Using Fast Fourier Transforms in Symbian C++

Comments

10 Sep
2009

Article Review by deepikagohil (20090910)

Fast Fourier Transform is very imortant algoritm in Digital Signal Processing(DSP). The FFT if used in S60, adds many additional features in our application. This article simply explains how to perform FFT in our S60 application.The abstract-expaination and source-code both are given in the article.

The article is essential for intermediate and experienced developers which will help them to perform more complicated tasks.

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Article

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.

1. Download FFT algorithm by Laurent de Soras for C++.
2. Port the algorithm for Symbian with the following minor modifications:
   • Set the used data type in fftreal.h:

     typedef TReal flt_t;

   • Create standard two-phased 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>

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 0-n><Complex values of coefficients 0-n>. 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;