pow2.cpp

实现3GPP的GSM中AMR语音的CODECS。

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 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
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 * express or implied.
 * See the License for the specific language governing permissions
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/****************************************************************************************
Portions of this file are derived from the following 3GPP standard:

    3GPP TS 26.073
    ANSI-C code for the Adaptive Multi-Rate (AMR) speech codec
    Available from http://www.3gpp.org

(C) 2004, 3GPP Organizational Partners (ARIB, ATIS, CCSA, ETSI, TTA, TTC)
Permission to distribute, modify and use this file under the standard license
terms listed above has been obtained from the copyright holder.
****************************************************************************************/
/*




 Filename: /audio/gsm_amr/c/src/pow2.c


     Date: 09/28/2000

------------------------------------------------------------------------------
 REVISION HISTORY



 Description: Updated template. Changed function interface to pass in a
              pointer to overflow flag into the function instead of using a
              global flag. Removed inclusion of "pow2.tab"

 Description:

------------------------------------------------------------------------------
*/

/*----------------------------------------------------------------------------
; INCLUDES
----------------------------------------------------------------------------*/
#include    "pow2.h"
#include    "basic_op.h"

/*----------------------------------------------------------------------------
; MACROS
; Define module specific macros here
----------------------------------------------------------------------------*/


/*----------------------------------------------------------------------------
; DEFINES
; Include all pre-processor statements here. Include conditional
; compile variables also.
----------------------------------------------------------------------------*/

/*----------------------------------------------------------------------------
; LOCAL FUNCTION DEFINITIONS
; Function Prototype declaration
----------------------------------------------------------------------------*/

/*----------------------------------------------------------------------------
; LOCAL STORE/BUFFER/POINTER DEFINITIONS
; Variable declaration - defined here and used outside this module
----------------------------------------------------------------------------*/


/*
------------------------------------------------------------------------------
 FUNCTION NAME: Pow2
------------------------------------------------------------------------------
 INPUT AND OUTPUT DEFINITIONS

 Inputs:
    exponent = Integer part whose valid range is: 0 <= value <= 30 (Word16)
    fraction = Fractional part whose valid range is 0 <= value < 1

    pOverflow = pointer to overflow flag

 Outputs:
    L_x = Result of the Pow2() computation (Word32)
    pOverflow -> 1 if the Pow2() function results in saturation

 Returns:
    None

 Global Variables Used:
    None

 Local Variables Needed:
    None

------------------------------------------------------------------------------
 FUNCTION DESCRIPTION

 This function computes  L_x = pow(2.0, exponent.fraction)

 The function Pow2(L_x) is approximated by a table and linear interpolation.

 1- i = bit10-b15 of fraction,   0 <= i <= 31
 2- a = bit0-b9   of fraction
 3- L_x = table[i]<<16 - (table[i] - table[i+1]) * a * 2
 4- L_x = L_x >> (30-exponent)     (with rounding)

------------------------------------------------------------------------------
 REQUIREMENTS

 None

------------------------------------------------------------------------------
 REFERENCES

 pow2.c, UMTS GSM AMR speech codec, R99 - Version 3.2.0, March 2, 2001

------------------------------------------------------------------------------
 PSEUDO-CODE

Word32 Pow2 (           // (o)  : result       (range: 0<=val<=0x7fffffff)
    Word16 exponent,    // (i)  : Integer part.      (range: 0<=val<=30)
    Word16 fraction     // (i)  : Fractional part.  (range: 0.0<=val<1.0)
)
{
    Word16 exp, i, a, tmp;
    Word32 L_x;

    L_x = L_mult (fraction, 32);        // L_x = fraction<<6
    i = extract_h (L_x);                // Extract b10-b16 of fraction
    L_x = L_shr (L_x, 1);
    a = extract_l (L_x);                // Extract b0-b9   of fraction
    a = a & (Word16) 0x7fff;

    L_x = L_deposit_h (table[i]);       // table[i] << 16
    tmp = sub (table[i], table[i + 1]); // table[i] - table[i+1]
    L_x = L_msu (L_x, tmp, a);          // L_x -= tmp*a*2

    exp = sub (30, exponent);
    L_x = L_shr_r (L_x, exp);

    return (L_x);
}

------------------------------------------------------------------------------
 RESOURCES USED [optional]

 When the code is written for a specific target processor the
 the resources used should be documented below.

 HEAP MEMORY USED: x bytes

 STACK MEMORY USED: x bytes

 CLOCK CYCLES: (cycle count equation for this function) + (variable
                used to represent cycle count for each subroutine
                called)
     where: (cycle count variable) = cycle count for [subroutine
                                     name]

------------------------------------------------------------------------------
 CAUTION [optional]
 [State any special notes, constraints or cautions for users of this function]

------------------------------------------------------------------------------
*/

/*----------------------------------------------------------------------------
; FUNCTION CODE
----------------------------------------------------------------------------*/

Word32 Pow2(            /* (o)  : result       (range: 0<=val<=0x7fffffff) */
    Word16 exponent,    /* (i)  : Integer part.      (range: 0<=val<=30)   */
    Word16 fraction,    /* (i)  : Fractional part.  (range: 0.0<=val<1.0)  */
    Flag *pOverflow
)
{
    Word16 exp, i, a, tmp;
    Word32 L_x;

    L_x = L_mult(fraction, 32, pOverflow);      /* L_x = fraction<<6    */

    /* Extract b10-b16 of fraction */

    i = (Word16)(L_x >> 16);
    L_x >>= 1;

    /* Extract b0-b9   of fraction */
    a = (Word16)(L_x);
    a = a & (Word16) 0x7fff;

    L_x = L_deposit_h(pow2_tbl[i]);             /* pow2_tbl[i] << 16       */

    /* pow2_tbl[i] - pow2_tbl[i+1] */
    tmp = sub(pow2_tbl[i], pow2_tbl[i + 1], pOverflow);
    L_x = L_msu(L_x, tmp, a, pOverflow);        /* L_x -= tmp*a*2        */

    exp = sub(30, exponent, pOverflow);
    L_x = L_shr_r(L_x, exp, pOverflow);

    return (L_x);
}