--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libgsmefr/levinson.c	Fri Nov 25 16:18:21 2022 +0000
@@ -0,0 +1,194 @@
+/*************************************************************************
+ *
+ *   FUNCTION:  Levinson()
+ *
+ *   PURPOSE:  Levinson-Durbin algorithm in double precision. To compute the
+ *             LP filter parameters from the speech autocorrelations.
+ *
+ *   DESCRIPTION:
+ *       R[i]    autocorrelations.
+ *       A[i]    filter coefficients.
+ *       K       reflection coefficients.
+ *       Alpha   prediction gain.
+ *
+ *       Initialisation:
+ *               A[0] = 1
+ *               K    = -R[1]/R[0]
+ *               A[1] = K
+ *               Alpha = R[0] * (1-K**2]
+ *
+ *       Do for  i = 2 to M
+ *
+ *            S =  SUM ( R[j]*A[i-j] ,j=1,i-1 ) +  R[i]
+ *
+ *            K = -S / Alpha
+ *
+ *            An[j] = A[j] + K*A[i-j]   for j=1 to i-1
+ *                                      where   An[i] = new A[i]
+ *            An[i]=K
+ *
+ *            Alpha=Alpha * (1-K**2)
+ *
+ *       END
+ *
+ *************************************************************************/
+
+#include "typedef.h"
+#include "basic_op.h"
+#include "oper_32b.h"
+#include "count.h"
+
+/* Lpc order == 10 */
+
+#define M 10
+
+/* Last A(z) for case of unstable filter */
+
+Word16 old_A[M + 1];
+
+void Levinson (
+    Word16 Rh[],    /* (i)     : Rh[m+1] Vector of autocorrelations (msb) */
+    Word16 Rl[],    /* (i)     : Rl[m+1] Vector of autocorrelations (lsb) */
+    Word16 A[],     /* (o)     : A[m]    LPC coefficients  (m = 10)       */
+    Word16 rc[]     /* (o)     : rc[4]   First 4 reflection coefficients  */
+
+)
+{
+    Word16 i, j;
+    Word16 hi, lo;
+    Word16 Kh, Kl;                /* reflexion coefficient; hi and lo      */
+    Word16 alp_h, alp_l, alp_exp; /* Prediction gain; hi lo and exponent   */
+    Word16 Ah[M + 1], Al[M + 1];  /* LPC coef. in double prec.             */
+    Word16 Anh[M + 1], Anl[M + 1];/* LPC coef.for next iteration in double
+                                     prec. */
+    Word32 t0, t1, t2;            /* temporary variable                    */
+
+    /* K = A[1] = -R[1] / R[0] */
+
+    t1 = L_Comp (Rh[1], Rl[1]);
+    t2 = L_abs (t1);                    /* abs R[1]         */
+    t0 = Div_32 (t2, Rh[0], Rl[0]);     /* R[1]/R[0]        */
+    test (); 
+    if (t1 > 0)
+        t0 = L_negate (t0);             /* -R[1]/R[0]       */
+    L_Extract (t0, &Kh, &Kl);           /* K in DPF         */
+
+    rc[0] = round (t0);                 move16 (); 
+
+    t0 = L_shr (t0, 4);                 /* A[1] in          */
+    L_Extract (t0, &Ah[1], &Al[1]);     /* A[1] in DPF      */
+
+    /*  Alpha = R[0] * (1-K**2) */
+
+    t0 = Mpy_32 (Kh, Kl, Kh, Kl);       /* K*K             */
+    t0 = L_abs (t0);                    /* Some case <0 !! */
+    t0 = L_sub ((Word32) 0x7fffffffL, t0); /* 1 - K*K        */
+    L_Extract (t0, &hi, &lo);           /* DPF format      */
+    t0 = Mpy_32 (Rh[0], Rl[0], hi, lo); /* Alpha in        */
+
+    /* Normalize Alpha */
+
+    alp_exp = norm_l (t0);
+    t0 = L_shl (t0, alp_exp);
+    L_Extract (t0, &alp_h, &alp_l);     /* DPF format    */
+
+    /*--------------------------------------*
+     * ITERATIONS  I=2 to M                 *
+     *--------------------------------------*/
+
+    for (i = 2; i <= M; i++)
+    {
+        /* t0 = SUM ( R[j]*A[i-j] ,j=1,i-1 ) +  R[i] */
+
+        t0 = 0;                         move32 (); 
+        for (j = 1; j < i; j++)
+        {
+            t0 = L_add (t0, Mpy_32 (Rh[j], Rl[j], Ah[i - j], Al[i - j]));
+        }
+        t0 = L_shl (t0, 4);
+
+        t1 = L_Comp (Rh[i], Rl[i]);
+        t0 = L_add (t0, t1);            /* add R[i]        */
+
+        /* K = -t0 / Alpha */
+
+        t1 = L_abs (t0);
+        t2 = Div_32 (t1, alp_h, alp_l); /* abs(t0)/Alpha              */
+        test (); 
+        if (t0 > 0)
+            t2 = L_negate (t2);         /* K =-t0/Alpha                */
+        t2 = L_shl (t2, alp_exp);       /* denormalize; compare to Alpha */
+        L_Extract (t2, &Kh, &Kl);       /* K in DPF                      */
+
+        test (); 
+        if (sub (i, 5) < 0)
+        {
+            rc[i - 1] = round (t2);     move16 (); 
+        }
+        /* Test for unstable filter. If unstable keep old A(z) */
+
+        test (); 
+        if (sub (abs_s (Kh), 32750) > 0)
+        {
+            for (j = 0; j <= M; j++)
+            {
+                A[j] = old_A[j];        move16 (); 
+            }
+
+            for (j = 0; j < 4; j++)
+            {
+                rc[j] = 0;              move16 (); 
+            }
+
+            return;
+        }
+        /*------------------------------------------*
+         *  Compute new LPC coeff. -> An[i]         *
+         *  An[j]= A[j] + K*A[i-j]     , j=1 to i-1 *
+         *  An[i]= K                                *
+         *------------------------------------------*/
+
+        for (j = 1; j < i; j++)
+        {
+            t0 = Mpy_32 (Kh, Kl, Ah[i - j], Al[i - j]);
+            t0 = L_mac (t0, Ah[j], 32767);
+            t0 = L_mac (t0, Al[j], 1);
+            L_Extract (t0, &Anh[j], &Anl[j]);
+        }
+        t2 = L_shr (t2, 4);
+        L_Extract (t2, &Anh[i], &Anl[i]);
+
+        /*  Alpha = Alpha * (1-K**2) */
+
+        t0 = Mpy_32 (Kh, Kl, Kh, Kl);           /* K*K             */
+        t0 = L_abs (t0);                        /* Some case <0 !! */
+        t0 = L_sub ((Word32) 0x7fffffffL, t0);  /* 1 - K*K        */
+        L_Extract (t0, &hi, &lo);               /* DPF format      */
+        t0 = Mpy_32 (alp_h, alp_l, hi, lo);
+
+        /* Normalize Alpha */
+
+        j = norm_l (t0);
+        t0 = L_shl (t0, j);
+        L_Extract (t0, &alp_h, &alp_l);         /* DPF format    */
+        alp_exp = add (alp_exp, j);             /* Add normalization to
+                                                   alp_exp */
+
+        /* A[j] = An[j] */
+
+        for (j = 1; j <= i; j++)
+        {
+            Ah[j] = Anh[j];                     move16 (); 
+            Al[j] = Anl[j];                     move16 (); 
+        }
+    }
+
+    A[0] = 4096;                                move16 (); 
+    for (i = 1; i <= M; i++)
+    {
+        t0 = L_Comp (Ah[i], Al[i]);
+        old_A[i] = A[i] = round (L_shl (t0, 1));move16 (); move16 (); 
+    }
+
+    return;
+}