## Forums Search for: FFT

## Please verify my "solution" to FFT scale factors

inDear DSP masters!! I'm using the FFTW C library to evaluate some signal processing ideas. As I have Richard Lyons Understanding ... book I...

Dear DSP masters!! I'm using the FFTW C library to evaluate some signal processing ideas. As I have Richard Lyons Understanding ... book I first tried his DFT example of paragraph 3.1.1 in order to check if the FFT results are matching. They did. But I would like to use the FFT to get a correct frequency domain representation, so the outputs must be properly scaled. Note: I'm using the FF...

## How to make FFT when having small amount of RAM?

inLet's say I have an embedded system with a small amount of RAM and I want to make an FFT of sampled data. But I don't need to make the FFT of...

Let's say I have an embedded system with a small amount of RAM and I want to make an FFT of sampled data. But I don't need to make the FFT of the whole frequency range. Is there some kind of shortcut one can make to achieve this and save RAM? And let's say that the system isn't fast enough to make the FFT on the fly on the samples.

## Matlab and VC++ values differ when determining FFTusing FFTW

inHi there, I have recently started using FFTW to determine fft in C++, so I am really not good at it. I am currently trying to just evaluate my...

Hi there, I have recently started using FFTW to determine fft in C++, so I am really not good at it. I am currently trying to just evaluate my fft values in both matlab and C++. I have used FFTW to determine the fft in C++ and fft2 in matlab for a 4*4 matrix [int values]. My values were same in both Matlab and C++. My real requirement is to use fft on complex matrices. But my values no longer m...

## FFT Radix 3

inI have been trying to code a normal C program for a radix 3 fft problem. My program works for 3,6,9,27 points but conks off for the 81 point...

I have been trying to code a normal C program for a radix 3 fft problem. My program works for 3,6,9,27 points but conks off for the 81 point fft onwards. I would like to know if there is a mistake in my program or is it because VC can't support such intensive calculations ;P. Well I really don't know how to go ahead.I thought maybe I had calculated my twiddle factors wrong and I have even tried wr...

## Real and Complex FFT responses

inHi Folks Just a quick question to verify if I'm correct. In a real only FFT, I get a mirror image of the peaks in the...

Hi Folks Just a quick question to verify if I'm correct. In a real only FFT, I get a mirror image of the peaks in the output symmetrical about the half way number of points of the FFT. I know this is correct. However with the complex FFT, I only get the peaks(on one side only)...no mirror image. They are at the correct position. Can someone tell me if this is correct or not, and expl...

## FFT for Non 2^N points

inI need to take FFT for 160 points. Of course zero padding it to 256 points will allow me to take radix 2 fft. But I need to do...

I need to take FFT for 160 points. Of course zero padding it to 256 points will allow me to take radix 2 fft. But I need to do redundant computation for 96 points. Will split radix help? If so give me links regarding theory behind and implementation of it. If you feel any other method efficient in this case kindly let me know.

## minimum cycles for fft, and limits of filling short sample out with zeros

inI need to get an FFT on as small a sample window as possible. I need to understand the relationship between wave period expressed in...

I need to get an FFT on as small a sample window as possible. I need to understand the relationship between wave period expressed in samples, minimum samples needed for FFT without zero-filling, what percentage of those samples can be zeros from filling and still get a good result. 1. What is the minimum number of periods of signal I need to get an accurate FFT (without zero filling)? I kno...

## MATLABs FFT and fftshifting the input

inI've tried looking around for an answer to these questions... and there are a few threads. Yet, I am still confused. My questions of this...

I've tried looking around for an answer to these questions... and there are a few threads. Yet, I am still confused. My questions of this thread are: A. Why should one do fft( fftshift(x) ) B. What assumptions does the MATLAB fft(.) routine have? B.1. Does it expect the DC bin to be the first element of the input signal? B.2. What other assumptions... ? Here's my understanding (or lack...

## Converting FFT to inverse-FFT

inI'm thinking about having a play with a software-based radio, based on a heavily hacked-up radio and an IF-to-AF down-converter (basically, tap...

I'm thinking about having a play with a software-based radio, based on a heavily hacked-up radio and an IF-to-AF down-converter (basically, tap the IF, downconvert to AF, then use a sound card to digitise it). In any case, enough about the hardware -- that's not the issue. I've got some code for an FFT from the O'Reilly "C++ Cookbook" (section 11.17, example 11-33): template

## need help understanding meaning of FFT

inHi Everyone, I observed an unexpected result in doing some FFT work recently, that I think relates to understanding the fundamentals of an FFT...

Hi Everyone, I observed an unexpected result in doing some FFT work recently, that I think relates to understanding the fundamentals of an FFT I'm hoping someone can help me understand better. I'm not a DSP expert. I captured five waveforms sequentially using an oscilloscope. Aside from capturing these 5 waveforms with a short time delay between each, these 5 waveforms are otherwise iden...

## Zero padded FFT

inI have a query on zero-padded FFT's.For calculating FFT of a signal with sampling rate 256 HZ what is the maximum zero padding that can be...

I have a query on zero-padded FFT's.For calculating FFT of a signal with sampling rate 256 HZ what is the maximum zero padding that can be done?And what does zeropadding actually do to the signal. I have observed frequency shift with zero padding.why so?

## Maximize speed when using canned fft routines

inI'm looking at the periodic components of GPS position errors. I have chosen to use fft as my _initial_ tool. I have it. I'm...

I'm looking at the periodic components of GPS position errors. I have chosen to use fft as my _initial_ tool. I have it. I'm comfortable with it. I think I understand the possible results. I believe my "problem" meets criteria discussed in thread "When is FFT of experimental numerical data logically valid/legitimate" I expect periods of interest to range from 1/hour to 1/mon...

## FFT and complex numbers

inHi all, I have been looking at some fft libraries libraries and since DSP is not my main area, there are some things that I am confused about....

Hi all, I have been looking at some fft libraries libraries and since DSP is not my main area, there are some things that I am confused about. I noticed that all major fft libraries, e.g. FFTW & kissfft, use complex data types in which to store both the input and the output of the fourier transform. I was wondering, must this always be the case, or is it possible to just use array of pri...

## Shifting data before FFT

inI'm doing some DSP for radar applications. My background in DSP is limited, so I'm trying to get up to speed. Performing a circular rotation...

I'm doing some DSP for radar applications. My background in DSP is limited, so I'm trying to get up to speed. Performing a circular rotation to move the beginning of data to the center of the dataset before a FFT and then rotating back after seems to be routine (almost taken for granted). Can anyone explain the benefit of this when compared to just a straight FFT of the data? Thank y...

## Some question regarding FFT

Hi, i have a question regarding the fft, my supervisor gave me a Matlab code, in the code there are something that i couldn't understand due to...

Hi, i have a question regarding the fft, my supervisor gave me a Matlab code, in the code there are something that i couldn't understand due to my poor understanding of DSP, fu1=fftshift(fft(fftshift(u1))); det=sum(fu1(ps:pe)); amp(jp)=real(det); pha(jp)=angle(det); from the code here i don't know what is fftshift(fft(fftshift(u1))) mean, i knew that fftshift is to shift the ...

## What purpose is for a sliding FFT on windowing data?

inHi, I read some codes from some formal demonstration sample project. It is found that there are 4 data blocks (one is the present data...

Hi, I read some codes from some formal demonstration sample project. It is found that there are 4 data blocks (one is the present data block). After the 4 block data windowing, it is sent to an FFT function. The interesting thing is that the FFT function is not the same with the windowing block. In fact, the FFT is cycled with a fixed data block position (as one data block is getting...

## How to calculate the frequency of 2D FFT Output image?

inHi, I have a surface image of 15x15 microns which is characterized by 512x512 matrix. (you can see the pic in...

Hi, I have a surface image of 15x15 microns which is characterized by 512x512 matrix. (you can see the pic in http://img363.imageshack.us/img363/7671/pic1tk8.jpg) Now I want to take a 2D FFT of that image. So in matlab I used "FFT = fftshift(fft2(InputImageData))" to get the FFT, which looks like in the pic http://img363.imageshack.us/img363/3346/pic2ne5.jpg But I am not able to judge ...

## Blackfin FFT precision

inThe 32 bit data types of the Blackfin processors should preserve more bits of precision than a 16 bit fixed point processor, if the FFT routines...

The 32 bit data types of the Blackfin processors should preserve more bits of precision than a 16 bit fixed point processor, if the FFT routines make use of the 32 bit data types. Can anyone comment on whether this is the case, and how well the precision is maintained in the calculations? How much precision would result from a library FFT routine of 2048 points of 10 bit data after an FFT i...

## phase of FFT

inI am new to FFT and have read some online notes and help on FFT. For a discrete time signal, I understand the FFT in Matlab will...

I am new to FFT and have read some online notes and help on FFT. For a discrete time signal, I understand the FFT in Matlab will give conjugate-symmetric phase response. Basically, the phases in the positive and negative frequencies have phases of the same value but opposite signs. However, I read it online at several places that the phase shift between the positive and negative frequencies is ...

## radial/circular average of 2D FFT

hello, I have a question about image processing and Matlab :). How is it possible to do radial/circular average of 2D FFT. I have an image, I...

hello, I have a question about image processing and Matlab :). How is it possible to do radial/circular average of 2D FFT. I have an image, I did 2D FFT on it, and now I would like to have amplitude over r (r = radius). So, the 2D FFT of my image represents a "spot" now, so I would like to take different values of radius r (to say 10-20, for example), for every radius to calculate the mean v...