preprocess.c

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/*
 * Copyright 1992 by Jutta Degener and Carsten Bormann, Technische
 * Universitaet Berlin.  See the accompanying file "COPYRIGHT" for
 * details.  THERE IS ABSOLUTELY NO WARRANTY FOR THIS SOFTWARE.
 */

/* $Header$ */

#include <stdio.h>
#include <assert.h>

#include "private.h"

#include "gsm.h"
#include    "proto.h"

/* 4.2.0 .. 4.2.3 PREPROCESSING SECTION
 *
 *    After A-law to linear conversion (or directly from the
 *    Ato D converter) the following scaling is assumed for
 *    input to the RPE-LTP algorithm:
 *
 *      in:  0.1.....................12
 *      S.v.v.v.v.v.v.v.v.v.v.v.v.*.*.*
 *
 * Where S is the sign bit, v a valid bit, and * a "don't care" bit.
 *    The original signal is called sop[..]
 *
 *      out:   0.1................... 12
 *      S.S.v.v.v.v.v.v.v.v.v.v.v.v.0.0
 */


void Gsm_Preprocess P3((S, s, so),
   struct gsm_state * S,
   word      * s,
   word      * so )     /* [0..159]    IN/OUT   */
{
   word        z1 = S->z1;
   longword    L_z2 = S->L_z2;
   word        mp = S->mp;
   word        s1;
   word     SO;
   ulongword   utmp;    /* for L_ADD */
   register int   k = 160;

   (void) utmp;

   while (k--) {

   /*  4.2.1   Downscaling of the input signal
    */
      /* SO = SASR( *s, 3 ) << 2;*/
      SO = SASR( *s, 1 ) & ~3;
      s++;

      assert (SO >= -0x4000); /* downscaled by     */
      assert (SO <=  0x3FFC); /* previous routine. */


   /*  4.2.2   Offset compensation
    *
    *  This part implements a high-pass filter and requires extended
    *  arithmetic precision for the recursive part of this filter.
    *  The input of this procedure is the array so[0...159] and the
    *  output the array sof[ 0...159 ].
    */
      /*   Compute the non-recursive part
       */

      s1 = SO - z1;        /* s1 = gsm_sub( *so, z1 ); */
      z1 = SO;

      assert(s1 != MIN_WORD);

   /* SJB Remark: float might be faster than the mess that follows */

      /*   Compute the recursive part
       */

      /*   Execution of a 31 bv 16 bits multiplication
       */
      {
      word     msp;
#ifndef __GNUC__
      word     lsp;
#endif
      longword L_s2;
      longword L_temp;

      L_s2 = s1;
      L_s2 <<= 15;
#ifndef __GNUC__
      msp = (word)SASR( L_z2, 15 );
      lsp = (word)(L_z2 & 0x7fff); /* gsm_L_sub(L_z2,(msp<<15)); */

      L_s2  += GSM_MULT_R( lsp, 32735 );
      L_temp = (longword)msp * 32735; /* GSM_L_MULT(msp,32735) >> 1;*/
      L_z2   = GSM_L_ADD( L_temp, L_s2 );
      /* above does L_z2  = L_z2 * 0x7fd5/0x8000 + L_s2 */
#else
      L_z2 = ((long long)L_z2*32735 + 0x4000)>>15;
      /* alternate (ansi) version of above line does slightly different rounding:
       * L_temp = L_z2 >> 9;
       * L_temp += L_temp >> 5;
       * L_temp = (++L_temp) >> 1;
       * L_z2 = L_z2 - L_temp;
       */
      L_z2 = GSM_L_ADD(L_z2,L_s2);
#endif
      /*    Compute sof[k] with rounding
       */
      L_temp = GSM_L_ADD( L_z2, 16384 );

   /*   4.2.3  Preemphasis
    */

      msp   = (word)GSM_MULT_R( mp, -28180 );
      mp    = (word)SASR( L_temp, 15 );
      *so++ = GSM_ADD( mp, msp );
      }
   }

   S->z1   = z1;
   S->L_z2 = L_z2;
   S->mp   = mp;
}