Using Fast Fourier Transforms in Symbian C++
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Created: mbrockma
(22 Aug 2007)
Last edited: hamishwillee
(03 Feb 2012)
The Fast Fourier transform (FFT) is one of the key algorithms of digital signal processing (DSP). This article shows how to port an open source FFT library and use it in your Symbian C++ applications.
Setup
Using FFT/IFFT algorithms in S60 is quite simple with the following instructions.
 Download FFT algorithm by Laurent de Soras for C++.
 Port the algorithm for Symbian with the following minor modifications:
 Set the used data type in fftreal.h:

typedef TReal flt_t;

 Create standard twophased constructors:

static CFFTReal* NewL(const TInt aLength);
void ConstructL();

 Add epoc32\include\libc to additional include directories and link against estlib.lib
 Modify FFTReal to inherit CBase and add #include <e32base.h>
 Set the used data type in fftreal.h:
Using the algorithm
Normal FFT:
fftreal >Fft(fft, x);
Inverse FFT:
fftreal>Ifft(fft, x);
fftreal>Rescale(x);
Important As the FFT coefficients may have complex values, they will be presented in format: <Real values of coefficients 0n><Complex values of coefficients 0n>. This way the length of the FFT result array is the same as the number of FFT points.
Example
#include "fftreal.h"
// FFT length must be a power of 2
TInt frame = 1024;
// Create objects
FFTReal* fftreal = FFTReal::NewL(frame);
FFTReal::flt_t * const x = new (ELeave) FFTReal::flt_t[frame];
FFTReal::flt_t * const fft = new (ELeave) FFTReal::flt_t[frame];
// Fetch the source signal from somewhere (eg. audio)
GetSourceSignal(x);
// Actual FFT calculation
fftreal >Fft(fft, x);
// Modify FFT coefficients etc.
ModifyFFT(fft);
// Convert back to time domain, note the important Rescale()
// after every Ifft() call!
fftreal>Ifft(fft, x);
fftreal>Rescale(x);
// Do something with the result
ProcessResult(x);
delete fft;
delete x;
delete fftreal;