FreeCalypso > hg > gsm-codec-lib

annotate libgsmefr/log2.c @ 282:9ee8ad3d4d30

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frtest: rm gsmfr-hand-test and gsmfr-max-out utils These hack programs were never properly documented and were written only as part of a debug chase, in pursuit of a bug that ultimately turned out to be in our then-hacky patch to osmo-bts-sysmo, before beginning of proper patches in Osmocom. These hack programs need to be dropped from the present sw package because they depend on old libgsm, and we are eliminating that dependency.
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 14 Apr 2024 05:44:47 +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 }