## Some Common FFT Misconceptions

Started by in comp.dsp11 years ago 20 replies

Just for fun, I thought I'd try and list many of the misconceptions that people seem to have regarding using the FFT, as seen in various...

Just for fun, I thought I'd try and list many of the misconceptions that people seem to have regarding using the FFT, as seen in various previous posts to comp.dsp. Feel free to disagree, or to add your own favorites. The FFT is a widely used algorithm because the result has something (not precisely understood) to do with frequencies. Some Common FFT Misconceptions: 1. The FFT will ...

## FFT Structure optimization

Started by in comp.dsp12 years ago 7 replies

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...

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 save half of the last stage butterflies calculation. is there any other FFT structure that i can use...

## Real vs. complex FFT

Started by in comp.dsp15 years ago 9 replies

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? ...

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, right?... but only the real part would be left at the end?. This wouldn't necessarily result in ...

## is FFT always approximate?

Started by in comp.dsp13 years ago 26 replies

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...

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...

## should FFT{x-mean(x)} = FFT{x}-FFT{mean(x)} ?

Started by in comp.dsp9 years ago 6 replies

Hi guys, The Linearity property of Fourier transform says: z(t)=af1(t)+bf2(t) the linear combination of the terms is unaffected by...

Hi guys, The Linearity property of Fourier transform says: z(t)=af1(t)+bf2(t) the linear combination of the terms is unaffected by the transform. Z(?)=aF1(?)+bF2(?) and Z(?) = FFT{ z(t) }. I synthesised a signal, sn as follows. I want to remove the mean values in the signal. According to the Fourier transform property, FFT{x-mean(x)} = FFT{x}-FFT{mean(x)} the left

## should FFT{x-mean(x)} = FFT{x}-FFT{mean(x)} ?

Started by in comp.dsp9 years ago 4 replies

Hi guys, The Linearity property of Fourier transform says: z(t)=af1(t)+bf2(t) the linear combination of the terms is unaffected by...

Hi guys, The Linearity property of Fourier transform says: z(t)=af1(t)+bf2(t) the linear combination of the terms is unaffected by the transform. Z(?)=aF1(?)+bF2(?) and Z(?) = FFT{ z(t) }. I synthesised a signal, sn as follows. I want to remove the mean values in the signal. According to the Fourier transform property, FFT{x-mean(x)} = FFT{x}-FFT{mean(x)} the left

## Analyzing ADC performance using FFT

Started by in comp.dsp11 years ago 1 reply

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...

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 for N samples? Guide me how to do? I am using DSP. I hope DSP GURU from this forum would cer...

## 1D Deconvolution

Started by in comp.dsp3 years ago 3 replies

Rosetta code has a 'C' example of 1D deconvolution. https://rosettacode.org/wiki/Deconvolution/1D It basically: - takes the FFT of the...

Rosetta code has a 'C' example of 1D deconvolution. https://rosettacode.org/wiki/Deconvolution/1D It basically: - takes the FFT of the convoluted signal. - takes the FFT of the original. - divides 'em. - Takes the FFT of the result. Is the FFT after the divide one of those cases where this fft is its own inverse - it is being used as an ifft? Thanks in advance. -- Les Carg...

## Help me a build the FFT of a signal please

Started by in comp.dsp14 years ago 3 replies

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...

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 peaks. I think, I should do some operation with output of the fft, but wich operation should I do? ...

## sub-Hz FFT

Started by in comp.dsp10 years ago 2 replies

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...

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 sampling times and long FFT lengths, for instance, Sampleing at 1 kHz and using 10000 FFT length will...

## Million-point FFT

Started by in comp.dsp6 years ago 16 replies

Hi all, I am interested in performing million-point FFT in FPGA. I've read some papers and concluded that the best way of performing...

Hi all, I am interested in performing million-point FFT in FPGA. I've read some papers and concluded that the best way of performing million-point FFT would be having multiple smaller size FFT in parallel, for example 8x 16K-point FFT, with twiddle factor multipliers (I think it will be radix-2^2 streaming FFT with multiple inputs) But, since I am a newbie in DSP, I want to check if there is a...

## Million-point FFT

Started by in comp.dsp6 years ago

Hi all, I am interested in performing million-point FFT in FPGA. I've read some papers and concluded that the best way of performing...

Hi all, I am interested in performing million-point FFT in FPGA. I've read some papers and concluded that the best way of performing million-point FFT would be having multiple smaller size FFT in parallel, for example 8x 16K-point FFT, with twiddle factor multipliers (I think it will be radix-2^2 streaming FFT with multiple inputs) But, since I am a newbie in DSP, I want to check if there ...

## using FFT as a channelizer

Started by in comp.dsp9 years ago

Hi, I am learning multi-channel DDC techniques using FFT. My design is referenced to http://www.pentek.com/tutorials/15_4/digdown.cfm Something...

Hi, I am learning multi-channel DDC techniques using FFT. My design is referenced to http://www.pentek.com/tutorials/15_4/digdown.cfm Something in this design I do not understand. The FFT will split the input stream into N(FFT size) bins equally spaced in frequency. Totally 4 FFTs are used so that the output data rate is fs/N*4. Followed by the FFT a compensation filter is used to flatten the...

## using FFT as a channelizer

Started by in comp.dsp9 years ago 2 replies

Hi, I am learning multi-channel DDC techniques using FFT. My design is referenced to http://www.pentek.com/tutorials/15_4/digdown.cfm Something...

Hi, I am learning multi-channel DDC techniques using FFT. My design is referenced to http://www.pentek.com/tutorials/15_4/digdown.cfm Something in this design I do not understand. The FFT will split the input stream into N(FFT size) bins equally spaced in frequency. Totally 4 FFTs are used so that the output data rate is fs/N*4. Followed by the FFT a compensation filter is used to flatten the...

## use of radix-2 ffts

Started by in comp.dsp15 years ago 13 replies

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...

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 for all replies. Bob Carter -- My ignorance is shameful, but I would rather be ashamed than igno...

## Question about FFT phase

Started by in comp.dsp7 years ago 18 replies

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...

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 noise, the FFT amplitude gets influenced and become 4 or 5 or just some values, and yet the phase cou...

## FFT Matlab - What is a N-point FFT?

Started by in comp.dsp13 years ago 11 replies

I have been collecting emg signal and would like to look at the frequency of the signal. In Matlab I know there is an FFT function which I have...

I have been collecting emg signal and would like to look at the frequency of the signal. In Matlab I know there is an FFT function which I have been using, but ran into a problem when using it. The example given in the help says it is using a 512-point FFT. I tried using a 512-point FFT and a 2056-point FFT and noticed that they look very different. Is the only difference that the 512 giv...

## Re: Execution times of FFT in MATLAB vs complex FFT in C

Started by in comp.dsp15 years ago

> I've written a relatively efficient radix 2 fft engine for a commercial > voice quality measurement product (while I was working) which...

> I've written a relatively efficient radix 2 fft engine for a commercial > voice quality measurement product (while I was working) which largely > modelled the one in Numerical Recipe. I've never seen a radix-2 FFT code that was efficient... > I did some research. Apparently, most people find integrating the fft in > Intel's library in general does indeed give 5 times of speed enhancement.

## Using KISS FFT with Ti's TMS320F2812 DSP

Started by in comp.dsp14 years ago 2 replies

I am trying to use the KISS FFT library found on www.sourceforge.net with the TMS320F2812 DSP from Texas Instruments. After a bit of work, I...

I am trying to use the KISS FFT library found on www.sourceforge.net with the TMS320F2812 DSP from Texas Instruments. After a bit of work, I have been able to compile the code and even run a couple of small-scale FFT calculations, but am having trouble with larger FFT sizes. I have a program set up to trigger the ADC periodically and commence the FFT calculation after enough samples have ...

## downsampling -> FFT -> upsampling

Started by in comp.dsp14 years ago 39 replies

Hi! I'm experimenting with FFT and inverse FFT, doing some filtering in real-time. It sounds good but it's a bit too CPU heavy so I decided to...

Hi! I'm experimenting with FFT and inverse FFT, doing some filtering in real-time. It sounds good but it's a bit too CPU heavy so I decided to downsample the signal and apply the FFT and inverse FFT on samplerate/2 and then interpolate the resulting upsampling the output again to the original samplerate. However, I get some strange overtones when I resample the signal. I've tried differe...