mpv/audio/filter/window.c

/*
 * Copyright (C) 2001 Anders Johansson ajh@atri.curtin.edu.au
 *
 * This file is part of MPlayer.
 *
 * MPlayer 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.
 *
 * MPlayer 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 MPlayer; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */

/* Calculates a number of window functions. The following window
   functions are currently implemented: Boxcar, Triang, Hanning,
   Hamming, Blackman, Flattop and Kaiser. In the function call n is
   the number of filter taps and w the buffer in which the filter
   coefficients will be stored.
*/

#include <math.h>
#include "dsp.h"

/*
// Boxcar
//
// n window length
// w buffer for the window parameters
*/
void af_window_boxcar(int n, FLOAT_TYPE* w)
{
  int i;
  // Calculate window coefficients
  for (i=0 ; i<n ; i++)
    w[i] = 1.0;
}


/*
// Triang a.k.a Bartlett
//
//               |    (N-1)|
//           2 * |k - -----|
//               |      2  |
// w = 1.0 - ---------------
//                    N+1
// n window length
// w buffer for the window parameters
*/
void af_window_triang(int n, FLOAT_TYPE* w)
{
  FLOAT_TYPE k1  = (FLOAT_TYPE)(n & 1);
  FLOAT_TYPE k2  = 1/((FLOAT_TYPE)n + k1);
  int      end = (n + 1) >> 1;
  int	   i;

  // Calculate window coefficients
  for (i=0 ; i<end ; i++)
    w[i] = w[n-i-1] = (2.0*((FLOAT_TYPE)(i+1))-(1.0-k1))*k2;
}


/*
// Hanning
//                   2*pi*k
// w = 0.5 - 0.5*cos(------), where 0 < k <= N
//                    N+1
// n window length
// w buffer for the window parameters
*/
void af_window_hanning(int n, FLOAT_TYPE* w)
{
  int	   i;
  FLOAT_TYPE k = 2*M_PI/((FLOAT_TYPE)(n+1)); // 2*pi/(N+1)

  // Calculate window coefficients
  for (i=0; i<n; i++)
    *w++ = 0.5*(1.0 - cos(k*(FLOAT_TYPE)(i+1)));
}

/*
// Hamming
//                        2*pi*k
// w(k) = 0.54 - 0.46*cos(------), where 0 <= k < N
//                         N-1
//
// n window length
// w buffer for the window parameters
*/
void af_window_hamming(int n,FLOAT_TYPE* w)
{
  int      i;
  FLOAT_TYPE k = 2*M_PI/((FLOAT_TYPE)(n-1)); // 2*pi/(N-1)

  // Calculate window coefficients
  for (i=0; i<n; i++)
    *w++ = 0.54 - 0.46*cos(k*(FLOAT_TYPE)i);
}

/*
// Blackman
//                       2*pi*k             4*pi*k
// w(k) = 0.42 - 0.5*cos(------) + 0.08*cos(------), where 0 <= k < N
//                        N-1                 N-1
//
// n window length
// w buffer for the window parameters
*/
void af_window_blackman(int n,FLOAT_TYPE* w)
{
  int      i;
  FLOAT_TYPE k1 = 2*M_PI/((FLOAT_TYPE)(n-1)); // 2*pi/(N-1)
  FLOAT_TYPE k2 = 2*k1; // 4*pi/(N-1)

  // Calculate window coefficients
  for (i=0; i<n; i++)
    *w++ = 0.42 - 0.50*cos(k1*(FLOAT_TYPE)i) + 0.08*cos(k2*(FLOAT_TYPE)i);
}

/*
// Flattop
//                                        2*pi*k                     4*pi*k
// w(k) = 0.2810638602 - 0.5208971735*cos(------) + 0.1980389663*cos(------), where 0 <= k < N
//                                          N-1                        N-1
//
// n window length
// w buffer for the window parameters
*/
void af_window_flattop(int n,FLOAT_TYPE* w)
{
  int      i;
  FLOAT_TYPE k1 = 2*M_PI/((FLOAT_TYPE)(n-1)); // 2*pi/(N-1)
  FLOAT_TYPE k2 = 2*k1;                   // 4*pi/(N-1)

  // Calculate window coefficients
  for (i=0; i<n; i++)
    *w++ = 0.2810638602 - 0.5208971735*cos(k1*(FLOAT_TYPE)i)
                        + 0.1980389663*cos(k2*(FLOAT_TYPE)i);
}

/* Computes the 0th order modified Bessel function of the first kind.
// (Needed to compute Kaiser window)
//
// y = sum( (x/(2*n))^2 )
//      n
*/
#define BIZ_EPSILON 1E-21 // Max error acceptable

static FLOAT_TYPE besselizero(FLOAT_TYPE x)
{
  FLOAT_TYPE temp;
  FLOAT_TYPE sum   = 1.0;
  FLOAT_TYPE u     = 1.0;
  FLOAT_TYPE halfx = x/2.0;
  int      n     = 1;

  do {
    temp = halfx/(FLOAT_TYPE)n;
    u *=temp * temp;
    sum += u;
    n++;
  } while (u >= BIZ_EPSILON * sum);
  return sum;
}

/*
// Kaiser
//
// n window length
// w buffer for the window parameters
// b beta parameter of Kaiser window, Beta >= 1
//
// Beta trades the rejection of the low pass filter against the
// transition width from passband to stop band.  Larger Beta means a
// slower transition and greater stop band rejection.  See Rabiner and
// Gold (Theory and Application of DSP) under Kaiser windows for more
// about Beta.  The following table from Rabiner and Gold gives some
// feel for the effect of Beta:
//
// All ripples in dB, width of transition band = D*N where N = window
// length
//
// BETA    D       PB RIP   SB RIP
// 2.120   1.50  +-0.27      -30
// 3.384   2.23    0.0864    -40
// 4.538   2.93    0.0274    -50
// 5.658   3.62    0.00868   -60
// 6.764   4.32    0.00275   -70
// 7.865   5.0     0.000868  -80
// 8.960   5.7     0.000275  -90
// 10.056  6.4     0.000087  -100
*/
void af_window_kaiser(int n, FLOAT_TYPE* w, FLOAT_TYPE b)
{
  FLOAT_TYPE tmp;
  FLOAT_TYPE k1  = 1.0/besselizero(b);
  int	   k2  = 1 - (n & 1);
  int      end = (n + 1) >> 1;
  int      i;

  // Calculate window coefficients
  for (i=0 ; i<end ; i++){
    tmp = (FLOAT_TYPE)(2*i + k2) / ((FLOAT_TYPE)n - 1.0);
    w[end-(1&(!k2))+i] = w[end-1-i] = k1 * besselizero(b*sqrt(1.0 - tmp*tmp));
  }
}