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FFT coding help

Hi, I've just coded an easy fft program. You can find it here: http://rapidshare.com/files/147982232/fft.tar.gz.html Compile it with gcc -lm -I. fft.c ep.c and then run a.out. As you'll see, it takes about 0.5 seconds to do a length-8192 FFT. I think that's a very bad time. Compared to other...

FFT with TMS320F2812

I use the TI FFT Library and I have two question : First, in the TI example RFFT32_brev(ipcb,ipcb,N); RFFT32_brev(ipcb,ipcb,N);// Input samples in Real Part fft.win(&fft); RFFT32_brev(ipcb,ipcb,N); RFFT32_brev(ipcb,ipcb,N);// Input after windowing fft.calc(&fft); fft.split(&fft); fft.mag(...

Re: Converting FFT to inverse-FFT

On Wed, 16 Dec 2009 13:52:04 -0800 (PST), dbd wrote: > Did you swap real and imaginary both before the FFT and after the FFT > as the cited reference on IFFT states? Thank you, Dale. Now it makes sense. I had relied entirely upon the original assertion: > For an inverse FFT, just r...

FFT question

Hi all, I always hear that the computational complexity of FFT is O(NlogN). However I learn that there are many FFT algorithms such as: Cooley-Tukey FFT algorithm ,Split-radix FFT algorithm ,Prime-factor FFT algorithm ,Bruun's FFT algorithm ,,Rader's FFT algorithm ,Bluestein's FFT algorithm ...

Wimax FFT size support question

I have been looking into the 802.16 Wimax system ( OFDMA ) to get a basic understanding. The system seems to be able to support multiple FFT sizes, such as 128, 256, 512, 1024 etc. I am not able to understand couple of issues: 1. Is the FFT size fixed for a basestation(BS)? If not when can i...

time domain zero padding VS fft interpolation between bins

Hi, Suppose I have N samples and I want to do one N point FFT and a 4N point FFT with zeros padding. Can do get the 4N-FFT by interpolating the N-FFT? Is there a formula? Is the interpolation fast than just do a zero-padded 4N-FFT? Thanks ...

FFT code for dsPIC

Hi Has anyone written their own fft function for the dsPIC ? (NOT the supplied library fft). I need a fft for measuring signal in noise and not being a DSP person would prefer to implement it rather than write it from scratch. The dsPIC library scales the data by 1/N which limits the dynamic ...

FFT for ADC output

Hi, I'm new to DSP and FFT. My question is how to do FFT for ADC digital output code,as FFT algorithms i've seen in Internet always ask for real and imaginary parts input for the FFT algorithm. So what is the input for FFT, if we want to do it for digital codes? Thanks ...

2D FFT window function

I have some questions about length of windowing functions applied to 1D and 2D FFT. For a N point 1D FFT, it is very straightforward, I can generate a N point window function, and multiply it with the signal before doing FFT. But for N*N point 2D FFT, what length of window function should I use? Sho...

Real vs. complex FFT

Hi DSP People, Just a quick question... is there really much difference between the processing of a normal, complex FFT and a real-only FFT? I thought there wouldn't be... The FFT/DFT math still requires that complex and imaginary parts of the FFT would be needed during the FFT processing, ...

is FFT always approximate?

If I have a time signal which is periodic, and I use FFT to obtain the spectrum, which should be discrete, will this FFT procedure be approximate? I am wondering about this because I've heard that FFT is only an approximation to the true Fourier transform... ...

FFT of 0s and 1s signal

Hello !! I have to FFT a signal made of 0s and 1s, i.e., my sequence is for example, 0,1,1,0,0,0,0,1,0,1,1,0 ... etc This is a true serie of 0s and 1s, not a number, written as binary. I am wondering if someone knows about a FFT algorithm able to handle such a cose. I'm not (far not!) an ex...

assemble 2K FFT by 2 times 1K FFT?

Hi folks, I heard about the possibility to obtain higher order FFTs by utilizing lower order FFTs. In fact, this IS the basic idea of FFT as far as i know. But since I am not that deep into the theory of FFT, I have a question to the experts. I want to perform a 2K FFT within FPGA. Now i ...

Is there a sample Java code for fixedpoint FFT?

Hi, I have now a Java code implementing real FFT. I need actually Java code for implementing integer (fixed-point) FFT. I mean with this that the FFT accepts Java int type as input and produces Java int type as output. If there is no Java fixed-point FFT, how can I convert the existing Java real ...

FFT processor - some issues

i am working on a FFT processor. i need to know that if i need to take the 1024 point FFT of a voltage time series then what should i do. i mean which one is correct way. 1) wait for 1024 samples and take their FFT. then wait for another 1024 samples and take their FFT and so on.... 2) wait fo...

use of radix-2 ffts

Hello again, Can a radix-4 fft be used to construct any type of radix-2 fft. e.g. can I manipulate a 64 pt radix-4 fft to give me a 128 pt radix-2 fft ? If so, is the radix-2 fft of any use in dsp as it usually results in larger and slower ffts than the radix-4 version. As always thanks f...

Large FFT on ADSP-21065L

Hi. I want to use a very large FFT (65536 samples) on a Sharc 21065L. But I encountered a problem with the ADI FFT (rfft65536) with my DSP: the twiddle-factors need to be in the PM, but that is far too much data for the internal memory of the 21065. I tried the FFT from "Numerical Recipies in C",...

Question about FFT phase

Hi. I understand that in FFT, we will get both the amplitude and the phase components. Is it possible to get the same phase value but yet the amplitudes of the FFT result are different? For example, a signal input produces a FFT amplitude of 10 and the phase computed is pi/4. If because of some n...

Analyzing ADC performance using FFT

I am new to FFT and very eager to learn FFT and explore various application use of FFT in real world application. I have a ADC and I have accumulated 4096 samples. Now I want to perform FFT and find out details on noise. Can you please Direct to web link which show steps on how to perform ...

FFT Radix

I am trying to implement a 128 point FFT. I have 16 8-bit samples aligned and ready for input into the FFT. I would like to know if the terms "mixed radix fft" and "split radix fft" are considered interchangeable. If not, what is different? Is it possible to use 64 radix 2 butterflys in front ...

Slight Modification of the FFT algorithm

Hello All, Suppose I have a non linear system with four inputs x1,x2,x3,x4 and one output y² x1(n) --> --> y1²(n)-------\ x2(n) --> (H) --> y2²(n)------ +-----> y²(n) = y1²(n)+y2²(n)+y3²(n)+y²4(n) x3(n) --> --> y3²(n)------/ x4(n) --> --> y4²(n)----...

combining small FFTs to make a larger FFT

Hello, When combining two smaller FFTs to make a larger one, what are the techniques. Example..I want to combine two 64 pt FFTs into a 128 pt FFT. I have fed the first 64 data samples into the first FFT and the second 64 into the 2nd FFT. The results are output from the two FFTs into a 128 array....

Relation between No of points of FFT and sampling frequency?

Hi Friends, I am new to this. My questions is very basic. How do i decide that i need to use 1024 point FFT or 2048 point FFT or N point FFT for a system. I am working on a project, where my sampling frequency is 80mhz and i need to decide how many point FFT to be use? What are the paramete...

FFT on dsPIC33F

Hi, Can anyone help with this ? :- I am using the fft function on the dsPIC from the Microchip library. When I do a fft I get a non linear result. The dsPIC function claims the result will be 1/N where N=fft size. When I do a 512 fft with a 20Hz sine signal sampled at 640Hz - I can extra...

FFT question!

Hi, When realising a FFT, can we get the frequency corresponding to each point calculated? Apparently, the results given by a FFT is only the [Re,Im] of each point? If we can't know the frequency by a FFT, how can we do it? The thing i'd like to do is detecting 2 different frequencies from ...

Using *real* FFT separably for multiple dimensions

Hello Comp.DSP, I need to perform forward and inverse FFT of multi-dimensional, real data. I am already successful to do this using the complex FFT separably. Now I would like to do this using the real FFT for computational savings. However, after I perform real FFT of the first dimension, ...

Help me a build the FFT of a signal please

Hello, I just started to work with DSP 320VC5416 and I try to build the FFT of the sine signal. If I have right, then I should get only one peak in FFT spectrum. but if I use fft function, and then show my result in "code composer studio" in praph/time:frequency, then I see a lot of peak...

How to cross-correlate in polar coordinate using FFT?

Hi, We all know the FFT-based cross-correlation in cartesian coordinate is given by: C = IFFT[ FFT[ f(x,y) ] * Conj[ FFT[ h(x,y) ] ] Here is the question: how is it possible to do this in polar? I do not think if this is the answer: C = IFFT[ FFT[ f(r,p) ] * Conj[ FFT[ h(r,p) ] ] wh...

Re: dsPIC 1024 or 2048 FFT

> Why getting stick to Microchip functions? There is a million of different > FFT realizations in C around. Just pick the one you like. > After reading the documentation, it appears to me that Microchip has tailored their FFT function for the dsPIC hardware. The way that they allocate m...

Unusual Transform Exist?

Suppose you have 8 discrete samples. If you apply a typical Cooley/Turkey FFT of size-8, you will get 4 real/even and 4 imag/odd values. Suppose you then take successive FFTs of size-4 for the different consecutive size-4 chunks from the original 8 time samples - you would have 5 size-4 FFT re...

FFT Algorithm and Time

I am new to DSP and FFT. I have implememented the FFT in Smith's book. What I am a little confused about is how do I determine what is the frequency scale of the X axis? thanks Don ...

COnvolution and FFT

Hello, I have a gaussian kernel of size [192,192] to be convoluted with an image of size [512,512] for smoothing. Using the FFT property that fft of a convolution is the product of the fft of the individual terms (fft(f(t) * g(t))) = f(w)g(w), i could say that smoothedimage = fftinverse(fft(...

dc component in fft query

Hi all, Just wondering about the dc component of an fft. If I have an input signal e.g a single cosine wave. When I sample it for say a 128 point fft, should it have a dc component value. In my fft, I sum the input samples and calculate the average. I half expected it to be zero (no dc c...

FFT SIZE DETECTION

Dear members, I am encountering a peculiar problem, wherein i have to dynamically detect size of the FFT of the signal taken. for example a signal is taken and fft of it is taken, it may be 128 or 256 or any size. utimately i dont have any iota of knowledge of the signal...

Zero padded FFt

Hi, In order to increase resolution of my FFT (reduce midbin loss), I want to use a 2N point FFT, N is a power of 2, last N points are zeroes. What is the efficient way to compute this? TIA BRMADHUKAR ...

FFT Structure optimization

Hi! I want to ask for your advice about a problem i have. I need to perform an FFT every specified amount of time but the FFT calculation takes more time than i can afford. as an optimization step i want to throw half of the output bins of the FFT but on the FFT structures that i know it'll only sa...

sub-Hz FFT

I am sampling voltage values. I need to convert it to the frequency domain using FFT . As I understand the frequency steps after FFT will be the integer multiples of Fs/N (where Fs is the data sample rate in Hz, and N is the FFT length). How can I have sub-Hz frequency steps? Do I use use long sa...

FFT Size calculation.

Hi, I have the following problem that I am trying to address. Lets say we take 64 numbers and apply FFT of size 64. Then we concatenate 100 of these 64 numbers, and perform D/A conversion. This signal is digitized at a rate other than the original sampling rate, say we have oversampling of 3/...

normalised cross correlation by fft

Hi, I have got a problem doing 2D normalised cross correlation by FFT. The normal 2D cross correlation seems very time consuming for long signal so I tried to use FFT. I can now get the equal result of cross correlation both by equation and by FFT, but I can not do the normalisation to get the re...

testing FFTs

Hi Folks, A few weeks ago, I posted a query on FFT scaling. After your kind replys, I got it working. I was able to get the real only FFT to work. By this I mean, there was no imaginary data input. However how does one test a complex FFT to make sure it is working? Is there a datafile th...