# 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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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 ...
561 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 ...
462 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 ...
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 ...
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 ...
2k 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 ...
127 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 ...
329 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 ...
634 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 ...
829 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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### 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 ...
460 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 ...
288 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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### 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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### 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 ...
13k 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 ...
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### 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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### 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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### 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 ...
546 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 ...
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 ...
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### 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 ...
18k 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 ...
754 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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### 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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### 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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### 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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### 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 ...
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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### 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 ...
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 ...
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### Filtering PCM data in java

I'm working on an audio based application for android platform , I'm trying to use a low pass filter to a fixed or variable max frequency "depends on the cost of development". first I've been playing ...
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### calculate root mean square, ratio of power, constant false alarm, … of a signal

I am new to Digital Signal Processing, and am reading a paper. I don't know how they can extract some features from signal, like root mean square (RMS), Constant False Alarm Rate - CFAR, Mean Value ...
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### FFT of High Frequency and Low Frequency?

When we plot FFT in a matlab it doesn't ask for Frequency , all we need to input the sequence and it simply shows the output using stem command. If i want to compute two different signal using FFT, ...
6k views

### breaking the input signal into segments while calculating DFT

Im reading a book about DSP and there is an example about investigating the sound that travel through the ocean. The sound was recorded as the time-domain signal and they tried to analyze it in ...
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### how to break the input signal into segments while calculating DFT [duplicate]

Possible Duplicate: breaking the input signal into segments while calculating DFT Im reading a book about DSP and there is an example about investigating the sound that travel through the ocean. ...
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### How does Fourier Transform manipulates image technically?

I need a little bit more explanation on technical details regarding Fourier Transform in Image Processing, mainly on the frequency aspect. I see that we can use Discrete Fourier Transform to change ...
315 views

### Can FFT Substitute Correlation for This Case?

I'm trying to determine frequency of a repeating pattern in a time series. The sampling rate 100Hz and the pattern repeats itself with periods between 0.5 second and 1.25 second. This corresponds to a ...
5k views

### Using FFT to convert to frequency domain, then IFFT back to time domain C++

I've posted the same question on stackoverflow.com with little success, so I thought I would try here! I'm using C++/C to perform forwards and reverse FFT on some data which is supposed to be the ...
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### Speech Recognition - Project Idea

Wondering if someone could offer me any advice or help. Sorry if this question is too broad or anything. I'm wanting to complete an algorithm in basic Speech analysis. The algorithm will detect ...
669 views

### Demodulation of BFSK signal with frequency hopping using Goertzel Algorithm

I am implementing a BFSK algorithm with frequency hopping on an embedded system and I am using the Goertezel algorithm in my demodulation approach. Basically, I use the goertzel alorithm to determine ...
868 views

### change the frequency component of a spectrum in an audio file

I am working on an audio dynamic range compressor using Matlab. After calculating the FFT of the audio file, I use several bandpass filters to amplify specific bands separately. What I want to do then ...
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I'm writing a program to emulate a vocoder and it obviously works, but any speech that comes through is very difficult to understand. I'm going to explain exactly how my implementation works and then ...
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### Time resolution of the Short Time Fourier Transform (STFT)

I'm struggling to figure out how the time points of an STFT are calculated, and I can't find a definitive answer. Let's say I have a 4Hz stationary signal and I'm going to use a 64 second window with ...
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### Why this two ways of FFT are different?

I have a small question that I am confused. N=16; x=[-pi:2*pi/N:pi-2*pi/N]; y=1.5*cos(x) z=fft(y)/N we will get negtive Fourier coefficients, however when we do ...
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### Demodulation of a BFSK frequency hopping signal using FFT

I am not very familiar with DSP techniques, and I am currently working on my research project to develop the firmware of a DSP processor, using C/Linux. It involves the modulation and demodulation of ...
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### How to calculate the gain in a bivariate fft in R?

In Statistica gain is defined as follows: Gain. The gain value is computed by dividing the cross-amplitude value by the spectrum density estimates for one of the two series in the analysis. ...
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### Can edge detection be done in the frequency domain?

Can we take advantage of the fact that high frequency components in the FFT of an image generally correspond to edges, to implement an edge detection algorithm in the fourier domain? I did try ...
I'm able to read wav files and its values. I need to find peaks and pits positions and their values. First time, I tried to smooth it by $\frac{(i-1 + i + i + 1)}{3}$ formula, then searching on ...