Questions tagged [fft]

The fast Fourier transform is an efficient algorithm to compute the discrete Fourier transform (DFT) and its inverse.

Filter by
Sorted by
Tagged with
0
votes
1answer
30 views

Equalisation of FFT spectra

The image shows the frequency spectra of a signal recorded using a SDR. The SDR is of DirectDownSampling type and has some compensation in the signal chain to compensate for the DC spike which would ...
0
votes
0answers
51 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 ...
0
votes
0answers
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. ...
3
votes
3answers
299 views

Does it make sense to use a wavelet that is equal to a sine of one period?

Would I get same results with STFT method (short-time Fourier transform) if I try this?
0
votes
1answer
26 views

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. ...
0
votes
1answer
35 views

What happens if I zero out the initial terms of the fft output and take inverse fft? [duplicate]

I am new to signal processing domain. When I run fft on data of length 1000, I get 1000 complex numbers. Now, if I want to extract the low frequency information or signal approximation, I take the ...
0
votes
0answers
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 ...
0
votes
1answer
22 views

SDR : Signal divided between the edges of the receiving window

I'm using a SDR (specifically the RTL-SDR) to analyse the spectrum and i noticed a phenomena that does not seem logical. The signal I want to capture is centred on 868.3 MHz and the central ...
1
vote
0answers
49 views

Additive Synthesis - clean playback of a spectrum frame

First of all - I have only a limited of Fourier transforms and the mathematics involved, so please pardon any potentially bad terminology or huge gaps in conceptual knowledge. I'm experimenting with ...
0
votes
0answers
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 ...
1
vote
1answer
93 views

Why resampling change this FFT output?

I have been studying the following code but still unable to explain its output. I really hope someone could shed some light. Let me begin with the baseline. Suppose I have a 20 seconds sinusoid with ...
2
votes
1answer
55 views

What is the proper way to filter out unwanted noise from accelerometers?

I'm a beginner in signal processing and have a hard time to filter out environmental noise from the floor vibration data collected using accelerometers. I have tried using butterworth filter (IIR) ...
0
votes
0answers
33 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 ...
1
vote
2answers
63 views

How to deal with the mirrored components of a FFT? And another question

I'm trying to simulate the behavior of my FFT spectrum analyzer, I am using Mathematica, but my questions are more conceptual than technical implementation of code...I hope. If we consider a simple ...
0
votes
0answers
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 ...
0
votes
0answers
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 ...
0
votes
1answer
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. ...
0
votes
0answers
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-...
0
votes
0answers
24 views

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 ...
0
votes
0answers
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 ...
0
votes
1answer
39 views

multiplication in frequency domain equals convolution in time domain mismatch

We know that a convolution in the time domain equals a multiplication in the frequency domain. As per the article below, https://www.dspguide.com/ch9/3.htm In order to multiply one frequency ...
1
vote
1answer
87 views

What is a sliding window?

I tried to find the answer by my own but unfortunately I couldn't. Let's say I have the discret values of a signal which is 10s long. What is the FFT which is performed on a sliding window with ...
1
vote
0answers
54 views

FFT of accelerometer signal: How to get a correct amplitude & noise reduction

I’m a student studying on vibration analysis & testing. I’ve performed low level sine sweep test of a simple rectangular PCB with clamped boundary conditions on two ends of it. That test was ...
0
votes
0answers
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 ...
1
vote
1answer
31 views

Extreme value at start of FFT proccesed signal

I have a signal with frequency of 1000 Hz, and it consist of velocities of an hot-wire. The measurements are taken behind a cylinder as air is blown on the cylinder. When i process the signal of each ...
0
votes
1answer
56 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 ...
0
votes
1answer
40 views

Prepare data for FFT

I'm reading the various FFT algorithms and the input consists usually of real and imaginary parts. The real data can be for example an audio input and the imaginary part should be the same sized array ...
0
votes
2answers
88 views

Interpolation and harmonics

A real valued causal sequence $x1[n]$ exists with length of the sequence being $N$. Valid indices of x conform to $0 \le n \le N-1 $ The DFT of x[n] is: $$ X1[k] = \sum_{n=0}^{N-1} x1[n].e^{-j.2.\...
0
votes
0answers
17 views

FFT time segment [duplicate]

When I use scipy.signal.spectrogram to calculate the spectrogram, with FFT size 2048, hop length 512, it return a time array which start with 1024 and increase 512 ...
0
votes
1answer
43 views

What is the result of fourier transform of an image

I am new to computer vision and Fourier transform concept. I have examined some codes. In those codes, an image is performed Fast Fourier transform by using the function ...
0
votes
0answers
20 views

How to get PSD from frequency domain representation of time-series data [duplicate]

Let's say we have a realization (time-series data) of a random process. I take fft() of that to get its frequency domain representation. Then how can I estimate the PSD of this random process by ...
0
votes
0answers
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 ...
2
votes
1answer
97 views

FFT-based coarse carrier recovery for QPSK

An FFT-based coarse frequency acquisition block is being tested in GNU Radio. The algorithm used can be found on the GOES satellite user manual and is summarized in the figure below. The algorithm ...
0
votes
0answers
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. ...
2
votes
0answers
66 views

Correct way of derivating in frequency domain with FFT

I believe I am very close to the answer and only need a small nudge to get to the answer. What I want: I want to take a signal, use FFT to transform it to the frequency domain (FD), multiply it by $...
0
votes
1answer
30 views

Any meaning to the magnitude of a FFT when audio is recorded using a PC?

I understand the theoretical meaning of the magnitude of a FFT from a mathematical sense. But what about when the sound audio is recorded using a PC microphone using a package like PyAudio, and FFT is ...
1
vote
1answer
62 views

Units for a PSD from square of FT?

If I am looking to do a PSD plot on a signal $x(t)$ with units $m$ then the units I am expecting in the PSD would be $\frac{m^2}{Hz}$ but I can't seem to get this result from the following reasoning: ...
0
votes
1answer
52 views

Why am I not getting the intended coefficients in this 2D Fourier demonstration?

I am trying to demonstrate how the 2D Fourier decomposition of an image works with Matlab and a very, very simple example. I create a 4 x 4 pixel image with cosine basis vectors as such: ...
0
votes
0answers
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 ...
0
votes
2answers
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 ...
0
votes
0answers
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 ...
1
vote
0answers
9 views

Complex Spectral Phase Evolution (CSPE) Performance depending on signal windowing?

I am look into CSPE. "Signal Analysis Using the Complex Spectral Phase Evolution (CSPE) Method" The method is simple. It compares the original signal's FFT and shifted signal FFT in phase domain so ...
0
votes
0answers
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 ...
0
votes
0answers
15 views

Spectrogram of chirp sequence radar signals

I am confused as to whether SFFT makes sense to process chirp sequence signals. The radar fires up a sequence of 100 linear ramps with that are only interrupted by the very short period in which the ...
0
votes
0answers
24 views

Calculating noise floor of digitization system using FFT

I am trying to calculate self noise of a digitization system with differential inputs.For this purpose, i select a differential channel, set input range to +/-5 V, short both differential inputs and ...
0
votes
0answers
21 views

IFFT, negative frequencies and noncausal impulse

When using the IFFT to compute an approximation to the IDTFT, frequency range is 0 to 2$\pi$ versus $-\pi$ to $\pi$. This produces a different impulse sequence(causal I think). How do you calculate ...
1
vote
1answer
34 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. ...
1
vote
4answers
71 views

Detecting a three-tone signal in real-time

I'm new to digital signal processing, and I've been looking into FFT, filtering, Matlab etc. What I'd like is to build a program which will detect when a certain BEEP is transmitted through a trunked ...
3
votes
0answers
50 views

Is it possible to approximate given skewness and kurtosis values using an IFFT?

I have time domain data for a signal that behaves randomly but has non-random (non-Gaussian?) skewness and kurtosis values of ~0.9 and ~7.4, respectively. The FFT of the signal shows that it's not ...
0
votes
1answer
32 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 ...