--- a/libgsmfr2/Makefile	Sun Apr 14 01:50:48 2024 +0000
+++ b/libgsmfr2/Makefile	Sun Apr 14 01:58:35 2024 +0000
@@ -1,8 +1,9 @@
 CC=	gcc
 CFLAGS=	-O2
 OBJS=	add.o comfort_noise.o dec_main.o ed_state.o enc_main.o long_term.o \
-	lpc.o pack_frame.o pack_frame2.o pp_bad.o pp_good.o pp_state.o prng.o \
-	sidclass.o silence_frame.o unpack_frame.o unpack_frame2.o xmaxc_mean.o
+	lpc.o pack_frame.o pack_frame2.o pp_bad.o pp_good.o pp_state.o \
+	preprocess.o prng.o sidclass.o silence_frame.o unpack_frame.o \
+	unpack_frame2.o xmaxc_mean.o
 HDRS=	ed_internal.h ed_state.h pp_internal.h pp_state.h tw_gsmfr.h typedef.h
 LIB=	libgsmfr2.a
 

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libgsmfr2/preprocess.c	Sun Apr 14 01:58:35 2024 +0000
@@ -0,0 +1,112 @@
+/*
+ * This C source file has been adapted from TU-Berlin libgsm source,
+ * original notice follows:
+ *
+ * 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.
+ */
+
+#include <stdint.h>
+#include <assert.h>
+#include "tw_gsmfr.h"
+#include "typedef.h"
+#include "ed_state.h"
+#include "ed_internal.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 (
+	struct gsmfr_0610_state * S,
+	const word	 * s,
+	word 		 * so )		/* [0..159] 	IN/OUT	*/
+{
+
+	word       z1 = S->z1;
+	longword L_z2 = S->L_z2;
+	word 	   mp = S->mp;
+
+	word 	   	s1;
+	longword      L_s2;
+
+	longword      L_temp;
+
+	word		msp, lsp;
+	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;
+		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);
+
+		/*   Compute the recursive part
+		 */
+		L_s2 = s1;
+		L_s2 <<= 15;
+
+		/*   Execution of a 31 bv 16 bits multiplication
+		 */
+
+		msp = SASR( L_z2, 15 );
+		lsp = L_z2-((longword)msp<<15); /* 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 );
+
+		/*    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;
+}