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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3answers
924 views

Determining the mean of of a sampled signal (improvements to time averaging method)

I am sampling a signal at 1kHz, which is essentially a single-frequency sinusoid with a DC offset (and a small amount of noise). I don't know the precise frequency of the sinusoid (it is at most 10Hz),...
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156 views

How to prevent Signal drifting when changing frequency

When I recreate a signal it seems to look great. But when I try and increase it's frequency (which is done in a for loop and the variable new_freq=2) it starts to drift. See Image below (The top ...
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1answer
4k views

retrieving original data from phase and magnitude of Fourier transform

I use this snippet of python code to transform data to Fourier phase and magnitude and then retrieving original data. ...
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2answers
1k views

Optimizing FFT code for 12 bit processing on microcontroller

I want to port the following FFT code on a microcontoller having a very little floating point capability. I want to perform FFT of the output from 12 bit ADC. Since my values are all 12 bit numbers it ...
4
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1answer
270 views

Discrete Fourier Transform a dummy approach

I'm trying to understand how DFT works exactly. However, when experimenting around, I compared both Matlab generated FFT result with a dummy approach result and I get similar result. However, the ...
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2answers
1k views

Solution for FFT size 1536

I have already asked this question 2 times, but still i am confused because this is not available anywhere on the internet and explained properly. I found some information in this document and i am ...
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1answer
5k views

Determining sampling rate in OFDM demodulation

I have seen that in OFDM, the sampling rate is much lesser than twice the bandwidth of signal. why is it so? How is sampling rate calculated in OFDM? Does it depend on the modulation type used inside ...
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1answer
10k views
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7k views

Effect of windowing on FFT magnitude

I have 2 sin waves: Fs = 1500; N = 100; t = (0:N-1)'/Fs; sinwave = sin(2*pi*15*t); sinwaveshifted = sin(2*pi*15*t + 1.3); If I get the magnitude at 15Hz using ...
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2answers
141 views

Do I get less accurate results if I do not use a power of 2 points for my FFT?

I heard that the padding with zeros is mainly for efficiency and speed, other than that, is there a downside to use fft(signal) instead of fft(signal, N) where N is a power of 2 assuming signal length ...
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3answers
9k views

Am I understanding the phase angle from FFT correctly?

Long story short, I have a sinusoidal wave with DC offset going in with a given frequency of say 100 Hz and $\phi = 0$ (which I'm guessing is really $\pi/2$ since it is a sine wave not cosine). I get ...
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2answers
208 views

Ways to analyze electrical signals without FFT?

I am collecting a 15Hz sine wave at 1500Hz. Therefore, every 100 points of data collected should contain a single complete sine wave. If I sample for 10 seconds, I will have 15000 points of data. If ...
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3answers
11k views

FFT Zero Padding - Amplitude Change

I'm just learning about Fourier Transforms and as an input I'm using a WAV file (Matlab) and taking one channel of it and performing a quick Fourier transform. I've zero padded the input, as I've been ...
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2answers
959 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 ...
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1answer
4k views

Why can the FFT always be mirrored in the middle of the x axis? [duplicate]

Plotting the FFT of a signal with Matlab e.g. by the code: y_fft = abs(fft(y)); f = fs*(0:length(y)-1)/length(y); plot(f,y_fft); where ...
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0answers
258 views

Points of interest in a spectrogram

We have several audio tracks, all of which are different versions of the same track under one type of distortion: the speed of the track is increased by a constant factor K (hence frequences are ...
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3answers
2k views

What is the relationship between frequency vector and the peak frequency in FFT?

I am fairly new to both Matlab and FFT. I would like someone to explain to me the relationship between the frequency of the wave being processed, the number of point (usually called NFFT), the Nyquist ...
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1answer
209 views

FFT function isn't working on imported signals greater than a second

I have an fft function in matlab/octave that works great on an imported mono wav/audio signal that is 1 second long, but when the signal imported is more than 1 second the frequency values become ...
4
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1answer
57 views

Transform Length and event duration

Does the length of a Fourier Transform affect the ability to discern a small event in the data? This is a tricky question to ask, let me clarify. I don't care about when the event happened and I don'...
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0answers
575 views

How to convert a sound file to any other data format which can be manipulated

I know this is very basic question and i am new to this site. But can anybody help me with my question. How to convert a sound file to other data format say using some type of transform (FFT or DFT) ...
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1answer
2k views

Extract sharpness from FFT

I've got in total 6 images, from 2 cameras (thus 3 per camera) storing 16 bit data. What I need to feedback to the user is like some sort of sharpness coefficient, preferably live while he is ...
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1answer
127 views

fft result order compared to direct implementation of DTFS using text book

To find the complex Fourier coefficients of this sequence, then The $k^{th}$ coeff. $c[k]$ is given by the DTFS formula $$ c[k]=% %TCIMACRO{\dsum \limits_{n=0}^{N-1}}% %BeginExpansion {\displaystyle\...
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1answer
804 views

Fast fourier transform and frequency domain spike in wrong area

I used MATLAB to produce a simple sine wave over time: ...
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0answers
137 views

Compensate for standard deviation loss

I am not sure if this question will be a little off-topic on this forum, that I will give it a try anyway, since it implies signal process arguments. By using ...
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1answer
841 views

Constant Magnitude Chirp across audio band (Parseval's Theorem?)

I want to excite a loudspeaker with a constant magnitude chirp that sweeps across all frequencies in the audio band. I downloaded a test file from this website to start with. I plot the log magnitude ...
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2answers
554 views

Autocorrelation Jitter

I'm doing autocorrelation (via fft) on pure, synthetic generated sinusoids and getting unexpected jitter in the results. The jitter depend on where in the sine the signal buffer is started. I get ...
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1answer
61 views

How to get some law parameters from its FFT figure?

Given an hidden law $$I(t)=\frac{1}{2}[A+B.cos(2\pi\nu_0t+\phi_0)]$$ I observe its signal with 256 samples and compute its FFT over this interval, and represent the FFT result from 0 to 1 (normalised ...
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1answer
2k views

About voice filtering

I have a waveform containing voice some children screaming some times when there is no noise at all some times there are cars driving by when talking I want to detect the start and ending times of ...
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1answer
150 views

Guitar String Replucks

I'm analyzing guitar string plucks and sustains. I'm having good success with auto correlation using FFT's. Now I'd like to detect plucks while the string is still vibrating. Since I already am ...
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3answers
1k views

Estimating Low Frequencies (including amplitude and phase)

For the past month I've been chasing the DFT/FFT rabbit. Along the way I've been cut-n-pasting the name of other rabbits to chase later: Autocorrelation, Phase Vocoder, Parabolic Approximation, ...
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2answers
256 views

FFT Data Array Indexing [closed]

I've been Googling "FIR filters" and "FFT convolution" etc. and I ran across something interesting. Array Index vs. Pointers: http://www.bores.com/courses/intro/program/7_array.htm http://www.dspguru....
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1answer
3k views

Finding periodicity of spikes, matlab

In my time series data, I have often spikes with regular periods. Sometimes, I get two or three different periodical spike sequences (two electrical noise sources), some with T1 seconds period, some ...
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0answers
752 views

What is the proper way to select the time-bandwidth product when doing multi-taper spectral estimation?

Frequently programs for calculating the spectrum of a signal using the multi-taper method require the user to specify the time-bandwidth product. What considerations must be made when selecting the ...
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2answers
3k views

How to predict the spectrum of an aliased/folded signal without looking at the FFT?

This seems like a trivial concept but I just can't get the hold of it. I have a signal with two complex sinusoids having frequencies -25MHz and 17MHz. Currently it is being sampled at 64MHz so I get ...
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2answers
4k views

Complex impulse response functions?

I think I must have misunderstood something regarding the relation between impulse and frequency responses. I have calculated the Impulse Response Function, IRF, from the Frequency Response Function,...
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0answers
376 views

Discrete Fourier transform in a multidimensional space

I want to measure the frequencies at which a point oscillates in a multidimensional space, let's take the example of a point on a 2d-surface. For now, I naïvely split the signal in two, along the ...
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1answer
997 views

Chirp z algorithm clarification

I am attempting to implement a chirp z algorithm to handle random sized DFTs, and I can not seem to obtain any meaningful results. I have gone over several write ups and "think" I have a handle on ...
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2answers
2k views

Comparing Image FFT with Sine wave FFT

So, When we take the FFT of any Composite or Single Sine Wave we get exact frequency on x-axis of the plot. The point where we get the original frequency in FFT is depend upon the Sampling Rate and ...
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1answer
1k views

Convolving With half-Hanning

The goal is to get a 200ms decaying delay of an audio signal while preserving the sharpness of attacks to mimick human perception of sound. The paper I'm following convolves a 200ms half-Hanning ...
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1answer
454 views

DFT as convolution question

I have tried to make this question as readable and consistent as possible. The short of it, is that I am trying to ascertain how one gets from the math equation shown, (which I understand), to the ...
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2answers
6k views

what's the baseband FFT size of LTE?

In standard of 3GPP LTE physical layer(3GPP TS 36.211), the FFT size for baseband signal generation for both Dl and UL is 2048 and CP size is also determined. However there is a table in several ...
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1answer
461 views

Dynamic time warping in percentage

I'd like to compare 2 speech similarity and show it in terms of 0-100%. Here's my approach: 1: Convert 2 audio waveform into audio spectrum by Fast Fourier Transform (FFT) 2: Use Dynamic Time Warping ...
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1answer
192 views

Given f(x) and its FFT F(u), I need to prove that df/dx = F(u)*2iu(pi)/n

Given $f(x)$ and its FFT $F(u)$, I need to prove that $df/dx = F(u) * 2iu\pi /n.$ $df/dx = f'(x)$ and $n$ is the number of pixels of the one dimensional image $f$. I tried to use the convolution ...
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2answers
9k views

FFT of a Square wave, what are frequency of each Harmonic here

After getting suggestion to first study FFT on square wave to understand FFT of discrete signal, I'm trying to understand the FFT of square wave. I've coded a program, here is the details, Frequency ...
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0answers
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Keep the exact frequency content - Multidimensional fft

In Matlab I've got to generate a Gaussian process embodying a certain frequency content. Not to go multidimensional, I have used reshape function. I explain better ...
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1answer
2k views

Audio fingerprint questions

I am developing an application that requires audio fingerprints. I have been reading a lot of articles and PDFs, now i think i have gotten myself confused. Based on my present understanding, I have ...
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0answers
479 views

How to find the phase lag between a sinusoidal input force and the response?

I have a system that is forced by a sinusoidal function with specific amplitude, phase and frequency. I get a deformation of sinusoidal form as well, and I would like to find the phase lag. I use FFT ...
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1answer
6k views

Working backwards from PSD to possible signal

I have been trying to reconstruct a random signal from its PSD and am running into trouble. I know that many different signals in the time or spatial domains can result in the same PSD-- I am ...
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1answer
3k views

How can I detect potholes using an accelerometer mounted in a vehicle?

I am currently studying DSP and FFT, I am very new to this and have been doing electronics for a long time with Arduino and hobby projects. Recently, I am doing a project with the goal to map potholes ...
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2answers
257 views

check power spectrum

my question is quite straight-forward to be grasped and handled. I have an 1D array, F_11 representing a target power spectrum. It is sketched in the following ...