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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1answer
1k views

Analysing 2500 frequencies using FFT with an input vector of 2048 samples?

I am currently reading the paper A Highly Robust Audio Fingerprinting System and on page 4 one can read about the technical parameters they use: Sampling rate of 5000 Hz, frames of 2048 samples as ...
24
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1answer
14k views

Overlap-Add versus Overlap-Save

What differences or other criteria can be used to help decide between using overlap-add and overlap-save for filtering? Both overlap-add and overlap-save are described as algorithms for doing FFT ...
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2answers
967 views

Sense of zeropadding in a time domain

I have the task related to Radon transform which contains a subtask which uses resampling by means of DFT. Let's consider the non-periodical discretized signal (Fig.1) (for example the string of ...
3
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1answer
287 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 ...
3
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3answers
4k views

Output of KISS FFT interpretation

I'm receiving PCM data trough socket connection in packets containing 320 samples. Sample rate of sound is 8000 samples per second. I am doing with it something like this: ...
5
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2answers
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,...
7
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1answer
2k views

Do FFT-based filtering methods add intrinsic latency to a real time algorithm?

In a current audio processing application, I am working entirely in the time domain using IIR filters. This is so I can use an audio buffer of just 3 or 4 samples and can guarantee that there will be ...
15
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3answers
3k views

Fast Fourier transform- non-integer number of cycles in the FFT aperture

There are a few excellent discussion threads and answers on this site (eletronics.se) on the theory of Fourier transforms. I tried implementing the same in a simulation tool (MS Excel :)). I have a ...
10
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2answers
4k views

How to do prediction using frequency domain data?

Both linear regression and Kalman filtering can be used to estimate and then predict from a time domain sequence of data (given some assumptions about the model behind the data). What methods, if ...
5
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2answers
3k views

Generating wavelet using amplitude spectra

Equation (4) of a recent paper (An improved peak-frequency-shift method for Q estimation) shows how to generate a wavelet using a zero-phase Ricker wavelet. $|B_1(f)| = |B_0(f)|\exp(-\pi t f / Q)$ ...
8
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3answers
2k views

FFT for line detection

I am trying use the FFT in a different way then most people ask about. I want to be able to take a picture of a graph with regular repeating vertical lines, and to process the image to determine how ...
10
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2answers
749 views

Is it valid to increase amplitude (and presumably FFT quality) by simply scaling the data?

I'm using a version of "KISS FFT" by Mark Borgerding. It accepts an array of 16-bit fixed-point input values and produces a 32-bit float result array. I've discovered that if the input amplitudes ...
16
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3answers
4k views

FFT with asymmetric windowing?

Common non-rectangular window functions all seem to be symmetric. Is there ever a case when one would want to use a non-symmetric window function before an FFT? (Say if the data on one side of the ...
4
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1answer
296 views

What sonic artifacts are caused by phase vocoders?

When using a phase vocoder to alter the spectral envelope of a signal, the input signal is split up into many (often hundreds of) bands. With many phase vocoders, the output levels these individual ...
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3answers
4k views

What is a good FFT window function to reject DC?

I'm using an FFT to analyze what is essentially the power envelope of a signal (see here for info on the containing project), and, since power numbers are always positive, to eliminate the DC ...
11
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3answers
5k views

How to Average Complex Responses (and Justification)?

I am developing software that calculates the response of a system by comparing the FFT of input and output signals. The input and output signals are divided into windows and, for each window, the ...
7
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3answers
568 views

FFT on non-rectangular part of image

I need to do a FFT on an image for noise reduction, but the problem is that I do not need the complete image, but only a circle in the middle. The borders are a fixed rig, thus I am not interested in ...
5
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3answers
4k views

I need advice about how to make an audio frequency analyzer

I just need some clarification of some very basic aspects of creating a frequency analyzer. I am writing a c++ program, so far it is just a media player, with a custom explorer. You can make ...
11
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2answers
3k views

What statistic is used to determine presence of a signal in noise?

This is a detector problem I believe: I am being stumped by what appears to be a simple problem. Basically, I have a band of interest. If signal energies exist within this band of interest, then I ...
17
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4answers
8k views

How do you calculate spectral flatness from an FFT?

Ok, the spectral flatness (also called Wiener entropy) is defined as the ratio of the geometric mean of a spectrum to its arithmetic mean. Wikipedia and other references say the power spectrum. Isn'...
7
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2answers
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 ...
3
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3answers
3k views

What's the best criterion for determining a frequency peak?

I'm writing an algorithm that analyzes results of discrete Fourier transform (DFT). The algorithm should detect amplitude peaks in predefined ranges of frequencies. This is an example of such a peak: ...
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1answer
283 views

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. ...
12
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4answers
18k views

Efficiently calculating autocorrelation using FFTs

I'm trying to calculate an autocorrelation on a platform where the only accelerated primitive I have available is the (I)FFT. I'm having a problem though. I prototyped it in MATLAB. I am, however, ...
22
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4answers
5k views

How to distinguish voice from snoring?

Background: I'm working on an iPhone application (alluded to in several other posts) that "listens to" snoring/breathing while one is asleep and determines if there are signs of sleep apnea (as a pre-...
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2answers
1k views

DFT with geometrically-spaced bins?

The traditional Discrete Fourier Transform (DFT) and its cousin, the FFT, produce bins that are spaced equally. In other words, you get something like the first 10 hertz in the first bin, 10.1 through ...
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2answers
1k views

Does down-mixing a signal before taking the fourier transform aid in signal detection?

Let us assume that we have a narrowband signal $s[n]$, at frequency $f_1$, and let us further assume that this $f_1$ is just below $\frac{f_s}{2}$, where $f_s$ is the sampling rate. Furthermore, let ...
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2answers
152 views

Help me understand this simple code snippet to get the magnitude frequency response

I inherited the following FFT code to get the magnitude frequency response of a sample window. I understand what fft() does. What I don't understand are the lines marked with a "???". What do they do? ...
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2answers
1k views

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 ...
19
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2answers
6k views

Does using the Goertzel algorithm actually give better frequency resolution?

I am reading this article, and I am getting a little confused by the author's liberal use of 'frequency resolution' regarding the Goertzel Algorithm. Basic question: Does using the Goertzel algorithm ...
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3answers
20k views

Frequency spectrum of a set of images in Matlab

I wish the compute the frequency distribution of a set of images. I basically am keen on finding out how the the frequency varies in the images. I know I can use the 'fft2' function to compute the 2-...
3
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2answers
6k 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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2answers
17k views

Improving spectrogram resolution in Python?

I'm using the specgram() function in matplotlib to generate spectrograms of speech wave files in Python, but the output is ...
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3answers
3k views

How to create a distortion compensation filter

I am creating a channelizer that consists of a complex mixer, CIC decimator, and a FIR compensation/decimation filter. The final FIR filter may, if it matters, be implemented as more than one filter. ...
2
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2answers
532 views

Embedding digital ID in audio signal, and retrieving it

The problem I am working on an interesting challenge -- to encode a 32 bit ID blip in an audio stream, and subsequently retrieve it. Something like this: http://gizmodo.com/5883215/why-lady-gaga-...
4
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2answers
222 views

Creating an array whose DFT is all whole numbers

I'm working on a simple FFT implementation and currently running some unit tests. Long floating point numbers in C and assembly become very hard to keep track of, and this whole thing is error prone. ...
3
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3answers
3k views

How to build a program that gives you on the fly the chords of the song while listening to it?

I am trying to think how can I program a SW that can recognize Chords on the fly. since I know nothing about music, Can I assume that the chords are orthogonal and unique? I guess I can since it is ...
16
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1answer
4k views

Find similar Music using FFT Spectrums

I have been experimenting some weeks to find a way to match/find similar Songs in a Library containing different Genres of Music. My first try was to Detect Features like Tempo or how much Bass there ...
5
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1answer
574 views

Spectrum Analysis using Windowed FFTs

I have a couple of questions regarding windowed FFTs: Why is the noise floor higher with windowed FFTs (according to Wikipedia's spectral leakage page, anyway), when the whole point of windowing is ...
3
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1answer
475 views

From a 44kHz audio signal, how could I extract 32 evenly spaced frequency bands?

Assume I have an array of floating points that contain the last 8192 samples played. How would I convert that array to 32 frequency bins that are spaced from 0Hz - Nqyst?
33
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9answers
12k views

Is there an algorithm for finding a frequency without DFT or FFT?

I was looking in the Android app store for a guitar tuner. I found a tuner app that claimed it was faster than other apps. It claimed it could find the frequency without using the DFT (I wish I still ...
9
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1answer
2k views

Can I use FFT to interpret accelerometer gestures?

I have the need to detect two different gestures that occur when looking at accelerometer data. Here's a run down (as brief as I can make it): Lets say an iPhone is being oscillated back and forth ...
10
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1answer
5k views

Creating a spectrogram

I've been trying to work out the logic for this task, and plan to use the KissFFT source package to perform the fast fourier transform. Please let me know if this looks about right: Allocate an FFT ...
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1answer
260 views

How Can I practically implement the FFT with size unequal to a power of two?

Can anybody explain how I can implement the Fast Fourier Transform (FFT) of length unequal to a power of two in hardware, e.g. FPGA? What is the algorithm for this implementation? Does anybody know ...
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1answer
2k views

Audio processing FFT of chunks of audio

Im new to Matlab. I need to split the time dimensions of an audio track into frames of 10mS and for every 10mS frame have to compute the FFT to generate the spectrograph. I need help on this.
5
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2answers
3k views

Confirmation on how to calculate phase differences at every frequency point between two wideband signals?

I have a problem that I thought was going to be simple, but it has become surprisingly stubborn and I question my method... I have used the method (described below), but I wanted to confirm that ...
8
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1answer
8k views

FFT of image data: “mirroring” to avoid boundary effects

I load and display an image of some rice in Matlab: g = imread('rice.png'); imshow(g); I take the FFT of this image and shift it: ...
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2answers
894 views

How can I implement a binary chirp signal containing zeros and ones?

I build a signal, $x$, which is linear in the frequency domain as (in MATLAB syntax) x = 1:0.1:1e3; Then I do: ...
4
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1answer
6k views

How do you plot an SPL vs frequency curve for a given set of pressure data?

I have a set of pressure data at a sampling rate of 262144 Hz for 3 seconds making it 786432 samples of pressure data. I want to plot a curve of the SPL (sound pressure level) vs frequency. I first ...
7
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3answers
10k views

What are high frequencies and low frequencies in a signal?

I'm new to signal processing. I'm plotting a signal in the time-domain in matlab. I don't understand which parts of the signal are high frequencies and low frequencies, can someone explain what a high ...