FreeCalypso > hg > gsm-codec-lib
annotate libgsmefr/log2.c @ 269:bd2271cb95d4
libgsmfr2: integrate preprocess.c from libgsm
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Sun, 14 Apr 2024 01:58:35 +0000 |
parents | 68f772469ba7 |
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rev | line source |
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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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27 |
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28 #include "log2.tab" /* Table for Log2() */ |
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29 |
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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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38 |
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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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48 |
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49 *exponent = sub (30, exp); move16 (); |
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50 |
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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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56 |
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57 i = sub (i, 32); |
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58 |
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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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62 |
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63 *fraction = extract_h (L_y);move16 (); |
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64 |
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65 return; |
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66 } |