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vorbis_lpc.c (4486B)

      1 /********************************************************************
      2 *                                                                  *
      3 * THIS FILE IS PART OF THE OggVorbis SOFTWARE CODEC SOURCE CODE.   *
      4 * USE, DISTRIBUTION AND REPRODUCTION OF THIS LIBRARY SOURCE IS     *
      5 * GOVERNED BY A BSD-STYLE SOURCE LICENSE INCLUDED WITH THIS SOURCE *
      6 * IN 'COPYING'. PLEASE READ THESE TERMS BEFORE DISTRIBUTING.       *
      7 *                                                                  *
      8 * THE OggVorbis SOURCE CODE IS (C) COPYRIGHT 1994-2009             *
      9 * by the Xiph.Org Foundation https://xiph.org/                     *
     10 *                                                                  *
     11 ********************************************************************
     12 
     13  function: LPC low level routines
     14 
     15 ********************************************************************/
     16 
     17 /* Some of these routines (autocorrelator, LPC coefficient estimator)
     18   are derived from code written by Jutta Degener and Carsten Bormann;
     19   thus we include their copyright below.  The entirety of this file
     20   is freely redistributable on the condition that both of these
     21   copyright notices are preserved without modification.  */
     22 
     23 /* Preserved Copyright: *********************************************/
     24 
     25 /* Copyright 1992, 1993, 1994 by Jutta Degener and Carsten Bormann,
     26 Technische Universita"t Berlin
     27 
     28 Any use of this software is permitted provided that this notice is not
     29 removed and that neither the authors nor the Technische Universita"t
     30 Berlin are deemed to have made any representations as to the
     31 suitability of this software for any purpose nor are held responsible
     32 for any defects of this software. THERE IS ABSOLUTELY NO WARRANTY FOR
     33 THIS SOFTWARE.
     34 
     35 As a matter of courtesy, the authors request to be informed about uses
     36 this software has found, about bugs in this software, and about any
     37 improvements that may be of general interest.
     38 
     39 Berlin, 28.11.1994
     40 Jutta Degener
     41 Carsten Bormann
     42 
     43 *********************************************************************/
     44 
     45 #include <stdlib.h>
     46 #include <string.h>
     47 #include <math.h>
     48 #include "os.h"
     49 #include "smallft.h"
     50 #include "lpc.h"
     51 #include "scales.h"
     52 #include "misc.h"
     53 
     54 /* Autocorrelation LPC coeff generation algorithm invented by
     55   N. Levinson in 1947, modified by J. Durbin in 1959. */
     56 
     57 /* Input : n elements of time doamin data
     58   Output: m lpc coefficients, excitation energy */
     59 
     60 float vorbis_lpc_from_data(float *data,float *lpci,int n,int m){
     61  double *aut=alloca(sizeof(*aut)*(m+1));
     62  double *lpc=alloca(sizeof(*lpc)*(m));
     63  double error;
     64  double epsilon;
     65  int i,j;
     66 
     67  /* autocorrelation, p+1 lag coefficients */
     68  j=m+1;
     69  while(j--){
     70    double d=0; /* double needed for accumulator depth */
     71    for(i=j;i<n;i++)d+=(double)data[i]*data[i-j];
     72    aut[j]=d;
     73  }
     74 
     75  /* Generate lpc coefficients from autocorr values */
     76 
     77  /* set our noise floor to about -100dB */
     78  error=aut[0] * (1. + 1e-10);
     79  epsilon=1e-9*aut[0]+1e-10;
     80 
     81  for(i=0;i<m;i++){
     82    double r= -aut[i+1];
     83 
     84    if(error<epsilon){
     85      memset(lpc+i,0,(m-i)*sizeof(*lpc));
     86      goto done;
     87    }
     88 
     89    /* Sum up this iteration's reflection coefficient; note that in
     90       Vorbis we don't save it.  If anyone wants to recycle this code
     91       and needs reflection coefficients, save the results of 'r' from
     92       each iteration. */
     93 
     94    for(j=0;j<i;j++)r-=lpc[j]*aut[i-j];
     95    r/=error;
     96 
     97    /* Update LPC coefficients and total error */
     98 
     99    lpc[i]=r;
    100    for(j=0;j<i/2;j++){
    101      double tmp=lpc[j];
    102 
    103      lpc[j]+=r*lpc[i-1-j];
    104      lpc[i-1-j]+=r*tmp;
    105    }
    106    if(i&1)lpc[j]+=lpc[j]*r;
    107 
    108    error*=1.-r*r;
    109 
    110  }
    111 
    112 done:
    113 
    114  /* slightly damp the filter */
    115  {
    116    double g = .99;
    117    double damp = g;
    118    for(j=0;j<m;j++){
    119      lpc[j]*=damp;
    120      damp*=g;
    121    }
    122  }
    123 
    124  for(j=0;j<m;j++)lpci[j]=(float)lpc[j];
    125 
    126  /* we need the error value to know how big an impulse to hit the
    127     filter with later */
    128 
    129  return error;
    130 }
    131 
    132 void vorbis_lpc_predict(float *coeff,float *prime,int m,
    133                     float *data,long n){
    134 
    135  /* in: coeff[0...m-1] LPC coefficients
    136         prime[0...m-1] initial values (allocated size of n+m-1)
    137    out: data[0...n-1] data samples */
    138 
    139  long i,j,o,p;
    140  float y;
    141  float *work=alloca(sizeof(*work)*(m+n));
    142 
    143  if(!prime)
    144    for(i=0;i<m;i++)
    145      work[i]=0.f;
    146  else
    147    for(i=0;i<m;i++)
    148      work[i]=prime[i];
    149 
    150  for(i=0;i<n;i++){
    151    y=0;
    152    o=i;
    153    p=m;
    154    for(j=0;j<m;j++)
    155      y-=work[o++]*coeff[--p];
    156 
    157    data[i]=work[o]=y;
    158  }
    159 }