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annotate libgsmefr/inv_sqrt.c @ 183:452c1d5a6268

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libgsmefr BFI w/o data: emit zero output after decoder reset In real-life usage, each EFR decoder session will most likely begin with lots of BFI frames before the first real frame arrives. However, because the spec-defined home state of the decoder is speech rather than CN, our regular logic for BFI w/o data would have to feed pseudorandom noise to the decoder (in the "fixed codebook excitation pulses" part), which is silly to do at the beginning of the decoder session right out of reset. Therefore, let's check reset_flag_old, and if we are still in the reset state, simply emit zero output.
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 03 Jan 2023 00:12:18 +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 }