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Real tech quesiton about FFT implementation

Official support for: sonicbirth.sourceforge.net

Moderators: dscro, makira

dscro
KVRist
 
92 posts since 28 Nov, 2007

Postby dscro; Sun Feb 17, 2008 8:28 pm Real tech quesiton about FFT implementation

As I understand the FFT implementation in SB, once I do an FFT, I can't seem to manipulate the FFT blocks the way I want.

Say for now that I think about the FFT output in its complex form. Then the real part consists of basically an array of numbers that represent the amplitudes of the various cosine elements of the Fourier series, the imaginary output gives the amplitudes of the sine components.

Any math I do to those outputs just changes the amplitudes. So say I multiply both [R] and [I] by 2. Then after inverse FFT I really have a waveform that just has twice the amplitude right?

Ok but what if I wanted to screw around with the frequencies of the sine and cosine components themselves, say . . . throw out the lowest one or shift them all up by some number to do pitch shift? Or say . . . multiply one the amplitudes of a specific harmonic by 2. I don't have access to those numbers in the output array right? The frequencies are just fixed multiples of some fundamental (2n, 3n, 4n, . . .) I can't change them in any way. This seems like such a basic functionality and the overall reason to do FFT in the first place.

I started to wonder if there is a way to do it with convolution (convolve an FFT of a single frequency sin wave with a signal or convolve a delta function/spike with the signal etc. but I haven't been successful.

Any tips here?
eigentone
KVRist
 
277 posts since 3 Apr, 2007

Postby eigentone; Mon Feb 25, 2008 12:02 pm

Hi dscro,

The frequency domain capabilities are presently much lower than the time domain - meaning, (AFAIK) much of the potential is not accessible at the circuit design level.

(Someday...)

J
SurvCons_Pres
KVRer
 
1 post since 9 Jun, 2008, from United States

Postby SurvCons_Pres; Mon Jun 09, 2008 3:27 pm

Hello, perhaps theres a "new member" area I should be saying "hi" in first, but I just want to jump off, kinda my personality.....

Anyone have knowledge of use if FFT in Optical Sigs Proc...???

Nice forum you guys have here, glad to visit. Best, Dave
Signature blocked until 5 posts made
Jee-Bee
KVRer
 
3 posts since 29 Nov, 2008, from Netherlands

Postby Jee-Bee; Tue Dec 06, 2011 9:36 am

Hey i am new with SB and my first try is to make a level and FFT analyzer
The level meter works basicly but the FFT anaylyzer i cant show it
i have tried the display meter and the osc but both are as far as i can see not what it meant to be
i have read that the fft is limited. is this one of the limitations or is this what i do wrong?

Gr Jan-bert

Moderators: dscro, makira

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