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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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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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1answer
2k views

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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494 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 ...
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1answer
7k 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 ...
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2answers
265 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 ...
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1answer
435 views

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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3answers
1k 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: ...
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1answer
74 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 ...
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2answers
432 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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5answers
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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1answer
1k views

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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1answer
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 ...
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2answers
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 ...
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1answer
189 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 ...
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1answer
438 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 ...
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0answers
366 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 ...
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1answer
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 ...
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2answers
846 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 ...
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1answer
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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2answers
8k views

radix-4 FFT implementation

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 ...
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1answer
568 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 ...
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1answer
463 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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2answers
5k views

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 ...
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1answer
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 ...
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1answer
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 ...
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2answers
133 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 ...
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2answers
338 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 ...
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1answer
644 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 ...
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1answer
844 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 ...
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1answer
4k views

Best way to resample a signal (image)?

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 ...
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2answers
475 views

Applying Window Function to Low Frequency Signals

I'm computing 1 sec. hanning windows from a signal that has a variable frequency between 1-4Hz. My first impression is that data loss caused by the distortion due to the window function is greater ...
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1answer
296 views

Channel Vocoder producing output with “click” sounds

I'm trying to make a channel vocoder that takes two inputs, one a frequency rich carrier (a musical sound) and the other a modulator (vocals). Applying operations involved in the channel vocoder ...
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1answer
125 views

Lower frequencies is not visible in FFT plot when adding into the original signal

I plotted a simple Cosine wave and After taking its FFT the frequency can be seen easily but after modifying the original signal to add more lower and higher frequencies, the higher ones are visible ...
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0answers
2k views

image registration using fft

I'm trying to register 2 images.First image don't change and second image is changing at each iteration by homography matrix and I'm using minimizer to find maximum of phase correlation, but for some ...
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4answers
14k views

Most efficient way to extract frequency components of audio signal with near-human accuracy in real time

I'm trying to work out how (if possible) to extract the frequency components of an arbitrary audio sample (typically music) in an FFT-like manner, but in my research on the FFT algorithm, I'm learning ...
3
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3answers
2k views

LPF and HPF Confusion - Rectangular and Circular Cut-Offs

Interpreting some homework, I'm a little confused on what it's asking for. I'm asked to filter in the frequency domain (easy enough, just a fft2 and ifft2 when I'm done) a 256x256 8bit grayscale tiff ...
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3answers
8k views

Window function and length of signal

I'm new to FFT and DSP and I want to ask you some questions about calculating FFT in Matlab. The following code is from Matlab help, I just removed the noise. What happens if I choose the length of ...
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1answer
2k views

Can you count signal energy equivalent in frequency domain?

I read about method of analyzing audio signals where FFT result is bandpass-filtered and the energy of this band is counted. Can you count the momentary energy in frequency domain? Does this mean ...
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1answer
554 views

A Working Signal Frequency Example?

I've been attempting to work on signal transformation and filtering for the past few months. It's a bit difficult, but I've been making progress. I was wondering if anyone had any simple examples to ...
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2answers
3k views

How to filter and downsample before FFT?

I would like to perform STFT on musical signal with defined sampling rate (44100Hz). I would like to get FFT result for smaller range of frequencies than are ...
2
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1answer
2k views

Harmonics in 2-D FFT Signal

In case of 1-D FFT it is far more easier to find number of harmonics and their Respective frequencies and amplitudes. Lets say if i have 8 points signal the harmonics of it then would be 7 with one DC ...
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19k views

How to shift the frequency spectrum?

Lets say we have a spectrum ranging from -X MHz to +X MHz. I would need to correct the frequency error in the spectrum by shifting the zero component to the middle (0 Hz). If the output (the ...
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1answer
801 views

Fractional Frequency and negative Frequencies

So, when we Sampled any signal at the rate lower than Nyquist Frequency we get aliasing, for instance, If the frequency of Cosine wave is 5Hertz and I had to make 5cycles in a second and if the ...
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4answers
4k views

Lower frequencies stronger in FFT?

I'm calculating FFT from microphone input. I notice that lower frequencies always seems to have more power (higher dB) than higher frequencies. I cut the data into frames of 24576 bytes (4096*6). ...
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2answers
26k views

Amplitude of the Signal in Frequency Domain different from Time Domain

How does the Amplitude of the Signal is changed when taking the FFT of a Signal, Have a look, The amplitude had changed from 2 to 30, and Here is my code for generating the above output, ...
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3answers
1k views

Understanding FFT of an Image

I am having trouble identifying the frequency components of an image, Here i simply generated, 256x256 binary-image. ...
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2answers
431 views

Correcting large variable dc bias in signal FFT

I am somewhat new to signal processing, but i'm hoping I can provide enough information in order to be pointed in the right direction. I am working on analyzing the frequency spectrum of audio signal ...
3
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1answer
9k views

understanding FFT2 function of Matlab

I am trying to understand what happen when we take FFT2 of a matrix in a matlab. first have a look at this simple example, a=ones(8); %8x8 Matrix of Ones fft2(a) ...
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1answer
2k views

Effect of Windowing Function on Autocorrelation Function

I need to compute FFT components and autocorrelation of a discrete signal. When I do not use any window function before taking FFT of the signal I get a reasonable AC function from FFT that includes ...
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1answer
2k views

Finding Peaks in an Autocorrelation Function

I'm trying to find the period of a signal. I've used FFT to compute the autocorrelation of the signal. As can be seen from the autocorrelation function (plotted below) I obtained, there are 70 sample ...