freeswitch/libs/silk/src/SKP_Silk_LPC_stabilize.c

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#include "SKP_Silk_typedef.h"
#include "SKP_Silk_SigProc_FIX.h"

#define LPC_STABILIZE_LPC_MAX_ABS_VALUE_Q16     ( ( (SKP_int32)SKP_int16_MAX ) << 4 )

/* LPC stabilizer, for a single input data vector */
void SKP_Silk_LPC_stabilize(
    SKP_int16       *a_Q12,         /* O    stabilized LPC vector [L]                       */
    SKP_int32       *a_Q16,         /* I    LPC vector [L]                                  */
    const SKP_int32  bwe_Q16,       /* I    Bandwidth expansion factor                      */
    const SKP_int    L              /* I    Number of LPC parameters in the input vector    */
)
{
    SKP_int32   maxabs, absval, sc_Q16;
    SKP_int     i, idx = 0;
    SKP_int32   invGain_Q30;

    SKP_Silk_bwexpander_32( a_Q16, L, bwe_Q16 );

    /***************************/
    /* Limit range of the LPCs */
    /***************************/
    /* Limit the maximum absolute value of the prediction coefficients */
    while( SKP_TRUE ) {
        /* Find maximum absolute value and its index */
        maxabs = SKP_int32_MIN;
        for( i = 0; i < L; i++ ) {
            absval = SKP_abs( a_Q16[ i ] );
            if( absval > maxabs ) {
                maxabs = absval;
                idx    = i;
            }
        }
    
        if( maxabs >= LPC_STABILIZE_LPC_MAX_ABS_VALUE_Q16 ) {
            /* Reduce magnitude of prediction coefficients */
            sc_Q16 = SKP_DIV32( SKP_int32_MAX, SKP_RSHIFT( maxabs, 4 ) );
            sc_Q16 = 65536 - sc_Q16;
            sc_Q16 = SKP_DIV32( sc_Q16, idx + 1 );
            sc_Q16 = 65536 - sc_Q16;
            sc_Q16 = SKP_LSHIFT( SKP_SMULWB( sc_Q16, 32604 ), 1 ); // 0.995 in Q16
            SKP_Silk_bwexpander_32( a_Q16, L, sc_Q16 );
        } else {
            break;
        }
    }

    /* Convert to 16 bit Q12 */
    for( i = 0; i < L; i++ ) {
        a_Q12[ i ] = (SKP_int16)SKP_RSHIFT_ROUND( a_Q16[ i ], 4 );
    }

    /**********************/
    /* Ensure stable LPCs */
    /**********************/
    while( SKP_Silk_LPC_inverse_pred_gain( &invGain_Q30, a_Q12, L ) == 1 ) {
        SKP_Silk_bwexpander( a_Q12, L, 65339 ); // 0.997 in Q16
    }
}

void SKP_Silk_LPC_fit(
    SKP_int16       *a_QQ,          /* O    Stabilized LPC vector, Q(24-rshift) [L]         */
    SKP_int32       *a_Q24,         /* I    LPC vector [L]                                  */
    const SKP_int    QQ,            /* I    Q domain of output LPC vector                   */
    const SKP_int    L              /* I    Number of LPC parameters in the input vector    */
)
{
    SKP_int     i, rshift, idx = 0;
    SKP_int32   maxabs, absval, sc_Q16;

    rshift = 24 - QQ;

    /***************************/
    /* Limit range of the LPCs */
    /***************************/
    /* Limit the maximum absolute value of the prediction coefficients */
    while( SKP_TRUE ) {
        /* Find maximum absolute value and its index */
        maxabs = SKP_int32_MIN;
        for( i = 0; i < L; i++ ) {
            absval = SKP_abs( a_Q24[ i ] );
            if( absval > maxabs ) {
                maxabs = absval;
                idx    = i;
            }
        }
    
        maxabs = SKP_RSHIFT( maxabs, rshift );
        if( maxabs >= SKP_int16_MAX ) {
            /* Reduce magnitude of prediction coefficients */
            sc_Q16 = 65470 - SKP_DIV32( SKP_MUL( 65470 >> 2, maxabs - SKP_int16_MAX ), 
                                        SKP_RSHIFT32( SKP_MUL( maxabs, idx + 1), 2 ) );
            SKP_Silk_bwexpander_32( a_Q24, L, sc_Q16 );
        } else {
            break;
        }
    }

    /* Convert to 16 bit Q(24-rshift) */
    SKP_assert( rshift > 0  );
    SKP_assert( rshift < 31 );
    for( i = 0; i < L; i++ ) {
        a_QQ[ i ] = (SKP_int16)SKP_RSHIFT_ROUND( a_Q24[ i ], rshift );
    }
}