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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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 ...
652 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 ...
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 ...
2k views

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 ...
152 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 ...
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, ...
264 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....
4k 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 ...
859 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 ...
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 ...
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,...
413 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 naĆÆvely split the signal in two, along the ...
1k 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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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 ...
527 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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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 ...
571 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 ...
195 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 ...
10k 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 ...
88 views

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 ...
536 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 ...
8k 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 ...
269 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 ...
483 views

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 ...
2k views

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: ...
75 views

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 ...
493 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,...
4k views

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 ...
2k views

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 ...
2k views

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 ...
194 views

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 ...
456 views

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 ...
369 views

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 ...
2k views

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 ...
878 views

Matching an LPC Magnitude Spectrum to FFT Magnitude Spectrum

I'm trying to get an LPC magnitude spectrum to match to an FFT magnitude spectrum. Basically I want the peak height in the LPC Spectrum to match the peak height in the FFT Spectrum. Thanks to ...
2k views

Failed to implement Goertzel algorithm in Python

After some questioning on stackoverflow, I tried to implement a Goertzel algorithm in Python. But it doesn't work : https://gist.github.com/4128537 ...
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I implemented a 4-point radix-4 FFT and found that I need to do some manipulation of the output terms to get it to match a dft. My code is a pretty direct implementation of the matrix-formulation so ...
610 views

Mapping the value of a sample in a 2-D DFT to cycles/pixel

If I have an image and its 2-D DFT of that image, what is the mapping between the value of the DFT at (u,v), and the frequency in the spatial domain in the x and y components, in cycles/pixel? I want ...
479 views

What is a Phase Delay filter?

I'm looking at a job spec, detailed below. I'm perplexed as to what is being asked for. Could someone offer a basic understanding of this material? I can imagine a solution may involve performing ...
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Why exactly is a bit/radix reverse required when calculating the fft?

When implementing the fft, you always include a bit reverse at the end of the process to put the data in the correct order. It seems to be because the fft proceeds by decimating the input array into ...
1k views

MFCC process confusion

I have read through many papers related to the Mel-frequency cepstrum coefficient (MFCC): I have some confusion about the MFCC process: Many of the papers say that I have to do a FT (Fourier ...
3k views

FFT spectre graph measurements y-axis

I am very new to this things. Sorry for probably stupid question. I don't understand what units and meaning have the values on Y-axis of Fourier Transform graph? On X-axis it is Frequency (Hz). Pretty ...
136 views

Lightweight audio listener for detecting blip

The challenge I'm looking at is to design an iPhone component that listens to microphone input and triggers when it hears a particular blip (preferably < 0.1sec duration). I can design the blip ...
357 views

FFT of audio, packet sampling and others

Hi I am just toying around with compression now that we are learning about Fourier transforms in my class (compression is not a part of the class but I became a bit curious). Trying to do this with a ...
656 views

High Level Process, Calculating Cepstrum When You Only Have Spectrum

I am not a DSP expert, so I would like someone to check my thought process: I have a text file representing the spectrum (calculated by some FFT hardware), and I'd like to calculate a cepstrum. I'm ...
863 views

Is there any example or existing resource for using matlab in audio comparison (cross correlation)?

I am new to MATLAB. I have the audio files A and B, B is recorded using a mic while playing audio file A. The length of file B is different from A. A does not always start from 0 second. My program is ...
I have an original image $\ I(x_i,y_j)$ where $\ x_i = i*\Delta x, 0<= i < n_i$ and $\ y_j = j*\Delta y, 0<= j < n_j$ I am trying to implement a function that resamples the image ...