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annotate libgsmefr/log2.c @ 494:aaf4dec8bee0

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libgsmhr1: implement perfect SID detection
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 17 Jun 2024 23:14:19 +0000
parents 68f772469ba7
children
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1 /*************************************************************************
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2 *
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3 * FUNCTION: Log2()
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4 *
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5 * PURPOSE: Computes log2(L_x), where L_x is positive.
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6 * If L_x is negative or zero, the result is 0.
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7 *
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8 * DESCRIPTION:
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9 * The function Log2(L_x) is approximated by a table and linear
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10 * interpolation. The following steps are used to compute Log2(L_x)
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11 *
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12 * 1- Normalization of L_x.
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13 * 2- exponent = 30-exponent
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14 * 3- i = bit25-b31 of L_x; 32<=i<=63 (because of normalization).
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15 * 4- a = bit10-b24
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16 * 5- i -=32
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17 * 6- fraction = table[i]<<16 - (table[i] - table[i+1]) * a * 2
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18 *
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19 *************************************************************************/
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20
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21 #include "gsm_efr.h"
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22 #include "typedef.h"
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23 #include "namespace.h"
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24 #include "basic_op.h"
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25 #include "no_count.h"
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26 #include "sig_proc.h"
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28 #include "log2.tab" /* Table for Log2() */
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30 void Log2 (
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31 Word32 L_x, /* (i) : input value */
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32 Word16 *exponent, /* (o) : Integer part of Log2. (range: 0<=val<=30) */
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33 Word16 *fraction /* (o) : Fractional part of Log2. (range: 0<=val<1) */
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34 )
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35 {
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36 Word16 exp, i, a, tmp;
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37 Word32 L_y;
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39 test ();
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40 if (L_x <= (Word32) 0)
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41 {
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42 *exponent = 0; move16 ();
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43 *fraction = 0; move16 ();
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44 return;
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45 }
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46 exp = norm_l (L_x);
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47 L_x = L_shl (L_x, exp); /* L_x is normalized */
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49 *exponent = sub (30, exp); move16 ();
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51 L_x = L_shr (L_x, 9);
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52 i = extract_h (L_x); /* Extract b25-b31 */
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53 L_x = L_shr (L_x, 1);
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54 a = extract_l (L_x); /* Extract b10-b24 of fraction */
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55 a = a & (Word16) 0x7fff; logic16 ();
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57 i = sub (i, 32);
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59 L_y = L_deposit_h (table[i]); /* table[i] << 16 */
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60 tmp = sub (table[i], table[i + 1]); /* table[i] - table[i+1] */
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61 L_y = L_msu (L_y, tmp, a); /* L_y -= tmp*a*2 */
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63 *fraction = extract_h (L_y);move16 ();
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65 return;
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66 }