fdk-aac/libFDK/include/fft.h

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   $Id$
   Author(s):   Josef Hoepfl, DSP Solutions
   Description: Fix point FFT

   This software and/or program is protected by copyright law and international
   treaties. Any reproduction or distribution of this software and/or program,
   or any portion of it, may result in severe civil and criminal penalties, and
   will be prosecuted to the maximum extent possible under law.

******************************************************************************/

#ifndef __FFT_H__
#define __FFT_H__

#include "common_fix.h"

/**
 * \brief Perform an inplace complex valued FFT of length 2^n
 *
 * \param length Length of the FFT to be calculated.
 * \param pInput Input/Output data buffer. The input data must have at least 1 bit scale headroom.
 *          The values are interleaved, real/imag pairs.
 * \param scalefactor Pointer to an INT, which contains the current scale of the input data,
 *                    which is updated according to the FFT scale.
 */
void fft(int length, FIXP_DBL *pInput, INT *scalefactor);

/**
 * \brief Perform an inplace complex valued IFFT of length 2^n
 *
 * \param length Length of the FFT to be calculated.
 * \param pInput Input/Output data buffer. The input data must have at least 1 bit scale headroom.
 *          The values are interleaved, real/imag pairs.
 * \param scalefactor Pointer to an INT, which contains the current scale of the input data,
 *                    which is updated according to the IFFT scale.
 */
void ifft(int length, FIXP_DBL *pInput, INT *scalefactor);


/*
 * Frequently used and fixed short length FFTs.
 */

LNK_SECTION_CODE_L1
static void FORCEINLINE fft_4(FIXP_DBL *x)
{
    FIXP_DBL a00, a10, a20, a30, tmp0, tmp1;

    a00 = (x[0] + x[4])>>1;  /* Re A + Re B */
    a10 = (x[2] + x[6])>>1;  /* Re C + Re D */
    a20 = (x[1] + x[5])>>1;  /* Im A + Im B */
    a30 = (x[3] + x[7])>>1;  /* Im C + Im D */

    x[0] = a00 + a10;       /* Re A' = Re A + Re B + Re C + Re D */
    x[1] = a20 + a30;       /* Im A' = Im A + Im B + Im C + Im D */

    tmp0 = a00 - x[4];       /* Re A - Re B */
    tmp1 = a20 - x[5];       /* Im A - Im B */

    x[4] = a00 - a10;       /* Re C' = Re A + Re B - Re C - Re D */
    x[5] = a20 - a30;       /* Im C' = Im A + Im B - Im C - Im D */

    a10 = a10 - x[6];       /* Re C - Re D */
    a30 = a30 - x[7];       /* Im C - Im D */

    x[2] = tmp0 + a30;       /* Re B' = Re A - Re B + Im C - Im D */
    x[6] = tmp0 - a30;       /* Re D' = Re A - Re B - Im C + Im D */
    x[3] = tmp1 - a10;       /* Im B' = Im A - Im B - Re C + Re D */
    x[7] = tmp1 + a10;       /* Im D' = Im A - Im B + Re C - Re D */
}

LNK_SECTION_CODE_L1
static void FORCEINLINE fft_8(FIXP_DBL *x)
{
      #define W_PiFOURTH STC(0x5a82799a)

      FIXP_DBL a00, a10, a20, a30;
      FIXP_DBL y[16];

      a00 = (x[0] + x[8])>>1;
      a10 =  x[4] + x[12];
      a20 = (x[1] + x[9])>>1;
      a30 =  x[5] + x[13];

      y[0] = a00 + (a10>>1);
      y[4] = a00 - (a10>>1);
      y[1] = a20 + (a30>>1);
      y[5] = a20 - (a30>>1);

      a00 = a00      - x[8];
      a10 = (a10>>1) - x[12];
      a20 = a20      - x[9];
      a30 = (a30>>1) - x[13];

      y[2] = a00 + a30;
      y[6] = a00 - a30;
      y[3] = a20 - a10;
      y[7] = a20 + a10;

      a00 = (x[2] + x[10])>>1;
      a10 =  x[6] + x[14];
      a20 = (x[3] + x[11])>>1;
      a30 =  x[7] + x[15];

      y[8]  = a00 + (a10>>1);
      y[12] = a00 - (a10>>1);
      y[9]  = a20 + (a30>>1);
      y[13] = a20 - (a30>>1);

      a00 = a00      - x[10];
      a10 = (a10>>1) - x[14];
      a20 = a20      - x[11];
      a30 = (a30>>1) - x[15];

      y[10] = a00 + a30;
      y[14] = a00 - a30;
      y[11] = a20 - a10;
      y[15] = a20 + a10;

      FIXP_DBL vr, vi, ur, ui;

      ur = y[0]>>1;
      ui = y[1]>>1;
      vr = y[8];
      vi = y[9];
      x[0] = ur + (vr>>1);
      x[1] = ui + (vi>>1);
      x[8] = ur - (vr>>1);
      x[9] = ui - (vi>>1);

      ur = y[4]>>1;
      ui = y[5]>>1;
      vi = y[12];
      vr = y[13];
      x[4]  = ur + (vr>>1);
      x[5]  = ui - (vi>>1);
      x[12] = ur - (vr>>1);
      x[13] = ui + (vi>>1);

      ur = y[10];
      ui = y[11];
      vr = fMultDiv2(ui+ur,W_PiFOURTH);
      vi = fMultDiv2(ui-ur,W_PiFOURTH);
      ur = y[2];
      ui = y[3];
      x[2]  = (ur>>1) + vr;
      x[3]  = (ui>>1) + vi;
      x[10] = (ur>>1) - vr;
      x[11] = (ui>>1) - vi;

      ur = y[14];
      ui = y[15];
      vr = fMultDiv2(ui-ur,W_PiFOURTH);
      vi = fMultDiv2(ui+ur,W_PiFOURTH);
      ur = y[6];
      ui = y[7];
      x[6]  = (ur>>1) + vr;
      x[7]  = (ui>>1) - vi;
      x[14] = (ur>>1) - vr;
      x[15] = (ui>>1) + vi;
}

/**
 * \brief FFT of fixed length 16
 */
inline void fft_16(FIXP_DBL *x);

/**
 * \brief FFT of fixed length 32
 */
inline void fft_32(FIXP_DBL *x);


#endif