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;

        longword        ltmp;           /* for   ADD */
        ulongword       utmp;           /* for L_ADD */

        register int            k = 160;

        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, lsp;
                longword L_s2;
                longword L_temp;
                
                L_s2 = s1;
                L_s2 <<= 15;
#ifndef __GNUC__ 
                msp = SASR( L_z2, 15 );
                lsp = 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   = GSM_MULT_R( mp, -28180 );
                mp    = SASR( L_temp, 15 );
                *so++ = GSM_ADD( mp, msp );
                }
        }

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

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