/src/fdk-aac/libFDK/include/fft.h

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/* -----------------------------------------------------------------------------
Software License for The Fraunhofer FDK AAC Codec Library for Android

© Copyright  1995 - 2018 Fraunhofer-Gesellschaft zur Förderung der angewandten
Forschung e.V. All rights reserved.

 1.    INTRODUCTION
The Fraunhofer FDK AAC Codec Library for Android ("FDK AAC Codec") is software
that implements the MPEG Advanced Audio Coding ("AAC") encoding and decoding
scheme for digital audio. This FDK AAC Codec software is intended to be used on
a wide variety of Android devices.

AAC's HE-AAC and HE-AAC v2 versions are regarded as today's most efficient
general perceptual audio codecs. AAC-ELD is considered the best-performing
full-bandwidth communications codec by independent studies and is widely
deployed. AAC has been standardized by ISO and IEC as part of the MPEG
specifications.

Patent licenses for necessary patent claims for the FDK AAC Codec (including
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already be covered under those patent licenses when it is used for those
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Commercially-licensed AAC software libraries, including floating-point versions
with enhanced sound quality, are also available from Fraunhofer. Users are
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Redistribution and use in source and binary forms, with or without modification,
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You must retain the complete text of this software license in redistributions of
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5.    CONTACT INFORMATION

Fraunhofer Institute for Integrated Circuits IIS
Attention: Audio and Multimedia Departments - FDK AAC LL
Am Wolfsmantel 33
91058 Erlangen, Germany

www.iis.fraunhofer.de/amm
amm-info@iis.fraunhofer.de
----------------------------------------------------------------------------- */

/******************* Library for basic calculation routines ********************

   Author(s):   Josef Hoepfl, DSP Solutions

   Description: Fix point FFT

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

#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.
 */

#ifndef FUNCTION_fft_4
/**
 * \brief Perform an inplace complex valued FFT of length 4
 *
 * \param pInput Input/Output data buffer. The input data must have at least 1
 * bit scale headroom. The values are interleaved, real/imag pairs.
 */
LNK_SECTION_CODE_L1
static inline void 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 */
}
#endif /* FUNCTION_fft_4 */

#ifndef FUNCTION_fft_8
LNK_SECTION_CODE_L1
static inline void fft_8(FIXP_DBL *x) {
  FIXP_SPK w_PiFOURTH = {{FIXP_SGL(0x5A82), FIXP_SGL(0x5A82)}};

  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];

  cplxMultDiv2(&vi, &vr, 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];

  cplxMultDiv2(&vr, &vi, 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;
}
#endif /* FUNCTION_fft_8 */

#endif

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Created: 2026-09-14 08:00