# Questions tagged [dft]

The Discrete Fourier Transform (DFT) is a mapping between a finite set of discrete points in a (primal) domain (time, space) and the dual frequency domain. DFT requires an input sequence which is discrete, such as a sampling from an analogue audio signal.

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### What does frequency domain denote in case of images?

I was just learning about the frequency domain in images. I can understand the frequency spectrum in case of waves. It denotes what frequencies are present in a wave. If we draw the frequency ...
82k views

### What is the difference between a Fourier transform and a cosine transform?

In speech recognition, the front end generally does signal processing to allow feature extraction from the audio stream. A discrete Fourier transform (DFT) is applied twice in this process. The first ...
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### Why is the FFT "mirrored"?

If you do an FFT plot of a simple signal, like: t = 0:0.01:1 ; N = max(size(t)); x = 1 + sin( 2*pi*t ) ; y = abs( fft( x ) ) ; stem( N*t, y ) 1Hz sinusoid + DC ...
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### 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 ...
10k views

### How can I compute a log-spaced power spectrum?

I would like to compute a power spectrum in which the frequencies are logarithmically spaced. In Welch's method there is a trade-off between the frequency resolution of the resulting power spectrum ...
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### Frequency-domain zero padding - special treatment of X[N/2]

Suppose we wish to interpolate a periodic signal with an even number of samples (e.g. N=8) by zero-padding in the frequency domain. Let the DFT X=[A,B,C,D,E,F,G,H] ...
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### Why are magnitudes normalised during synthesis (IDFT), not analysis (DFT)?

In most examples and FFT code that I've seen, the output (frequency magnitudes) of the forward DFT operation is scaled by N -- i.e. instead of giving you the magnitude of each frequency bin, it gives ...
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### Why Does the DFT Assume the Transformed Signal Is Periodic?

In many signal processing books, it is claimed that the DFT assumes the transformed signal to be periodic (and that this is the reason why spectral leakage for example may occur). Now, if you look at ...
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### Discrete Fourier Transform: What is the DC Term really?

I am currently toying around with the Discrete Fourier Transform (DFT) in Matlab to extract features from images. I like to fully understand the concepts that I use. I have read several explanations, ...
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### What are some of the differences between DFT and FFT that make FFT so fast?

I'm trying to understand FFTs, here's what I have so far: In order to find the magnitude of frequencies in a waveform, one must probe for them by multiplying the wave by the frequency they are ...
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### 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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### Why do we say that "zero-padding doesn't really increase frequency resolution"

Here is a sinusoid of frequency f = 236.4 Hz (it is 10 milliseconds long; it has N=441 points at sampling rate ...
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### Could a DCT be used for an audio magnitude spectrum rather than DFT?

From what I understand, the DCT has half the bin size as a DFT of the same size N. The DFT also includes phase information, but often this is not needed when only the magnitude spectrum is desired. ...
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### When to use the DTFT vs the DFT (and their inverses) in analysis?

In many of my readings, whenever some author speaks about working in the frequency (transform) domain (of a digital signal), they often times take the DFT, or the DTFT, (and of course their ...
27k views

### What is normalized frequency

I am working on DSP and finding difficulty to understand the term Normalized frequency often used with DFT & DTFT. What is normalized frequency in DSP ? and how it is different from analog ...
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### How to demodulate an AFSK signal in software

I am trying to transmit binary data from one device to another over an audio channel (speaker/mic). I use AFSK (Audio Frequency Shift Keying) as in Packet Radio, with $1200 \text{ Baud}$ and two ...
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### STFT and DWT (Wavelets)

STFT can be successfully used on sound data (with a .wav soundfile for example) in order to do some frequency-domain modifications (example : noise removal). With ...
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### DFT - Removing window effect in spectral domain with convolution

I was thinking about the DFT windowing subject and a thought came to my mind. A DFT will yield the spectrum of a signal convoluted with spectrum of the window used, therefore having a main lobes and ...
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### Discrete Fourier transform symmetry

I was reading the chapter on discrete Fourier transforms in Lyons' book -- Understanding Digital Signal Processing -- and could not understand the last paragraph about symmetry. There’s an ...
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### FFT vs DFT Run Time Comparison (Complexity Analysis) in MATLAB

So we were given an assignment to plot the time taken by the FFT algorithm by MATLAB and a DFT algorithm written by me in MATLAB.G The expected output should have been the DFT algorithm following the ...
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### Artifacts in FFT

I recently realized that FFT's aren't perfect. Meaning if I take a signal and then take it's FFT, and then do an inverse FFT, the resulting output isn't exactly the same as the input. Here's an image ...
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### Implementation of Wikipedia Equation for the DFT

I was writing a simple fourier transform implementation and looked at the DFT equation on wikipedia for reference, when I noticed that I was doing something differently, and after thinking about it ...
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### Number of DFT (FFT) Points Required for a Specific Frequency Resolution for an Oversampled Signal

I have a bandpass signal centered at 2 MHz and bandwidth of 50 kHz (the signal frequency varies from 2 MHz - 25 kHz to 2 MHz + 25 kHz). This signal is being sampled at 10 MHz. I want a frequency ...
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### What is the meaning of the DFT? [duplicate]

Possible Duplicate: Real Discrete Fourier Transform What is the most lucid, intuitive explanation for the various FTs - CFT, DFT, DTFT and the Fourier Series? Discrete-time Fourier transform ...
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### How can I decompose a signal into square waves?

I am dealing with signals that are a superposition of different square waves with different amplitudes and phases. Normally, one would decompose a signal into sine waves with help of the Fourier ...
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### Log-Polar DFT Based Scale-Invariant Image Registration

I'm trying to do image registration using phase correlation as described in the Reddy Chatterji paper. In my case, the images may be scaled and translated relative to each other. The algorithm for ...
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Suppose there is a DFT vector $\mathbf{X}$ with length N, which presents complex conjugate symmetry around its middle point, i.e., $X(1) = X(N-1)^*$, $X(2) = X(N - 2)^*$ and so forth. $X(0)$ and $X(N/... 9 votes 5 answers 2k views ### DFT-like transform using triangle waves instead of sin waves We know that DFT (discrete Fourier transform) breaks down a signal into multiple frequencies of sine waves. Does there exist a transform that does the same thing, but for triangle waves? For my ... 9 votes 1 answer 13k views ### Symmetry of real and imaginary parts in FFT [duplicate] ... 9 votes 1 answer 2k views ### Zero Padding of FFT There are many question related to the zero padding a time domain signal to get more frequency bins after performing Fourier transform. As I understand this process is equivalent to trigonometric ... 9 votes 1 answer 527 views ### Determining Power of White Noise Component of an Incomplete Time Signal I want to determine the power of the noise component of a time dependend signal x(t) (similar to picture below) With constant sampling rate I use Parceval's Therorem and calculate the standard ... 8 votes 3 answers 5k views ### Why calculate negative frequencies of DFT? This answer says that when calculating the DFT on a real valued signal, the (roughly) second half of the bins are the complex conjugates of the first half of the bins. Firstly I was curious if that is ... 8 votes 5 answers 16k views ### How to get Fourier coefficients to draw any shape using DFT? I'm teaching myself about Fourier Series and the DFT and trying to draw a stylised$\pi$symbol by fourier epicycles as detailed by Mathologer on youtube (from 18:39 onwards), and the excellent ... 8 votes 4 answers 5k views ### What is Frequency Resolution? Im trying to tackle the following problem while still not having a firm idea on what "frequency resolution" means : Suppose we sample a continuous time signal with sampling period Ts = 1/2000, and ... 8 votes 5 answers 798 views ### Estimators for improved spectral subtraction of noise Real zero-mean Gaussian white noise, independent of a clean signal$x$and of known variance is added to$x$producing a noisy signal$y.$Discrete Fourier transform (DFT)$Y$of the noisy signal is ... 8 votes 2 answers 2k views ### Difference Between Convolution and Multiplication of Frequency Response and Frequency Spectrum Suppose I have impulse response like [1/25,2/25,3/25,4/25,5/25,4/25,3/25,2/25,1/25]. I did convolution with 600 samples of test signal (it seems that I did some filtering). Then I calculated the ... 8 votes 3 answers 1k views ### Spectral Leakage in layman's terms I'm trying to understand the concept of spectral leakage for the DFT, without going deep to the mathematical intricacies (it's for practical purposes). I've read from the book "Introduction to Digital ... 8 votes 1 answer 9k views ### What happens when N increases in N-point DFT [duplicate] I am curious about DFT, and I wrote a simple MATLAB code to test what happens when$N$increases. I took a rectangular signal with length$L=15$, an then found th DFT of 16, 32 and 64 points. I looked ... 8 votes 2 answers 12k views ### Frequency resolution and timestep in DFT I am learning DFT and I am not sure I understand it correctly. I am just going to write out the process of DFT and I am asking you to tell me if I am thinking the right way. Suppose we have a signal ... 8 votes 4 answers 2k views ### The Number of Sine and Cosine Waves in an$ N \$ Point DFT

This is bound to be an embarrassingly simple question, but here it goes... I was reading the chapter on discrete Fourier transforms (DFT) of this really didactic online book, The Scientist and ... 