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annotate libgsmefr/pow2.c @ 550:de333989a12b

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document gsm[e]fr-decode-tw5[-r] utilities
author Mychaela Falconia <falcon@freecalypso.org>
date Sat, 05 Oct 2024 02:16:48 +0000
parents 33714b36841a
children
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1 /*************************************************************************
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2 *
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3 * FUNCTION: Pow2()
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4 *
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5 * PURPOSE: computes L_x = pow(2.0, exponent.fraction)
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6 *
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7 * DESCRIPTION:
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8 * The function Pow2(L_x) is approximated by a table and linear
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9 * interpolation.
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10 * 1- i = bit10-b15 of fraction, 0 <= i <= 31
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11 * 2- a = bit0-b9 of fraction
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12 * 3- L_x = table[i]<<16 - (table[i] - table[i+1]) * a * 2
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13 * 4- L_x = L_x >> (30-exponent) (with rounding)
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14 *
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15 *************************************************************************/
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16
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17 #include "gsm_efr.h"
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18 #include "typedef.h"
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19 #include "namespace.h"
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20 #include "basic_op.h"
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21 #include "no_count.h"
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22 #include "sig_proc.h"
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24 #include "pow2.tab" /* Table for Pow2() */
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26 Word32 Pow2 ( /* (o) : result (range: 0<=val<=0x7fffffff) */
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27 Word16 exponent, /* (i) : Integer part. (range: 0<=val<=30) */
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28 Word16 fraction /* (i) : Fractional part. (range: 0.0<=val<1.0) */
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29 )
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30 {
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31 Word16 exp, i, a, tmp;
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32 Word32 L_x;
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34 L_x = L_mult (fraction, 32);/* L_x = fraction<<6 */
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35 i = extract_h (L_x); /* Extract b10-b16 of fraction */
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36 L_x = L_shr (L_x, 1);
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37 a = extract_l (L_x); /* Extract b0-b9 of fraction */
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38 a = a & (Word16) 0x7fff; logic16 ();
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40 L_x = L_deposit_h (table[i]); /* table[i] << 16 */
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41 tmp = sub (table[i], table[i + 1]); /* table[i] - table[i+1] */
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42 L_x = L_msu (L_x, tmp, a); /* L_x -= tmp*a*2 */
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44 exp = sub (30, exponent);
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45 L_x = L_shr_r (L_x, exp);
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47 return (L_x);
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48 }