1 /*
2 * Copyright (c) 2011 The WebRTC project authors. All Rights Reserved.
3 *
4 * Use of this source code is governed by a BSD-style license
5 * that can be found in the LICENSE file in the root of the source
6 * tree. An additional intellectual property rights grant can be found
7 * in the file PATENTS. All contributing project authors may
8 * be found in the AUTHORS file in the root of the source tree.
9 */
10
11 /******************************************************************
12
13 iLBC Speech Coder ANSI-C Source Code
14
15 WebRtcIlbcfix_Lsp2Lsf.c
16
17 ******************************************************************/
18
19 #include "defines.h"
20 #include "constants.h"
21
22 /*----------------------------------------------------------------*
23 * conversion from LSP coefficients to LSF coefficients
24 *---------------------------------------------------------------*/
25
WebRtcIlbcfix_Lsp2Lsf(int16_t * lsp,int16_t * lsf,int16_t m)26 void WebRtcIlbcfix_Lsp2Lsf(
27 int16_t *lsp, /* (i) lsp vector -1...+1 in Q15 */
28 int16_t *lsf, /* (o) Lsf vector 0...Pi in Q13
29 (ordered, so that lsf[i]<lsf[i+1]) */
30 int16_t m /* (i) Number of coefficients */
31 )
32 {
33 int16_t i, k;
34 int16_t diff; /* diff between table value and desired value (Q15) */
35 int16_t freq; /* lsf/(2*pi) (Q16) */
36 int16_t *lspPtr, *lsfPtr, *cosTblPtr;
37 int16_t tmp;
38
39 /* set the index to maximum index value in WebRtcIlbcfix_kCos */
40 k = 63;
41
42 /*
43 Start with the highest LSP and then work the way down
44 For each LSP the lsf is calculated by first order approximation
45 of the acos(x) function
46 */
47 lspPtr = &lsp[9];
48 lsfPtr = &lsf[9];
49 cosTblPtr=(int16_t*)&WebRtcIlbcfix_kCos[k];
50 for(i=m-1; i>=0; i--)
51 {
52 /*
53 locate value in the table, which is just above lsp[i],
54 basically an approximation to acos(x)
55 */
56 while( (((int32_t)(*cosTblPtr)-(*lspPtr)) < 0)&&(k>0) )
57 {
58 k-=1;
59 cosTblPtr--;
60 }
61
62 /* Calculate diff, which is used in the linear approximation of acos(x) */
63 diff = (*lspPtr)-(*cosTblPtr);
64
65 /*
66 The linear approximation of acos(lsp[i]) :
67 acos(lsp[i])= k*512 + (WebRtcIlbcfix_kAcosDerivative[ind]*offset >> 11)
68 */
69
70 /* tmp (linear offset) in Q16 */
71 tmp = (int16_t)((WebRtcIlbcfix_kAcosDerivative[k] * diff) >> 11);
72
73 /* freq in Q16 */
74 freq = (k << 9) + tmp;
75
76 /* lsf = freq*2*pi */
77 (*lsfPtr) = (int16_t)(((int32_t)freq*25736)>>15);
78
79 lsfPtr--;
80 lspPtr--;
81 }
82
83 return;
84 }
85