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      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 http://www.xiph.org/                  *
     10  *                                                                  *
     11  ********************************************************************
     12 
     13   function: LPC low level routines
     14   last mod: $Id: lpc.c 16227 2009-07-08 06:58:46Z xiphmont $
     15 
     16  ********************************************************************/
     17 
     18 /* Some of these routines (autocorrelator, LPC coefficient estimator)
     19    are derived from code written by Jutta Degener and Carsten Bormann;
     20    thus we include their copyright below.  The entirety of this file
     21    is freely redistributable on the condition that both of these
     22    copyright notices are preserved without modification.  */
     23 
     24 /* Preserved Copyright: *********************************************/
     25 
     26 /* Copyright 1992, 1993, 1994 by Jutta Degener and Carsten Bormann,
     27 Technische Universita"t Berlin
     28 
     29 Any use of this software is permitted provided that this notice is not
     30 removed and that neither the authors nor the Technische Universita"t
     31 Berlin are deemed to have made any representations as to the
     32 suitability of this software for any purpose nor are held responsible
     33 for any defects of this software. THERE IS ABSOLUTELY NO WARRANTY FOR
     34 THIS SOFTWARE.
     35 
     36 As a matter of courtesy, the authors request to be informed about uses
     37 this software has found, about bugs in this software, and about any
     38 improvements that may be of general interest.
     39 
     40 Berlin, 28.11.1994
     41 Jutta Degener
     42 Carsten Bormann
     43 
     44 *********************************************************************/
     45 
     46 #include <stdlib.h>
     47 #include <string.h>
     48 #include <math.h>
     49 #include "os.h"
     50 #include "smallft.h"
     51 #include "lpc.h"
     52 #include "scales.h"
     53 #include "misc.h"
     54 
     55 /* Autocorrelation LPC coeff generation algorithm invented by
     56    N. Levinson in 1947, modified by J. Durbin in 1959. */
     57 
     58 /* Input : n elements of time doamin data
     59    Output: m lpc coefficients, excitation energy */
     60 
     61 float vorbis_lpc_from_data(float *data,float *lpci,int n,int m){
     62   double *aut=alloca(sizeof(*aut)*(m+1));
     63   double *lpc=alloca(sizeof(*lpc)*(m));
     64   double error;
     65   double epsilon;
     66   int i,j;
     67 
     68   /* autocorrelation, p+1 lag coefficients */
     69   j=m+1;
     70   while(j--){
     71     double d=0; /* double needed for accumulator depth */
     72     for(i=j;i<n;i++)d+=(double)data[i]*data[i-j];
     73     aut[j]=d;
     74   }
     75 
     76   /* Generate lpc coefficients from autocorr values */
     77 
     78   /* set our noise floor to about -100dB */
     79   error=aut[0] * (1. + 1e-10);
     80   epsilon=1e-9*aut[0]+1e-10;
     81 
     82   for(i=0;i<m;i++){
     83     double r= -aut[i+1];
     84 
     85     if(error<epsilon){
     86       memset(lpc+i,0,(m-i)*sizeof(*lpc));
     87       goto done;
     88     }
     89 
     90     /* Sum up this iteration's reflection coefficient; note that in
     91        Vorbis we don't save it.  If anyone wants to recycle this code
     92        and needs reflection coefficients, save the results of 'r' from
     93        each iteration. */
     94 
     95     for(j=0;j<i;j++)r-=lpc[j]*aut[i-j];
     96     r/=error;
     97 
     98     /* Update LPC coefficients and total error */
     99 
    100     lpc[i]=r;
    101     for(j=0;j<i/2;j++){
    102       double tmp=lpc[j];
    103 
    104       lpc[j]+=r*lpc[i-1-j];
    105       lpc[i-1-j]+=r*tmp;
    106     }
    107     if(i&1)lpc[j]+=lpc[j]*r;
    108 
    109     error*=1.-r*r;
    110 
    111   }
    112 
    113  done:
    114 
    115   /* slightly damp the filter */
    116   {
    117     double g = .99;
    118     double damp = g;
    119     for(j=0;j<m;j++){
    120       lpc[j]*=damp;
    121       damp*=g;
    122     }
    123   }
    124 
    125   for(j=0;j<m;j++)lpci[j]=(float)lpc[j];
    126 
    127   /* we need the error value to know how big an impulse to hit the
    128      filter with later */
    129 
    130   return error;
    131 }
    132 
    133 void vorbis_lpc_predict(float *coeff,float *prime,int m,
    134                      float *data,long n){
    135 
    136   /* in: coeff[0...m-1] LPC coefficients
    137          prime[0...m-1] initial values (allocated size of n+m-1)
    138     out: data[0...n-1] data samples */
    139 
    140   long i,j,o,p;
    141   float y;
    142   float *work=alloca(sizeof(*work)*(m+n));
    143 
    144   if(!prime)
    145     for(i=0;i<m;i++)
    146       work[i]=0.f;
    147   else
    148     for(i=0;i<m;i++)
    149       work[i]=prime[i];
    150 
    151   for(i=0;i<n;i++){
    152     y=0;
    153     o=i;
    154     p=m;
    155     for(j=0;j<m;j++)
    156       y-=work[o++]*coeff[--p];
    157 
    158     data[i]=work[o]=y;
    159   }
    160 }
    161