preprocess.c File Reference

#include <stdio.h>
#include <assert.h>
#include "private.h"
#include "gsm.h"
#include "proto.h"

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Functions
void Gsm_Preprocess	P3 ((S, s, so), struct gsm_state *S, word *s, word *so)

Function Documentation

P3()

void Gsm_Preprocess P3	(	(S, s, so)	,
		struct gsm_state *	S,
		word *	s,
		word *	so
	)

Definition at line 34 of file preprocess.c.

{
    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;
}

References GSM_ADD(), GSM_L_ADD, GSM_MULT_R, MIN_WORD, S, and SASR.