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annotate libgsmefr/inv_sqrt.c @ 553:ebcf414b7d99

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doc/TFO-transform: describe details for FRv1, both modes
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 07 Oct 2024 08:24:24 +0000
parents 2f0828ba0725
children
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1 /*************************************************************************
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2 *
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3 * FUNCTION: Inv_sqrt
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4 *
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5 * PURPOSE: Computes 1/sqrt(L_x), where L_x is positive.
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6 * If L_x is negative or zero, the result is 1 (3fff ffff).
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7 *
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8 * DESCRIPTION:
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9 * The function 1/sqrt(L_x) is approximated by a table and linear
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10 * interpolation. The inverse square root is computed using the
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11 * following steps:
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12 * 1- Normalization of L_x.
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13 * 2- If (30-exponent) is even then shift right once.
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14 * 3- exponent = (30-exponent)/2 +1
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15 * 4- i = bit25-b31 of L_x; 16<=i<=63 because of normalization.
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16 * 5- a = bit10-b24
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17 * 6- i -=16
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18 * 7- L_y = table[i]<<16 - (table[i] - table[i+1]) * a * 2
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19 * 8- L_y >>= exponent
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20 *
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21 *************************************************************************/
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22
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23 #include "gsm_efr.h"
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24 #include "typedef.h"
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25 #include "namespace.h"
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26 #include "basic_op.h"
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27 #include "no_count.h"
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28 #include "sig_proc.h"
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30 #include "inv_sqrt.tab" /* Table for inv_sqrt() */
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32 Word32 Inv_sqrt ( /* (o) : output value */
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33 Word32 L_x /* (i) : input value */
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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 return ((Word32) 0x3fffffffL);
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43 exp = norm_l (L_x);
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44 L_x = L_shl (L_x, exp); /* L_x is normalize */
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46 exp = sub (30, exp);
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47 test (); logic16 ();
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48 if ((exp & 1) == 0) /* If exponent even -> shift right */
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49 {
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50 L_x = L_shr (L_x, 1);
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51 }
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52 exp = shr (exp, 1);
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53 exp = add (exp, 1);
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55 L_x = L_shr (L_x, 9);
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56 i = extract_h (L_x); /* Extract b25-b31 */
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57 L_x = L_shr (L_x, 1);
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58 a = extract_l (L_x); /* Extract b10-b24 */
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59 a = a & (Word16) 0x7fff; logic16 ();
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61 i = sub (i, 16);
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63 L_y = L_deposit_h (table[i]); /* table[i] << 16 */
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64 tmp = sub (table[i], table[i + 1]); /* table[i] - table[i+1]) */
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65 L_y = L_msu (L_y, tmp, a); /* L_y -= tmp*a*2 */
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67 L_y = L_shr (L_y, exp); /* denormalization */
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69 return (L_y);
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70 }