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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Vertical lines noise in spectrogram

I have started looking into DSP and made an implementation of short-time fourier transform, using kissFFT to perform the FFT. I then render a spectrogram. An example image that I have generated can ...
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3answers
4k views

Adequate representation of frequency domain amplitude/magnitude of FFT of a signal

I'm quite new to the subject and am having fun playing around with the FFT. What I am currently doing is trying to sample an audio signal and display its frequency spectrum at the same time. This ...
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3answers
174 views

FFT doesn't always have frequency in all frames

I'm trying to do an FFT on live audio to see its spectrum. So I have a sampling rate of 44100 Hz and from this take out 1024 bins on my FFT So to test it I play a continuous tone at 15000 Hz and ...
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2answers
918 views

FFT plot in Matlab, accuracy

I have a small input signal of 60Hz sine wave from signal generator, which is corrupted with 50Hz mains supply frequency. I want to measure the amplitude of the 60Hz signal using FFT because it is ...
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1answer
147 views

FFT of 2 D images

Branching off from this question - https://dsp.stackexchange.com/questions/6020/mapping-the-value-of-a-sample-in-a-2-d-dft-to-cycles-pixel, I have a few more questions related to 2D FFT of images. I ...
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1answer
539 views

How can I use time-series transforms for detecting malware? [closed]

I am student of network security, and very unfamiliar with the domain of signal processing. Do pardon my naivety at asking very basic stuff. My work deals with detecting malicious activity in network ...
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0answers
453 views

FFT of complex envelope

I have I and Q samples of a time domain signal, sampled at a rate T, both of length L. I create the complex envelope, C = sampleI+j*sampleQ. Then take the FFT of ...
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0answers
879 views

bearing frequency detection

I am working on bearing faults detection. I am doing the following steps: signal->mean zero->high pass->hilbert->y->fft->cepstrum of y. my code is: ...
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0answers
128 views

discrete fourier transform in Matlab - theoretical confusion

I have a periodic term $V(x) = \sum_K \exp(iKx) V_K$ where $K =2\pi n/a$ where $a$ is the periodicity of the term and $n =0,1,2,3....$ Now I want to find the Fourier coefficient $V_K$ corresponding ...
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1answer
150 views

What's the frequency transform of a random matrix?

Assume the following: img = 255*rand(512); %Generating a 2D matrix with random numbers between 0 to 255. fftimg = abs(fft2(img)); %FFT of the image. The frequency transform of this image is a ...
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2answers
539 views

Spectrum analysis : how to represent FFT results

I'm trying to program a spectrum analyser in C++ but I'm faced with a conceptual problem : how to map the results of the FFT to an image ? Let me detail : I'm doing all this in a dynamic way. I ...
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2answers
145 views

Using DFT to determine control system bandwidth from user input

I am currently working on a race car project where steering and braking is to be done by a fully electronic x-by-wire system. To roughly determine the minimum bandwidth of the controllers, I took user ...
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1answer
3k views

How to make this cool Audio Spectrum Visualizer

I want to make/create this spectrum visualization on a physical Led_Bar_Panel(s). Here's what I think I should do: Take sample's in real-time( Sampling freq = Fs KHz, only voice) Do a FFT( As far I ...
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3answers
796 views

FFT method input argument have to be $2^n$ ?

Does FFT method input argument have to be power of 2, i.e, $2^n$ I just realized there are many algorithm for FFT implementation,...
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1answer
250 views

How is the complexity of applying Short term fourier transform over n samples with a moving window of size m amount to O(nmlogm)

I have been trying to understand a publication here where Short time fourier transform is applied over n samples in steps of m samples each (m is the size of the moving window). I understand that the ...
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2answers
459 views

Is $O (N \log N)$ FFT speed the fastest we can ever attain?

I am wondering about whether or not there is a theoretical limit as to the speed at which we can compute a DFT. We all know that the FFT executes in $O (N \log N)$ time. However, is this a lower bound ...
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7answers
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Downsampling/Decimation - When is well done?

I am working on decimation of signal, and I want to know which is the best way to understand if the downsampling is well done or not. At the moment I am comparing the FFT of the source signal and the ...
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3answers
7k views

Why does a narrower window in a Fourier transform or STFT give poor frequency resolution?

In theory of FT(Fourier transform) and STFT(Short Time Fourier Transform) it is said that "A wide window gives better frequency resolution but poor time resolution. A narrower window gives good time ...
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1answer
170 views

Why is it possible to frequency sweep with just one sinusoid?

One way to measure the frequency response of a filter is to input separate cosine/sine wave at the desired frequency, measure the steady state output, stack the inputs/outputs into complex arrays, and ...
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1answer
50 views

Signal spectrum when carrier width changes

If I have spectrum $X_k$ of the signal evaluated at frequencies $k \delta f$ and I want to know what is the value of that same signal at some other frequencies , i.e., $k' \delta f'$.I know that $\...
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2answers
641 views

How do you alter the phase relationship between I and Q in a quadrature signal?

I am collection a quadrature signal from a radio. The I and Q signals go through a sound card at a sample rate of 192000, and then a FFT is performed on the signals. Due to various reasons, the ...
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2answers
128 views

Is there any relationship between the FFT and linear prediction?

Is there any relationship between the FFT and linear predictive methods? Can an FFT result or the input to an FFT be modified to do non-circular prediction/extrapolation from the FFT results.
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1answer
5k views

Wavelet Transfrom + Power Spectral Density (using Matlab)

I don't have background knowledge about signal processing before and new at Matlab too. I have EEG data (with noise removed) 1x128; sampling rate = 128 Hz, It's means that I have 1 sec. data right? ...
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1answer
105 views

Understanding Hertz

This question is in two parts, both related in some way or another. Suppose I have a signal where the sample rate is 44100 and compute a normal 1D DFT on this signal. The size of this signal is ...
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1answer
534 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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2answers
282 views

Need help with FFT results

I am playing around with FFT and cant seem to get the expected results. I have a sound clip that i recorded from a mic placed on an analogue clock. So it has a sound every second. ...
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2answers
1k views

Working around FFT windowing?

I have the following problem, that I ran into recently, when calculating the spectra of data that I obtain from a measurement technique, we are using in our group. In short what we do in the ...
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1answer
1k views

Python scipy.fftpack.rfft frequency bin mapping

I'm trying to get the correct FFT bin index based on the given frequency. The audio is being sampled at 44.1k Hz and the FFT size is ...
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1answer
6k views

FFT in MATLAB is shifted in frequency

I have a signal in MATLAB on which I'd like to perform an FFT. My signal is stored in s, and I use the code below (inspired by the MATLAB help): ...
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1answer
2k views

Implementation of FFT algorithm in C Programming [closed]

Does anyone know any FFT algorithm that can be implemented in C? I am trying to convert a built-in matlab FFT api to a function in C. Appreciate any help!
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1answer
175 views

1-D Fourier Transform Of A 2-D Image But At An Arbitrary Orientation

If one has a 2-D array and would like to take the 1-D Fourier transform along a direction $\theta$ degrees off the horizontal, is there a better/faster way to do this rather than rotating the image by ...
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4answers
1k views

FFT - Effects of Windowing,Improvisation approaches on RealTime FFT Hardware

Fore-word :Kindly Observe this question has multiple updates take the latest update as question ! here below is my 256pt FFT on an 2Khz sinusoid signal,on a fixed point DSP tms320c5515 please ignore ...
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2answers
638 views

Plotting Spectrum Analysis

I am working on a spectrum analyzing simulator, which is meant to compute and plot the FFT of a sine wave carrier signal of $16 Hz$. From my practicals with a spectrum analyzer, I know that the spike ...
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1answer
1k views

Phase factors for an 32 point fft

I have just started working on FFT and its related phase factors. I have read for an N=2 point FFT, We have two phase factors, one on the positive axis and one on negative axis. similarly for an 4 ...
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1answer
2k views

Interpret numpy.fft.fft2 output

My goal is to obtain a plot with the spatial frequencies of an image - kind of like doing a fourier transformation on it. I don't care about the position on the image of features with the frequency f (...
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1answer
1k views

PSD - Is this formula correct?

Trying to understand whether or not I am calculating the PSD correctly. Below is the equation that I am using: $$ PSD_{rms}(f_{m} = m \cdot f_{res}) = \frac{PS_{rms} }{ENBW} = \frac{2 \cdot \left | ...
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1answer
647 views

Easy Filter with FFT and Convolution

I'am pretty new to this topic so i dont have much experience with DSP. I want to filter(highpass) a WAVE file, i have already programmed the FFT, and now want to filter the FFT vector via convolution (...
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2answers
4k views

Carrier frequency offset (CFO) estimation using cyclic prefix

I understand the cyclic prefix, but in (1) it's said that: "If we consider that the channel effect is minimal and can be neglected. then the phase difference of the CP and the OFDM symbol which the ...
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0answers
94 views

Signal Relationships and Conversion: Transforming one Signal into another?

I'm trying to relate a near shore tidal signal (point A) to 3 points along a long model boundary (points B C D). I want to possibly have a relationship between B C D with which we can convert A ...
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1answer
285 views

PSD - Correct Formula for removing Noise

Ok, so finally some things are making sense. I'm trying to scale the results, which I have done by the following equation: $$ PS_{rms} = \frac{2 \cdot \left |y _{...
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2answers
200 views

STFT - Putting it all together

I'm currently reading this paper: http://www.rssd.esa.int/SP/LISAPATHFINDER/docs/Data_Analysis/GH_FFT.pdf On page 21, it has a section called "Putting it all together", it states the ...
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2answers
802 views

position of spike in the spectrum of a sin wave

I have a sin wave of 16Hz, sampled at 2048 samples persecond. I performed the FFT of the sequence and did a spectrum plot. My plot gave me two spikes at both ends of the graph. But i was told the ...
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3answers
596 views

strange phenomenon in spectrogram of Windowed Fourier Transform [edited] [closed]

I am trying to understand how the Windowed Fourier transform works, so I used Matlab to create an artificial signal, and plotted the Windowed Fourier transform using a gaussian window. $x(t) = \left\...
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1answer
440 views

Inverse Short time fourier transform

I am I am working with 'wav' files in audio signal processing. I am able to find out STFT by using the function " spectrogram(file name, window length). Now I want to apply Inverse Short time fourier ...
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1answer
7k views

Power Spectral Density estimate (PSD)

I'm trying to understand PSD, and, it's functions. So far I have carried out the following: Segmented a signal with NFFT = 256 and ...
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2answers
1k views

Help understanding FFT Phase: code and results provided

I'm trying to figure out how FFT's phase works. Here's my code: ...
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0answers
1k views

Frequency shift and phase shift (Doppler)

I'm trying to apply a frequency shift to a generated despreading code in order to find and track a broadcast signal CDMA. Expected doppler shift range of the signal from center frequency would be ...
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1answer
96 views

How to find the Freqs bins

Ok, so I'm kind of moving forward with implementing the STFT. I realise that I was taking the entire FFT and not the first half. The problem that I'm having now is, understanding how to get the Freq ...
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4answers
9k views

physical meaning of fourier transform

I have a question regarding the fourier transform. If I plot the frequency against the fourier transform for a periodic signal and I get a peak, What is the physics behind it? I want to know the ...
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2answers
202 views

how to make fft ignore phase shift when only interested in magnitude

I have a signal that consists of a sine wave, on which I apply a rectangular window and then an FFT. I'm only interested in the magnitude of the sine wave, and I want it to stay the same even if the ...

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