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/*
* Program: REALFFT.C
* Author: Philip VanBaren
* Date: 2 September 1993
*
* Description: These routines perform an FFT on real data.
* On a 486/33 compiled using Borland C++ 3.1 with full
* speed optimization and a small memory model, a 1024 point
* FFT takes about 16ms.
* This code is for integer data, but could be converted
* to float or double simply by changing the data types
* and getting rid of the bit-shifting necessary to prevent
* overflow/underflow in fixed-point calculations.
*
* Note: Output is BIT-REVERSED! so you must use the BitReversed to
* get legible output, (i.e. Real_i = buffer[ BitReversed[i] ]
* Imag_i = buffer[ BitReversed[i]+1 ] )
* Input is in normal order.
*
* Copyright (C) 1995 Philip VanBaren
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/
/*
* For Borland C++/TASM you may define the flag ASMFFTCODE in order to use the
* assembly code for the FFT. This will skip the compilation of the C version
* of the routine; you must be sure to add realffta.asm to your makefile
*/
#include <math.h>
#include <stdlib.h>
#include <stdio.h>
int *BitReversed;
short *SinTable;
int Points = 0;
/*
* Initialize the Sine table and Twiddle pointers (bit-reversed pointers)
* for the FFT routine.
*/
void InitializeFFT(int fftlen)
{
int i;
int temp;
int mask;
/*
* FFT size is only half the number of data points
* The full FFT output can be reconstructed from this FFT's output.
* (This optimization can be made since the data is real.)
*/
Points = fftlen;
if((SinTable=(short *)malloc(Points*sizeof(short)))==NULL)
{
puts("Error allocating memory for Sine table.");
exit(1);
}
if((BitReversed=(int *)malloc(Points/2*sizeof(int)))==NULL)
{
puts("Error allocating memory for BitReversed.");
exit(1);
}
for(i=0;i<Points/2;i++)
{
temp=0;
for(mask=Points/4;mask>0;mask >>= 1)
temp=(temp >> 1) + (i&mask ? Points/2 : 0);
BitReversed[i]=temp;
}
for(i=0;i<Points/2;i++)
{
register double s,c;
s=floor(-32768.0*sin(2*M_PI*i/(Points))+0.5);
c=floor(-32768.0*cos(2*M_PI*i/(Points))+0.5);
if(s>32767.5) s=32767;
if(c>32767.5) c=32767;
SinTable[BitReversed[i] ]=(short)s;
SinTable[BitReversed[i]+1]=(short)c;
}
}
/*
* Free up the memory allotted for Sin table and Twiddle Pointers
*/
void EndFFT()
{
free(BitReversed);
free(SinTable);
Points=0;
}
/* Include this only if you don't use the REALFFTA.ASM routine */
/* This is for DOS only */
#ifndef ASMFFTCODE
short *A,*B;
short *sptr;
short *endptr1,*endptr2;
int *br1,*br2;
long HRplus,HRminus,HIplus,HIminus;
/*
* Actual FFT routine. Must call InitializeFFT(fftlen) first!
*/
void RealFFT(short *buffer)
{
int ButterfliesPerGroup=Points/4;
endptr1=buffer+Points;
/*
* Butterfly:
* Ain-----Aout
* \ /
* / \
* Bin-----Bout
*/
while(ButterfliesPerGroup>0)
{
A=buffer;
B=buffer+ButterfliesPerGroup*2;
sptr=SinTable;
while(A<endptr1)
{
register short sin=*sptr;
register short cos=*(sptr+1);
endptr2=B;
while(A<endptr2)
{
long v1=((long)*B*cos + (long)*(B+1)*sin) >> 15;
long v2=((long)*B*sin - (long)*(B+1)*cos) >> 15;
*B=(*A+v1)>>1;
*(A++)=*(B++)-v1;
*B=(*A-v2)>>1;
*(A++)=*(B++)+v2;
}
A=B;
B+=ButterfliesPerGroup*2;
sptr+=2;
}
ButterfliesPerGroup >>= 1;
}
/*
* Massage output to get the output for a real input sequence.
*/
br1=BitReversed+1;
br2=BitReversed+Points/2-1;
while(br1<=br2)
{
register long temp1,temp2;
short sin=SinTable[*br1];
short cos=SinTable[*br1+1];
A=buffer+*br1;
B=buffer+*br2;
HRplus = (HRminus = *A - *B ) + (*B << 1);
HIplus = (HIminus = *(A+1) - *(B+1)) + (*(B+1) << 1);
temp1 = ((long)sin*HRminus - (long)cos*HIplus) >> 15;
temp2 = ((long)cos*HRminus + (long)sin*HIplus) >> 15;
*B = (*A = (HRplus + temp1) >> 1) - temp1;
*(B+1) = (*(A+1) = (HIminus + temp2) >> 1) - HIminus;
br1++;
br2--;
}
/*
* Handle DC bin separately
*/
buffer[0]+=buffer[1];
buffer[1]=0;
}
#endif
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