webrtc / src / webrtc / f54860e9ef0b68e182a01edc994626d21961bc4b / . / modules / audio_coding / codecs / ilbc / lsp_to_lsf.c

/* | |

* 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 "defines.h" | |

#include "constants.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; | |

} |