## DFT Blind Spot?

Started by in comp.dsp16 years ago 18 replies

I am trying to build up some flash programs to demonstrate the DFT and I am pondering something. I am looking at a 16 point DFT and am...

I am trying to build up some flash programs to demonstrate the DFT and I am pondering something. I am looking at a 16 point DFT and am focusing on the eigth (or ninth, depending on how you look at it) basis function: e^-j2pi*8*n/16. This is also the basis function that is at the folding frequency of the DFT (half the sample rate). what I am realizing is that the DFT will catch cosine en...

## Recycling FFTs

Started by in comp.dsp18 years ago 1 reply

Friends, assume x[n] is a comlex vector of length 2N, and I want to compute the 2N-point DFT X[k] of x[n]. Also, I am given X1[k] = DFT(x1[n])...

Friends, assume x[n] is a comlex vector of length 2N, and I want to compute the 2N-point DFT X[k] of x[n]. Also, I am given X1[k] = DFT(x1[n]) with x1 = x(0:N-1) and X2[k] = DFT(x2[n]) with x2 = x(N:2N-1). I want to save time computing X by using X1 and X2. DIF gives me X(2 k) = X1(k) + X2(k), so the even indices of X are very smple to compute. Unfortunately, X(2 k + 1) = DFT((x1[n] - x...

## dft general Q

Started by in comp.dsp19 years ago 9 replies

Hello When a statement like "take 16 point DFT" of the input x[n]. Does that mean the max value of n is 16, I mean an input sequence with...

Hello When a statement like "take 16 point DFT" of the input x[n]. Does that mean the max value of n is 16, I mean an input sequence with say 40000 points, you then take the DFT of this input signal, does that mean you get back 40000 spectral points from the DFT, if yes, than why the literature say 16, or 8 or what ever number pint DFT, why not just say DFT if the input signal. but if ...

## six DFT window functions applied to a 64 point DFT

Started by in comp.dsp16 years ago 2 replies

Hi, I have created a new flash program that shows how windowing is used to reduce leakage in a DFT. The program also discussed magnitude...

Hi, I have created a new flash program that shows how windowing is used to reduce leakage in a DFT. The program also discussed magnitude and power representation of the DFT coefficients. The program allows you to run a 64 point DFT. The DFT allows two frequency inputs with variable gain on the input levels of each frequency. You can apply six window functions and vary the signal...

## Smart DFT

Started by in comp.dsp11 years ago 3 replies

Hello, I am trying to estimate a frequency of power system at the moment, but I have got some problems with methods I have used. Last week...

Hello, I am trying to estimate a frequency of power system at the moment, but I have got some problems with methods I have used. Last week I found out, the smart DFT exists. I hope, it can be a appropriate way for me. But on the net there is not much information about this, I mean, very interesting part of DFT. Can somebody explain me the algorithm of smart DFT? Thank you very much. ...

## Doubt regarding DFT and sinc function

Started by in comp.dsp17 years ago 4 replies

Hello, I am kind of new to DSP and presently going through the book " Understnading DSP" by Richard lyons. On the topic on DFT it says that...

Hello, I am kind of new to DSP and presently going through the book " Understnading DSP" by Richard lyons. On the topic on DFT it says that the max amplitude/gain of the main lobe in the magnitude response of DFT increases with N ( N-point DFT) and the bandwidth reduces. Makes sense. The it goeson derive the DFT of a rectangular function whose magnitude rsponse is simalar to that of a sin...

## M point DFT using higher radix-2 FFT

Started by in comp.dsp16 years ago 6 replies

I want to compute an M point DFT of a sequence x(n)where M is not a power of 2. One way to do this would be: 1. Pad (N - M) zeros and compute...

I want to compute an M point DFT of a sequence x(n)where M is not a power of 2. One way to do this would be: 1. Pad (N - M) zeros and compute an N point DFT where N = power of 2 & N > = M. 2. Find the (continuous) DTFT (discrete time fourier transform) of x(n) from the N DFT samples by interpolating the DFT with the sinc function. X(w) = Summation {Y(k)* sinc[N/2 * (w - 2*pi

Started by in comp.dsp12 years ago 15 replies

Hi everyone, I have a question about DFT. Let's say the DFT's outputs are F(m). As we all know the m in the DFT equation is an integer (bin...

Hi everyone, I have a question about DFT. Let's say the DFT's outputs are F(m). As we all know the m in the DFT equation is an integer (bin number). My question is: Can I turn this integer m to a floating point number? My point is, if I want to do a 1024 point DFT, and just want to examine a specific range of frequencies (e.g. m = 50 - 51), instead of calculating m[0, 1023], can I do m[...

## Computing M point DFT/FFT using radix-2 FFT

Started by in comp.dsp16 years ago 6 replies

I want to compute an M point DFT of a sequence x(n)where M is not a power of 2. One way to do this would be: 1. Pad (N - M) zeros and compute an...

I want to compute an M point DFT of a sequence x(n)where M is not a power of 2. One way to do this would be: 1. Pad (N - M) zeros and compute an N point DFT where N = power of 2. 2. Find the (continuous) DTFT (discrete time fourier transform) of x(n) from the N DFT samples by interpolating the DFT with the sinc function. X(w) = Summation {Y(k)* sinc[N/2 * (w - 2*pi*k/N)] } ...

## DFT z-transform help needed

Started by in comp.dsp19 years ago 13 replies

Hi, I have the following questions that I am stuck on. Could you please help in solving them or at least point me in the direction that can help...

Hi, I have the following questions that I am stuck on. Could you please help in solving them or at least point me in the direction that can help me? Thanks. A given sequence x has x[n]=0 for n outside the range 0

## DFT Leakage

Started by in comp.dsp19 years ago 10 replies

Hi, When the DFT of an integer multiples of the input signal is taken, then the mahgnitude response shows that there are no...

Hi, When the DFT of an integer multiples of the input signal is taken, then the mahgnitude response shows that there are no sidelobes. However, when the DFT of a non-interger multiples of the input signal is taken, we get the sidelobes, which we call the DFT leakage. Ex: when input signal has 5 cycles, and you obtain the DFT at any analysis frequency(m*fs/N), we get only the main l...

## application of discrete fourier series and discrete fourier transform

Started by in comp.dsp8 years ago 5 replies

Hello, I am able to appreciate what exactly are DFS and DFT, but I would like to know the actual applications of these. I do know that we...

Hello, I am able to appreciate what exactly are DFS and DFT, but I would like to know the actual applications of these. I do know that we do design/analysis of LTI systems using DFT, mostly using response to impulse inputs. But what is the use of DFS? If yes, wouldn't DFT do the same thing? If not, what would be effect of taking DFT of a discrete signal? Thanks a lot ... ---------------...

## DFT Leakage

Started by in comp.dsp19 years ago 2 replies

> Hi, > > When the DFT of an integer multiples of the input signal is taken, > then the > mahgnitude response shows that there are no...

> Hi, > > When the DFT of an integer multiples of the input signal is taken, > then the > mahgnitude response shows that there are no sidelobes. > > However, when the DFT of a non-interger multiples of the input signal > is taken, > we get the sidelobes, which we call the DFT leakage. > > Ex: > when input signal has 5 cycles, and you obtain the DFT at any analysis > frequency(m*fs/

## DFT symmetry

Started by in comp.dsp12 years ago 10 replies

I've just discovered that the DFT is symmetrical about the N/2 value and does not include DC, something which the institution which is supposed...

I've just discovered that the DFT is symmetrical about the N/2 value and does not include DC, something which the institution which is supposed to have told me this (the university I go to) had not! Anyway, for an even N how can the DFT be symmetrical about the Im-axis when there is always going to be one extra bin in either the positive or negative frequency side? This means that t...

## DCT vs. DFT

Started by in comp.dsp7 years ago 10 replies

Hi! Can anyone explain me, why you use DCT instead of DFT for transformation of image-data (like it is done for JPEG). I know that the DCT...

Hi! Can anyone explain me, why you use DCT instead of DFT for transformation of image-data (like it is done for JPEG). I know that the DCT can be performed by a DFT with a post-rotation. But what are the advantages of the DCT compared to the DFT? Are there any benefits from the post-rotation? Regards, Matthias

## DFT or DFS: Are they the same thing?

Started by in comp.dsp14 years ago 83 replies

Hello, I have a question, it may be simple but its bugging me, so I need your help. What is the difference between Discrete Fourier Series...

Hello, I have a question, it may be simple but its bugging me, so I need your help. What is the difference between Discrete Fourier Series (DFS) and the Discrete Fourier Transform (DFT) both theoretically and practically Now, I know that we use DFS for the spectral analysis of Periodic sequences and DFT for Aperiodic. (My concept is that in DFT, we have an aperiodic sequence so we decide on ...

## spectral estimation question

Started by in comp.dsp18 years ago 49 replies

OK, here's one of those "This should be simple..." questions. We know that many (all?) methods of spectral estimation of discrete-time data...

OK, here's one of those "This should be simple..." questions. We know that many (all?) methods of spectral estimation of discrete-time data rely fundamentally on the DFT (the N-point DFT). However, the output of a specific bin of the DFT can be viewed as a bandpass filter. This DFT bandpass filter is not particularly "good," i.e., it has high side lobes and poor stop-band rejection. So...

## Equalization using DFT

Started by in comp.dsp15 years ago 25 replies

Hi, I'm wondering, if there is no problem in equalization with DFT. Simply window the source signal, DFT, mutliply each frequency bin by a...

Hi, I'm wondering, if there is no problem in equalization with DFT. Simply window the source signal, DFT, mutliply each frequency bin by a real value defining it, and IDFT. This would even be zero-phase filtering wouldn't it? Or am I missing something? Thanks! dmnc

## How to caulate the Frequency Sample Response of IIR Filter?

Started by in comp.dsp8 years ago 7 replies

This has probably been answered already but how does one the 'DFT' samples of an IIR filter from the 'b' and 'a' coefficients. For a FIR,...

This has probably been answered already but how does one the 'DFT' samples of an IIR filter from the 'b' and 'a' coefficients. For a FIR, this is trivial. if b_n are the coefficients which are by definition finite in length. The DFT would be easily calculated as DFT = F*b_n where F is the DFT matrix. How would one specify and calculate each frequency bin amplitude and phase for an I...

## DFT and bandpass filters

Started by in comp.dsp18 years ago 58 replies

I've a program that does a bandpass filtering using DFT. Firstly I do a real-to-half-complex DFT, then, i set to zero all the samples that are...

I've a program that does a bandpass filtering using DFT. Firstly I do a real-to-half-complex DFT, then, i set to zero all the samples that are not in my wanted frequency range, and then do a half-complex-to-real DFT. It works, the problem is that it creates artifacts at the two limit frequencies, due to the first and last samples of the transformed wave that are rarely near zero. I we...