# 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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### 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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### 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 ...
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 ...
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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### 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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### 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 ...
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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### 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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### 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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### 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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### 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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### 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 ...
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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### 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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### 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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### 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 ...
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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### 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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### 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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### 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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### Help understanding FFT Phase: code and results provided

I'm trying to figure out how FFT's phase works. Here's my code: ...
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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### 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 ...