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;
}