# 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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332 views

### Current harmonics via FFT — do peaks matter?

Typically, one would perform an FFT on a current waveform and measure it's magnitude at multiples of the line frequency (e.g. 50Hz, 60Hz) to get the harmonics, and subsequently the Total Harmonic ...
1k views

### Obtaining power spectrum from ACF, FFT using Matlab and FFTW

I am using Matlab R2012b 64-bit on Windows 7 in order to estimate the power spectrum of a simple signal that is: $$\cos(10t) + \sin(20t)$$defined in the time interval from 0.0 to 10.0 Here is what ...
766 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 ...
475 views

### discrete sine transform using FFT

i have made a subroutine to calculate 1d fft of a series of size 2^n. I need to calculate discrete sine transform of a series using the fft subroutine. I am not sure what to do.
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### How can I “smooth” square wave/pulse data to remove/reduce harmonics?

First, I have reviewed close-match questions, but can't find something that seems to match what I'm looking for. Also, it's been 30 years since college, so forgive me for forgetting stuff.. :) I am ...
10 views

### SNR computation in frequency domain with scipy.fftpack.fft

I have a noisy time series (gaussian coloured noise) to which I add a signal. I call the sum of noise+signal a segment. In the picture you can see an instance: I want to compute the signal-to-noise ...
24 views

### How to remove sum and difference terms while computing frequency from fft?

I am currently operating in vhf band AND trying to detect frequencies using Fast Fourier transform thresholding method. While detection of multiple frequencies , i received spurs(May not appropriate ...
31 views

### FFT and Power Spectrum Normalization

On many websites, including MathWorks, it was suggested to normalize the fft spectrum (MATLAB or numpy) by dividing it by the total number of samples ($N$). For a sinusoidal signal, for example: x(...
20 views

### Classifier for detecting abnormal noise using FFT

As a simple machine learning application for quality evaluation, I want to detect abnormal sound in a product. A good product would make a signature humming sound, a bad product might make noises due ...
25 views

### how to cut off frequencies above 10 Hz with Welch method

This is the first time I have worked with any code so I am rather unfamiliar with all the lingo and processes involved. I am trying to convert a signal to the frequency domain and want to find the ...
26 views

### Measuring group delay through a linear phase filter

I'm having a difficult time wrapping my head around a phase/ group delay problem and it corresponding to where you start sampling so I built up a system to test some of these issues. The system passes ...
68 views

### Have you seen these kind of DFT artifacts?

we've been working with the Spiral DFT Implementation on an FPGA and managed to get it working. Unfortunately we see these strange artifacts and have no idea where they come from. Our input is a ...
41 views

### Using fourier coefficients to reconstruct data in matlab

When doing a discrete fourier transform on some data using matlab's fft function, its output is a set of fourier coefficients but I was wondering how do I go about converting these into an and bn so I ...
10 views

### How to preserve amplitude of original data in Lomb-scargle periodogram

To my knowledge, peaks in Lomb-Scargle periodogram does not preserve the amplitude of the original data. It just shows how much variance each frequency explain about the original data. For example: ...
14 views

### cpsd calculation help

I have several channels from where I am reading sensor signals in time domain. I have 9 channels where I am reading time domain signals from each and every channel for 10 seconds. Now if I perform ...
16 views

### Does FFT magnitude of recorded sound depends on the sound card used?

Based on some reading, here's my understanding of how sound recording works: Microphone outputs a voltage based on sound pressure. The microphone voltage is amplified and the amplified voltage goes ...
12 views

### How to preserve amplitude of original data in Lomb-Scargle transformation?

I have unevenly sampled data. I tried FFT, and failed. I tried Lomb-Scargle (LS) transformation, and it succeeded. Here's what I understand about FFT vs LS. Correct me if I'm wrong. FFT requires ...
21 views

### Dual FFT - difficulties extracting phase

I am performing dual FFT analysis on a system. 1) I generate a Log Sine Sweep signal and save it as my reference array 2) I then produce an inverse filter to correct for amplitude and save this as ...
17 views

### Question about FMCW range-velocity plot

I am working on plotting range-velocity heatmap of the receiving FMCW signal. I actually have already plotted. However, I do not really understand the mapping relationship between Doppler FFT bin and ...
30 views

### Double Sided FFT Power Spectrum on Matlab

I'm trying to plot the power spectrum of a signal but I cannot understand clearly the units of the amplitude. ...
26 views

### Phase measurement with FFT for DMM Keithley 7510

I am currenly working on a measurement driver for the DMM Keithley 7510. I implemented a harmonics measurement using FFT (from MathNet library). First I simply sample my input voltage signal and ...
43 views

### How to find frequency and corresponding time for an audio signal?

I want to read in a sound file and get the frequency at each time interval of the signal. I thought that the best way to approach this is by applying FFT to small chunks of the signal. Because FFT ...
31 views

### Window function correction when using frequency domain likelihood

I was wondering if someone could tell me or point to resources which discuss what kind of of windowing corrections factors one should use when using a frequency domain Whittle likelihood for spectral ...
36 views

### FFT command line application?

Are there any command line programs for Windows, preferably free and stand-alone, which can report the peak/strongest frequency within a given range of frequencies? I need something like this to ...
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### Why do I need this modulo tweak to bandpass filter an I/Q signal

I have an I/Q signal with $f_c=2.06\text{MHz}$ and $f_s=50\text{KHz}$. I am looking the first 256 samples of the signal, which is a NumPY array of complex numbers. If I do a spectrogram of this ...
20 views

### Detecting changes in signal due to server delay

I'm currently working on an application that can be used to determine when a signal changes due to a server delay. Essentially, I have an API that is used to output data to a UI. However, for reasons ...
38 views

### How do I narrow the frequency from a time domain Gaussian signal?

I'm trying to get a frequency domain Gaussian signal that starts from 180.045 THz and ends in 180.65 THz. This is the matlab code I used to get the FFT of time domain Gaussian signal: ...
12 views

### What's the significance of lomb-scargle power?

What is the significance of Lomb-scargle power (y-axis)? I have two data sets. For each plot, above plot is Lomb-scargle periodogram of the lower plot (original data). The first data set has an ...
53 views

### How to normalize PSD to get the same magnitude as FFT peak

I am trying to use FFT and power spectra density estimation with python (np.fft.fftand scipy.signal.periodogram). And trying to ...
10 views

### Symmetry in Lomb-Scargle transformation

I'm observing weird symmetry and repeating pattern on my unevenly sampled time series data after Lomb-Scargle transformation. I used astropy lomb-scargle. ...
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### How can I graph the amplitude for a given frequency window over time?

I have simple audio files of single plucked notes from a guitar: I want to graph the amplitude of each partials over time, rather than just have this color-coded "dB" intensity graphing I have here. ...
26 views

### Optimizing Spectrogram Parameters

I'm looking for references that explain the trade-offs between more averages with shorter fft sizes or longer fft sizes with fewer averages when generating a spectrogram to perform energy detection of ...
27 views

### Correctly interpret Butterworth filter frequencies

I am using the butter function from the signal package in R to filter my data. The data is Fourier-transformed (complex numbers) - and I want to filter out specific frequencies (I have a 256Hz ...
34 views

### Plot Frequency Spectrum of Binary Sequence in Matlab

I am new to Matlab and I am trying to implement a section of a published paper, the basic idea of the part that i am implementing is to show the frequency spectrum of camera aperture. The shutter ...
23 views

### Time gating of FFT for audio Measurements or Multi Time Windowing

I am learning more about DSP specifically for audio measurement purposes. I can an app, where I can create a log sine sweep from start frequency to end frequency over a certain length. I can perform ...
21 views

### Causal bandpass using fft?

I would like to take a time series of several spikes or narrow Gaussians and bandpass using an ift/ifft together with a frequency domain mask to create a series containing band-limited spikes. This is ...
41 views

### Silence removal from the audio

Suppose i've a mp3 file. I performed FFT of that mp3 file. Then is it good to state the following: 1. Silence occurs by that particular DFT point whose magnitude is lesser than some threshold. 2. ...
29 views

### How to generate wireless O-QPSK modulated signal in Matlab?

I am trying to create a small simulation where I would need to transmit a wireless signal from one wireless node to another and then transmit one more message back to node 1. Considering this is LR-...
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### How to compare two 802.15.4 O-QPSK modulated digitals signals?

I am working on wireless sensors project and I am trying to simulate a transmission between two nodes. So let's say I have two nodes which transmit the message to each other, i.e. node 1 transmits ...
27 views

### Comparing two audio (Phone operator messages)

I have this two wavs. Same audible audio differente data. The audios are phone operator messages, so they are always the same. Wav:http://instaud.io/3Ezu Wav:http://instaud.io/3Ezw Using this ...
32 views

### Find a simple sine wave buried within noise, FFT or cross-correlation?

My recorded signal x = s + v, where s = A*sin(ωt) and v = noise. SNR < 1 or ~ 1. So the sine wave is buried under a lot of noise. I need to compare the relative amplitudes of s for each recording ...
57 views

### Inverse FFT of complex exponential with time delay - window leakage?

I am working in MATLAB with a frequency domain signal that is a sum of complex exponentials, e.g. $X(\omega) = \sum_i e^{-j\omega t_i}$ where $t_i > 0$ $\forall$ $i$ I expect the output to ...
14 views

### Finding subimage using correlation and FFT

For my assignment I have to locate subimages on larger image (simple OCR). I have text and sample image (e. g. single letter) and have to locate it's positions on larger image (which will be converted ...
8 views

### Wavelet Analysis Magnitude Scale

I am trying to plot magnitude against frequency averaged over a time window. To do this I make a morlet wavelet Filterbank. And multiply the dft of each kernel by the dft of my incoming data set. ...
33 views

### Power, energy and amplitude spectrum from fft in MATLAB - scaling

I've looked at previous answers and none of the answers seem to agree (perhaps I'm missing something obvious): I would like to know what the correct scaling to use is for amplitude, energy and power ...
52 views

### Overlapping in real time fourier transform?

I have an algorithm and I need to record audio and perform short time Fourier transform to obtain which frequency is the most common. I am using a Hanning window to try and reduce spectral leakage as ...
15 views

### Methodology for log some sweep audio measurement impulse response

I am trying to implement a method for measuring the response of a loudspeaker in a room using a log sine sweep stimulus. To check my method: 1. I create the stimulus using: Make recording length ...
28 views

### Remove constant noise from Goertzel output

I'm trying to analyze a signal recorded from an analogue phone line and do DTMF recognition using a Goertzel algorithm. However, due to a fault in the hardware, there is a constant 50Hz power line ...