# 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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204 views

### Understanding why spectral flatness cannot be computed

I have a real signal recorded 512Hz for 1s. After resampling the signal at 256Hz, I would like to compute its spectral flatness. To do so, I start by taking its discrete Fourier transform with ...
236 views

### What is the algorithmic latency of STFT processing?

I was reading this paper, and in section 3 I got a bit confused. It claims that the algorithmic latency is equal to synthesis frame length. If the overlap between two consecutive frames is 50%, the ...
85 views

### Compute hilbert transform from DCT- FFTW3 REDFT00

My problem is similar to this but not the same. Edit: Here is a description of Neumann boundary conditions for differential equations: Neumann-type boundary conditions means derivatives are ...
557 views

### Symmetry and mirror of inverse FFT

I am trying to do an inverse FFT to get the real signal from a derived formula which describes the Fourier transformed signal. I am doing this with FFT. I am fairly new to the theory of FFTs, so my ...
2k views

I'm working on the processing of raw FMCW radar data. In my case the radar works in the [6.014-7.76] GHz band (I have 187 frequency points starting from 6.014 GHz and ended with 7.76GHz) and the data ...
54 views

### Increase power for particular frequencies

I am taking an FFT of two signals ( A very clean signal, with little noise, and a very noisy signal). Signal A The signal that has very little noise has the following FFT output: Signal B The ...
90 views

### Ideal channel in OFDM simulation

I am trying to simulate an OFDM based system in MATLAB where the estimated channel in the reciever is fed back to transmitter. As a proof of concept I wanted to return the ideal channel. I used the ...
567 views

I saw the Winograd radix-8 kernel algorithm below, shown in the image. Comparing to the mathematical formula of Cooley-Tukey, there is a multiplication by $\cos$ and $\sin(\pi/8)$, which can't be ...
153 views

### Correct scaling for AWGN noise in OFDM system after adding Preambles

I have a two-part question: Question 1 I wonder if anyone can explain about the scaling factor that is used for AWGN noise in OFDM system. For example I know about the scaling factor for normal OFDM ...
64 views

### Real-time DAQ (Data AcQuisition) device and filtering of voltage signals in audio frequencies

I'm working on a frequency mixer project in which two analog voltage signals at two different frequencies (each <5 kHz) are considered. I'd like to add the two fundamental frequencies and return an ...
202 views

### How to use frame based audio features for machine learning

I have an audio recording (wav file) that is 2 seconds long. That is my sample and it needs to be classified as [class_A] or [class_B]. To extract MFCC features, I divided the sample into frames (183 ...
467 views

### Window size and DFT size

I am using pwelch function of Matlab which returns the power spectral density (PSD) estimate. This got me confused about DFFT size and Windows size. Here my code: ...
348 views

### Filter image in Fourier space imaginary components after ifft (R)

I want to high-pass filter an image in Fourier space. It works but the retransformed image has an imaginary part, what shouldn't be the case http://math.stackexchange.com/questions/867337/how-to-...
641 views

### Get spectrum from autocorrelation function using fft()

I would like to observe Fourier Spectrum, got through Autocorrelation function. This is my code: ...
137 views

### Feature recognition within a single song

I want to write an algorithm in C++ which is capable of identifying specific features within a single song, e. g. the sound of a drum which is played 100 times during 5 min. State of the project So ...
64 views

### One period with transient

Assume I have only access to only one period of a signal that there is a transient effect on the signal as well. I am pretty sure that there is a particular frequency in the signal. Is there a way ...
161 views

Why do we need 2N-FFT at the receiver when we want to demodulate ZP-OFDM signal ?
35 views

### Perturbation test for a real plant

How should we conduct a perturbation test for an LTI unknown system by perturbing the input to see the output response i.e. How many different perturbation step size should be given to the input?
215 views

### Efficient computation of Chirp Z Transform

Chirp Z Transform (1, 2, 3) is more powerful than zooming techniques (I use it to actually trace non-stationary chirp signals) and very usable in signal processing, but it's flexibility comes at price ...
116 views

### Are wavelets for audio spectrograms available as a simple FIR filter?

I have read that "Debauchies Wavelet is simply a short FIR" so i am searching for a single FIR that i can make into a filterbank, a precise "bandpass filter" that detects narrow frequencies, i.e. 1Hz ...
2k views

### How to design an interpolated notch filter in Python to filter powerline harmonics

I have a a time domain data set that records the magnetic field vs time, which must be processed to reveal an embedded signal. This data also contains power line harmonics (i.e. multiples of 60 Hz) ...
91 views

### Window function on large arays

Hello I'm reading the following document to better understand windowing and how to use it. "Spectrum and spectral density estimation by the Discrete Fourier transform (DFT), including a ...
331 views

### How to calculate energy of an OFDM signal?

What is the difference between power and energy of OFDM signal. How to calculate energy of an OFDM signal, if we consider OFDM symbol size is 64, and QPSK is used as modulation scheme?
127 views

### FFT Convolution: missing sample somewhere

For a piece of software I've been working on lately I need to implement FFT convolution for performance reasons. Doing so has been mildly succesfull but there is still a error somewhere causing lots ...
43 views

### Is there a way to go from a set of (F/D)FT values to FIR coefficients?

I'm not yet well educated on the DSP subject, but I've initiated a project where I will do some audio filtering. My intuition tells me that there is a link between the coefficients of a FIR filter and ...
393 views

### Short VHF pulse detection from SDR

I need to detect a short (10ms) VHF pulse that occurs every 1.5s at a specific frequency (e.g., 150.20 MHz). I have an SDR that support IQ sample rates up to 10MSPS but due to computing constraints 2....
440 views

### How to compute DFT of IIR

This question pertains to frequency domain filtering. I'd like to multiply an IIR filter's frequency response by a buffer of samples that have been FFT'd. I get how I'd evaluate an FIR, I'd FFT and ...
140 views

### Averaging different frequencies

How does one average a group of frequencies? Frequencies taken for example using FFT. That is, given n frequencies over m time intervals (n $\leq$ m), how should one "interpolate" between those ...
594 views

### Subtracting one FFT from another for noise reduction

I am using an FFT/iFFT library in c#, and working with arrays of audio & complex data. What I am hoping to do is take a small amount of 'background' audio signal, and use it as a fingerprint to ...
258 views

### sound FFT data to PCM data

I have some FFT data, 257 dimensions, every 10 ms, with 121 frames, i.e. 1.21 secs. I think the first dimension is probably something else and the remaining are the FFT coefficients, I guess. From ...
71 views

### Is negative frequency important in calculating Spectral Moments

A follow on question form this one (What is meant by "spectral moment"?) In answers to the above referenced question spectral moment is defined as:  m_k=\int_{-\infty}^{\infty} f^k|X(f)|^...
586 views

### Overlap add / overlap save using FFTW

I'm working on a realtime FFT processing algorithm in C using PortAudio. Currently I'm just trying to test if I can do OLA / OLS in (semi) real time using FFTW. I'm getting some distortion in the ...
2k views

### Compare two voices and outline their similarity

I am trying to develop an application which records the voice of someone through a microphone. I will have a preset recorded voices for the same words. I then need to compare the real time recorded ...
646 views

### How to process the .wav file to get meaningful information like the number of inhales/exhales

The plot of the data from the wav file. Link to the sound file - https://drive.google.com/open?id=0BxAPlY66uX7qWWtDZUw3WkpjSGs&authuser=0 Also the FFT plot is like this I am new to the field of ...
82 views

### Measuring typical seperation between patches using 2D Fourier Transform

I need to extract information about the typical distance between the black patches in an image like the one I attached here. Typical distance would be the the most frequent separation between two ...
2k views

### Relationship between FRF, FFT, AutoPower, & CrossPower

Given A is an input signal & B is an output signal: I know that the frequency response function (FRF) is defined as the CrossPower of AxB divided by the AutoPower AxA. Logic tells me I should be ...
156 views

### Question About Linear and Circular Convolution - 1D and 2D

We know that applying a filter on an image is called correlation or convolution depending on the filter angle. In Gonzalez I have read that we can apply linear convolution on an image. Here I have a ...
532 views

### How to estimate PSD, and time delay of a random signal?

I'm trying to estimate the power spectrum and time delay for random signals. the signals were taken from two sensors, 10 cm apart. My aim is to estimate the PSD first then estimate the time delay in ...
465 views

### Using Fourier Transform on Gyroscope

The original idea is to calculate distance from accelerometer input. However, accelerometer reading also contains the gravitational values, thus to remove gravity, I tried using Gyroscope. The idea ...
153 views

### filtering out specific waves in scattering pattern

I have the following (experimental) image, with its correponding raw data in .csv format here. The pattern represents the scattering of a laser field off a point defect located in the center. One can ...
94 views

### Can the Fast Fourier transform be used to compute the magnitude of moiré

I am experimenting with image down-sampling and would like to calculate a numerical magnitude for alias-induced moiré patterns. Is it possible to do this by analyzing the frequency-domain (FFT) of ...
376 views

### Extracting features from a signal: a 21st century approach?

I have a discrete signal of random length (let's say between 500 and 800 in time). This signal has an important information which I want to collect and to put it together in a feature vector to train ...
96 views

### transmitting data using phase modulation of two sine waves passed through distortion

I have two sine signals with frequencies f1 and f2, and phase p1 and p2. I modulate the phase of these signals to transmit two symbols simultaneously, chosen from the set [0,1,2,3]. For example, I ...
57 views

### determining walsh function components of signal after passing through non linear process

When a sum of sine waves is passed through a non linear process, harmonics and intermodulation distortion products are created, which are visible when applying an FFT to the signal. This causes ...
143 views

### Causal bandstop filter

I have a vector of zeros and ones with length ~2000. I need to detect and filter out the once, that appear periodically. There may be none to 3 such periodic signal's components. Typical periods are ...
640 views

### What is the best pitch (frequency detector) for a Voice Activity Detection Algorithim?

I am programming a Voice Activity Detection algorithm. After researching several methods, I implemented a simple method that determines whether or not the person is speaking based off the RMS of the ...
76 views

### Get excitation from two signals

I have two signals, one whisper data and one normal voice data of the same sentence. I'm trying to build a voice conversion, but just want to start off with some simple signal manipulations. I ...
90 views

### how to plot amplitude and phase of IMDs from their fft values?

i have my output as vector of 60000 complex number representing 10 sequence in frequency domain , each of length 6000. i need to plot the phase and amplitude of intermodulation distortion from this ...