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,
27Technische Universita"t Berlin
28
29Any use of this software is permitted provided that this notice is not
30removed and that neither the authors nor the Technische Universita"t
31Berlin are deemed to have made any representations as to the
32suitability of this software for any purpose nor are held responsible
33for any defects of this software. THERE IS ABSOLUTELY NO WARRANTY FOR
34THIS SOFTWARE.
35
36As a matter of courtesy, the authors request to be informed about uses
37this software has found, about bugs in this software, and about any
38improvements that may be of general interest.
39
40Berlin, 28.11.1994
41Jutta Degener
42Carsten 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
61float 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
133void 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