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/*
* Copyright (c) 2011 The WebRTC project authors. All Rights Reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
/******************************************************************
iLBC Speech Coder ANSI-C Source Code
WebRtcIlbcfix_Lsp2Lsf.c
******************************************************************/
#include "modules/audio_coding/codecs/ilbc/lsp_to_lsf.h"
#include "modules/audio_coding/codecs/ilbc/constants.h"
#include "modules/audio_coding/codecs/ilbc/defines.h"
/*----------------------------------------------------------------*
* conversion from LSP coefficients to LSF coefficients
*---------------------------------------------------------------*/
void WebRtcIlbcfix_Lsp2Lsf(
int16_t *lsp, /* (i) lsp vector -1...+1 in Q15 */
int16_t *lsf, /* (o) Lsf vector 0...Pi in Q13
(ordered, so that lsf[i]<lsf[i+1]) */
int16_t m /* (i) Number of coefficients */
)
{
int16_t i, k;
int16_t diff; /* diff between table value and desired value (Q15) */
int16_t freq; /* lsf/(2*pi) (Q16) */
int16_t *lspPtr, *lsfPtr, *cosTblPtr;
int16_t tmp;
/* set the index to maximum index value in WebRtcIlbcfix_kCos */
k = 63;
/*
Start with the highest LSP and then work the way down
For each LSP the lsf is calculated by first order approximation
of the acos(x) function
*/
lspPtr = &lsp[9];
lsfPtr = &lsf[9];
cosTblPtr=(int16_t*)&WebRtcIlbcfix_kCos[k];
for(i=m-1; i>=0; i--)
{
/*
locate value in the table, which is just above lsp[i],
basically an approximation to acos(x)
*/
while( (((int32_t)(*cosTblPtr)-(*lspPtr)) < 0)&&(k>0) )
{
k-=1;
cosTblPtr--;
}
/* Calculate diff, which is used in the linear approximation of acos(x) */
diff = (*lspPtr)-(*cosTblPtr);
/*
The linear approximation of acos(lsp[i]) :
acos(lsp[i])= k*512 + (WebRtcIlbcfix_kAcosDerivative[ind]*offset >> 11)
*/
/* tmp (linear offset) in Q16 */
tmp = (int16_t)((WebRtcIlbcfix_kAcosDerivative[k] * diff) >> 11);
/* freq in Q16 */
freq = (k << 9) + tmp;
/* lsf = freq*2*pi */
(*lsfPtr) = (int16_t)(((int32_t)freq*25736)>>15);
lsfPtr--;
lspPtr--;
}
return;
}
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