FreeCalypso > hg > gsm-codec-lib
view libgsmefr/log2.c @ 193:cf39a90f5196
libgsmefr/dtx_enc.c: perf opt
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Wed, 04 Jan 2023 03:22:35 +0000 |
parents | 68f772469ba7 |
children |
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/************************************************************************* * * FUNCTION: Log2() * * PURPOSE: Computes log2(L_x), where L_x is positive. * If L_x is negative or zero, the result is 0. * * DESCRIPTION: * The function Log2(L_x) is approximated by a table and linear * interpolation. The following steps are used to compute Log2(L_x) * * 1- Normalization of L_x. * 2- exponent = 30-exponent * 3- i = bit25-b31 of L_x; 32<=i<=63 (because of normalization). * 4- a = bit10-b24 * 5- i -=32 * 6- fraction = table[i]<<16 - (table[i] - table[i+1]) * a * 2 * *************************************************************************/ #include "gsm_efr.h" #include "typedef.h" #include "namespace.h" #include "basic_op.h" #include "no_count.h" #include "sig_proc.h" #include "log2.tab" /* Table for Log2() */ void Log2 ( Word32 L_x, /* (i) : input value */ Word16 *exponent, /* (o) : Integer part of Log2. (range: 0<=val<=30) */ Word16 *fraction /* (o) : Fractional part of Log2. (range: 0<=val<1) */ ) { Word16 exp, i, a, tmp; Word32 L_y; test (); if (L_x <= (Word32) 0) { *exponent = 0; move16 (); *fraction = 0; move16 (); return; } exp = norm_l (L_x); L_x = L_shl (L_x, exp); /* L_x is normalized */ *exponent = sub (30, exp); move16 (); L_x = L_shr (L_x, 9); i = extract_h (L_x); /* Extract b25-b31 */ L_x = L_shr (L_x, 1); a = extract_l (L_x); /* Extract b10-b24 of fraction */ a = a & (Word16) 0x7fff; logic16 (); i = sub (i, 32); L_y = L_deposit_h (table[i]); /* table[i] << 16 */ tmp = sub (table[i], table[i + 1]); /* table[i] - table[i+1] */ L_y = L_msu (L_y, tmp, a); /* L_y -= tmp*a*2 */ *fraction = extract_h (L_y);move16 (); return; }