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      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 
     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)WEBRTC_SPL_MUL_16_16_RSFT(WebRtcIlbcfix_kAcosDerivative[k],diff, 11);
     72 
     73     /* freq in Q16 */
     74     freq = (int16_t)WEBRTC_SPL_LSHIFT_W16(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