FreeCalypso > hg > gsm-codec-lib
annotate libtwamr/sqrt_l.c @ 457:9bcf65088006
doc/AMR-EFR-philosophy: document implemented solution
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Fri, 10 May 2024 18:20:06 +0000 |
parents | 8dfb7cbe6b59 |
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1 /* |
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2 ******************************************************************************** |
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3 * |
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4 * GSM AMR-NB speech codec R98 Version 7.6.0 December 12, 2001 |
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5 * R99 Version 3.3.0 |
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6 * REL-4 Version 4.1.0 |
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7 * |
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8 ******************************************************************************** |
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9 * |
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10 * File : sqrt_l.c |
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11 * Purpose : Computes sqrt(L_x), where L_x is positive. |
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12 * : If L_var is negative or zero, the result is 0 |
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13 * Description : |
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14 * The function sqrt(L_x) is approximated by a table and linear |
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15 * interpolation. The square root is computed using the |
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16 * following steps: |
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17 * 1- Normalization of L_x. |
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18 * 2- If exponent is even then shift right once. |
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19 * 3- exponent = exponent/2 |
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20 * 4- i = bit25-b31 of L_x; 16<=i<=63 because of normalization. |
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21 * 5- a = bit10-b24 |
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22 * 6- i -=16 |
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23 * 7- L_y = table[i]<<16 - (table[i] - table[i+1]) * a * 2 |
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24 * 8- return L_y and exponent so caller can do denormalization |
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25 * |
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26 ******************************************************************************** |
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27 */ |
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28 /* |
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29 ******************************************************************************** |
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30 * MODULE INCLUDE FILE AND VERSION ID |
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31 ******************************************************************************** |
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32 */ |
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33 #include "namespace.h" |
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34 #include "sqrt_l.h" |
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35 |
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36 /* |
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37 ******************************************************************************** |
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38 * INCLUDE FILES |
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39 ******************************************************************************** |
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40 */ |
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41 #include "typedef.h" |
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42 #include "basic_op.h" |
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43 #include "no_count.h" |
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44 |
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45 /* |
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46 ******************************************************************************** |
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47 * LOCAL VARIABLES AND TABLES |
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48 ******************************************************************************** |
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49 */ |
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50 #include "sqrt_l.tab" /* Table for sqrt_l_exp() */ |
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51 |
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52 /* |
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53 ******************************************************************************** |
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54 * PUBLIC PROGRAM CODE |
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55 ******************************************************************************** |
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56 */ |
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57 |
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58 Word32 sqrt_l_exp (/* o : output value, Q31 */ |
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59 Word32 L_x, /* i : input value, Q31 */ |
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60 Word16 *exp /* o : right shift to be applied to result, Q1 */ |
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61 ) |
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62 { |
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63 /* |
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64 y = sqrt(x) |
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65 |
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66 x = f * 2^-e, 0.5 <= f < 1 (normalization) |
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67 |
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68 y = sqrt(f) * 2^(-e/2) |
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69 |
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70 a) e = 2k --> y = sqrt(f) * 2^-k (k = e div 2, |
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71 0.707 <= sqrt(f) < 1) |
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72 b) e = 2k+1 --> y = sqrt(f/2) * 2^-k (k = e div 2, |
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73 0.5 <= sqrt(f/2) < 0.707) |
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74 */ |
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75 |
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76 |
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77 Word16 e, i, a, tmp; |
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78 Word32 L_y; |
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79 |
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80 test (); |
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81 if (L_x <= (Word32) 0) |
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82 { |
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83 *exp = 0; move16 (); |
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84 return (Word32) 0; |
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85 } |
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86 |
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87 e = norm_l (L_x) & 0xFFFE; logic16 (); /* get next lower EVEN norm. exp */ |
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88 L_x = L_shl (L_x, e); /* L_x is normalized to [0.25..1) */ |
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89 *exp = e; move16 (); /* return 2*exponent (or Q1) */ |
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90 |
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91 L_x = L_shr (L_x, 9); |
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92 i = extract_h (L_x); /* Extract b25-b31, 16 <= i <= 63 because |
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93 of normalization */ |
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94 L_x = L_shr (L_x, 1); |
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95 a = extract_l (L_x); /* Extract b10-b24 */ |
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96 a = a & (Word16) 0x7fff; logic16 (); |
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97 |
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98 i = sub (i, 16); /* 0 <= i <= 47 */ |
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99 |
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100 L_y = L_deposit_h (table[i]); /* table[i] << 16 */ |
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101 tmp = sub (table[i], table[i + 1]); /* table[i] - table[i+1]) */ |
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102 L_y = L_msu (L_y, tmp, a); /* L_y -= tmp*a*2 */ |
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103 |
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104 /* L_y = L_shr (L_y, *exp); */ /* denormalization done by caller */ |
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105 |
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106 return (L_y); |
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107 } |