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# 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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### 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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### 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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### 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 ...
211 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 ...
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'...
584 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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### 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 ...
127 views

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### 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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### 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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### 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, ...
257 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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### 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 ...
770 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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### 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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### 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,...
385 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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### 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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### 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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### 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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### 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 ...
7k 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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 ...
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### Avoiding padding during FFT calculation

I have a signal X(n) of length 24. But I want to find 1024 point FFT of this signal. I can do this by padding 1000 0s in x(n). But this requires huge memory which actually will be having only 24 valid ...
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### best window for close frequency components

I'm currently designing a receiver which has to determine whether a signal contains specific frequencies or not. The frequencies are at constant 215Hz difference: ...
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### Prepare Buffer for Inverse FFT

How should buffer value at N/2 get initialized prior to performing an inverse FFT on a buffer, size N, that contains a complex frequency response in only values 0 to N/2-1? It is pretty clear the ...
399 views

### change FFT bins position

I'm sampling a signal at 44100kHz, and I have 1024 samples on which I perform an FFT. My bin resolution is therefore around 43 hz/bin, so the frequencies I get energy for are multiples of 43hz: 43Hz,...
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### Piano pitch detection

I'm working on a piano tuning program and part of it requires real-time pitch detection. Here is the scheme I have so far which works to some degree but could probably use some refinement. I'm ...
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### How to properly perform convolution on bitmap image?

Correct me if I'm wrong, I should read bitmap into matrix of float values where I would get numbers up to little more than 16*10^6, then I should expand my image matrix to size 2^k putting the ...
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### How to find the envelope of a wide-band signal?

I would like some feedback on possible techniques that one may use to determine the envelope of a broad-band time domain signal. I have heard anecdotally, that it is not as straight-forward as it ...
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### Intuition for how a Fourier transform handles signals where the frequency varies with time?

I created a discrete signal sampled at 44.1kHz that is 330Hz for 1 second, 360Hz for 0.5s, and 390Hz for 0.25s. I did an FFT transformation of this, and got the magnitude graph below: I don't ...
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### Commenting frequency spectrum of a sampled data

the question below asked in my latest signal processing exam. I have no idea about it and i could not find any solution in literature. Could you please help me finding out the answer. Actually i ...
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### Time -Domain to Frequency Domain

I am constructing a MFCC and come to a section which is kind of confusing me. It is: Step 4: Fast Fourier Transform and has this equation: Now I understand that FFT needs to be carried out, I ...
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### What causes High Intensity Noise after Acoustic Echo Cancellation for a VoIP?

I am trying to implement a Acoustic Echo cancellation algorithm in Java for a VoIP system. This AEC will help to cancel the echo (coming from Speaker) in microphone input at the same PC. The sound ...
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### Showing frequency and amplitude after doing an FFT on a signal

I have executed an FFT on a .wav file of a person saying "ahhh" which shows the frequency and amplitude of the audio file. When I run it on a .wav file it shows the frequency 0 Hz as having the ...