transform. The results are quantised and then encoded. ...
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48 views

Bandpass filter equivalent for amplitude

I have a signal where I am using a bandpass filter to limit the frequency range. I am also interested in filtering the remaining frequencies (after bandpass filtering) within the signal by their ...
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1answer
30 views

Belt tension measurement (36 Hz): What should my FFT size be?

These are my setting that I can adjust and I need to measure 36 Hz. +- 1 Hz. What settings do you recommend?
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How to use Pffft FFT library

I am interested to use the pffft library, available here: https://bitbucket.org/jpommier/pffft/src/master/ I am currently using Intel MKL library, as well as a naive optimized reference implementation....
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How to 'multiply' FFTs

In certain algorithms its required to 'multiply' FFTs. How does one actually do that when you have real/imaginary pair data? Say I have twos FFTs a and b, should I for each bin just multiply a[k]....
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76 views

Finding Frequencies from Log Indexed FFT

I have FFT data that has logarithmically scaled at x-axis as shown on the image. But the problem is I do have only array of float numbers that has 1024 element in it, but I know that is already not a ...
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What does it mean by “mean energy for each FFT bin”?

I have a sequence $x(n)$ of data in time domain that I have taken the FFT of, which have yielded me a corresponding sequence $X(k)$ in frequency domain. I'm trying to implement a method from a paper, ...
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50 views

Apply FFT to positive spiky signal

I have a signal that looks like this It has regularly occurring peaks 50 units apart, and offset from 0 by approximately 20. I thought I could apply the DTFT to this signal and extract the period (...
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53 views

How to compute convolution using the Discrete Hartley Transform

It's easy to compute the Discrete Hartley Transform of a 1D signal: ...
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Is there an example of fft where on the y axis wouldn't be power? [duplicate]

my question is the same as in title. Is there an example of signal which after fft operation wouldn't have power on the it's y axis?
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71 views

DFT after the Nyquist limit

I usually do the DFT using the fft in Matlab. After the Nyquist frequency I don't see any result. Is it possible to perform a dft looking after the Nyquist frequency. I am asking this because I have ...
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89 views

1ppm Sampling clock offset in Matlab

How to emulate a sampling clock offset (between the transmitter and the receiver) of 1 part per million (SCO=1ppm) in Matlab? Theoretically, I can upsample the signal by a factor of 1,000,001 (one ...
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2answers
54 views

Deconvolution of system response in Python/Matlab

I had two sets of data, the output function of the system (time series with a length of 1292 entries) and the transfer function (similar to a gaussian with a length of 681 entries). I would like to ...
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1answer
35 views

How to get rid of noise in audio files?

Is there any method to get rid of noise in wite circles of the spectrogram? I have done the high pass filter for a cut-off frequency of 1 kHz. And I'v also done a adptive noise reduction in adbobe ...
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fft result with the number of integer multiples of fs

With the cosine signal fc=3Hz and the fs=13Hz as below, fc=3; fs=13; Ts=1/fs; N=101*13; % 101 times of fs n=0:N-1; x=cos(2*pi*fc*n*Ts); and then fft with the ...
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1answer
24 views

How to get coordinates(x(time),y(frequency),z(intensity(db))) from spectrogram?

I have created this spectrogram from a wav file. Please have a look: enter image description here I want to get the x = time, y= frequency, z=intensity(db) of every pixel of the spectrogram. How can ...
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3answers
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Why does FFT size equals the numbers of samples in the time domain?

My question is simple. I think I kind of understand FFT and DFT. what I dont understand is why, in Python or matlab, do we use FFT size as the number of samples? why does every sample taken in the ...
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OFDM signal in time and frequency domain and its PSD

My question is on OFDM signals for LTE. I been reading a tutorial on LTE where ofcourse the modulation is OFDM and it is explained that an OFDM signal is a rect function in time domain. The duration ...
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Plotting amplitude and phase spectrum in MATLAB [closed]

We are supposed to write a code that will help us plot the following signal and their amplitude and phase spectrum. unit step exponential unit ramp sinc rectangular I believe I can scrap the code ...
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filtering large wiggles in signals

I am trying to filter out large wiggles showing in a signal. Those wiggles are happening due to many signal processing layers and a final sine Fourier transform (no phase) Here's the python code i am ...
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Create a Range Doppler Map using “Arbitrary” phase coded Waveforms

This is my first questions I am asking to this community! I want to understand how to create a Range-Doppler Map (RDM) for a phase/frequency coded radar return using an arbitrary waveform generator (...
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2answers
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unsymmetric fft in matlab

This is the FFT of a 4khz sin signal. I wonder why the negative part and positive part of X axis is not symmetrical? ...
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1answer
53 views

Magnitude Spectrum of a Linear Chirp Signal

I am trying to understand intuitively the fft of a signal that changes in frequency in time. Suppose I have the fft of a linear chirp signal sampled at 1 kHz for 2 seconds. The instantaneous frequency ...
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57 views

STFT with varying window lengths (like the continuous wavelet transform)

I was wondering if it is possible to use windows of varying lengths when making a spectrogram based on the short-time Fourier transform (STFT). That is, for higher frequencies I would use shorter ...
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How to multiply two FFTs in MATLAB?

I want to find the output signal(y) from input signal(x) and impulse response(h). To do that first I take the fft of both x and h, then multiply them, and finally taking the ifft of the result. The ...
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Waveform independent frequency analysis

If a musician (or a vocal coach) hears a human voice at a certain note, he/she can easily find the same note on piano. Not only for human voice, if the waveform is square, or created with a synth, ...
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Trying to do frequency analysis of an accelerometer output

I am new to signal processing. I have a 3 axis MEMS accelerometer. I have read its output into $\mbox{Matlab}^{\tt TM}$. My problem is that the signal I am trying to isolate has a frequency of about 0....
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42 views

Fast Sine Wave Recognition

I have a Sine wave at frequency F1. This signal changes and shifts to frequency F2. In frequency domain I like to find this shift as quickly as possible. (Signal has a lot of random noise and hard to ...
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116 views

Is there mathematical relationship between the FFT and DCT transforms

I need to model the relationship between the DCT and DFT transforms (If it exists). I mean real signal $x \to y = \textrm{DCT}(x) \to z = \textrm{DFT}(y)$, so I need to get the relationship between ...
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Mathematically expressing FFT and STFT on a matrix

I have a matrix $q(m,n)$, where $m$ represents the vertical direction and $n$ is the horizontal direction. I need to perform the following three operations: take FFT of matrix $q$ in the vertical ...
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27 views

FFT Frequency-Spectrum has noise

I am plotting a Frequency-Amplitude Graph using openGL for the graph and Rosetta FFT implementation for the FFT calculations, I generated a 2 waveform, applied Hanning window, applied FFT and ...
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47 views

Use Fourier Transformation to compare 2 short audio files [duplicate]

I want to use Fourier Transformation to compute the similarity of 2 audios. I am dealing with small audios having the same size: 1 second length, mono channel. My question is how can I use the Fourier ...
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24 views

getting different output when using FFTW3 Library than using Rosetta FFT

I have the following array containing sine waveform that I want to compute FFT: { 0.0, 0.114124, 0.370557, 0.0, -0.576201, -0.370557, 0.0, 0.0 }; When using FFTW3 ...
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Estimating Average HR from PPG sensor

I am reading Smartphone based Blood Pressure Monitoring via the Oscillometric Finger Pressing Method, which is trying to estimate blood pressure from a PPG sensor and a small applied force finger ...
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92 views

The FFT of a sine wave with varying frequency and constant amplitude does not show a plateau, why?

I want to use python to see what the audio response of a speaker is. This would be done by comparing the output of the speaker measured with a microphone to the given input. That means that for both ...
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41 views

Obtaining real numbers from FFT with same length as original signal

In this article the real valued time domain signal is transformed to the frequency domain to extract some features like mean or variance. But after transform to the frequency domain I calculate ...
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1answer
34 views

FFT requirements of real time signal from sensors

I have an electrical current sensor which generates measurements every second. I want to calculate the FFT of this signal. Since my real-time values are received every second, I think that the fastest ...
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2answers
31 views

Deconvolution of shifted gaussian function in the frequency range

I have a signal defined as $$A(t)\cdot\exp\left(-i\omega_0t\right)$$ with $A$ the envelope function and $\omega_0$ the carrier frequency. I would like to transfer this signal into the fourier space ...
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Understanding why (at least) 2 points of the FFT of a real signal are real

I am aware that the FFT of a real signal is Hermitian, i.e. $\text{FFT}(f)[i] = \text{FFT}(f)[-i]^*$, where $\text{FFT}(f)[i]$ is the i-th component of the FFT of $f$. The logical consequence of this ...
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103 views

Why would a time shifted signal have a higher maximum frequency than an unshifted one?

I´m reading the book Numerical Simulation of Optical Wave Propagation by Jason Schmidt. In section 2.1-2.2, the author performs the FFT of a centered and a shifted Gaussian pulse and makes the ...
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41 views

What does Cooley–Tukey FFT algorithm calculates in order to compute DFT

0 I'm relative new to this subject, I've watched many videos explaining FFT and DFT and read some articles. I wanted to see how I could implement FFT in C++ and then I found this code, it works but I ...
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150 views

Amplitude of the fft is less than what is expected

This is my MATLAB code ...

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