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27
28 /* *
29 * SKP_Silk_schur.c *
30 * *
31 * Calculates the reflection coefficients from the correlation sequence *
32 * *
33 * Copyright 2008 (c), Skype Limited *
34 * Date: 080103 *
35 * */
36 #include "SKP_Silk_SigProc_FIX.h"
37
38 /* Faster than schur64(), but much less accurate. */
39 /* uses SMLAWB(), requiring armv5E and higher. */
SKP_Silk_schur(SKP_int16 * rc_Q15,const SKP_int32 * c,const SKP_int32 order)40 void SKP_Silk_schur(
41 SKP_int16 *rc_Q15, /* O: reflection coefficients [order] Q15 */
42 const SKP_int32 *c, /* I: correlations [order+1] */
43 const SKP_int32 order /* I: prediction order */
44 )
45 {
46 SKP_int k, n, lz;
47 SKP_int32 C[ SKP_Silk_MAX_ORDER_LPC + 1 ][ 2 ];
48 SKP_int32 Ctmp1, Ctmp2, rc_tmp_Q15;
49
50 /* Get number of leading zeros */
51 lz = SKP_Silk_CLZ32( c[ 0 ] );
52
53 /* Copy correlations and adjust level to Q30 */
54 if( lz < 2 ) {
55 /* lz must be 1, so shift one to the right */
56 for( k = 0; k < order + 1; k++ ) {
57 C[ k ][ 0 ] = C[ k ][ 1 ] = SKP_RSHIFT( c[ k ], 1 );
58 }
59 } else if( lz > 2 ) {
60 /* Shift to the left */
61 lz -= 2;
62 for( k = 0; k < order + 1; k++ ) {
63 C[ k ][ 0 ] = C[ k ][ 1 ] = SKP_LSHIFT( c[k], lz );
64 }
65 } else {
66 /* No need to shift */
67 for( k = 0; k < order + 1; k++ ) {
68 C[ k ][ 0 ] = C[ k ][ 1 ] = c[ k ];
69 }
70 }
71
72 for( k = 0; k < order; k++ ) {
73
74 /* Get reflection coefficient */
75 rc_tmp_Q15 = -SKP_DIV32_16( C[ k + 1 ][ 0 ], SKP_max_32( SKP_RSHIFT( C[ 0 ][ 1 ], 15 ), 1 ) );
76
77 /* Clip (shouldn't happen for properly conditioned inputs) */
78 rc_tmp_Q15 = SKP_SAT16( rc_tmp_Q15 );
79
80 /* Store */
81 rc_Q15[ k ] = (SKP_int16)rc_tmp_Q15;
82
83 /* Update correlations */
84 for( n = 0; n < order - k; n++ ) {
85 Ctmp1 = C[ n + k + 1 ][ 0 ];
86 Ctmp2 = C[ n ][ 1 ];
87 C[ n + k + 1 ][ 0 ] = SKP_SMLAWB( Ctmp1, SKP_LSHIFT( Ctmp2, 1 ), rc_tmp_Q15 );
88 C[ n ][ 1 ] = SKP_SMLAWB( Ctmp2, SKP_LSHIFT( Ctmp1, 1 ), rc_tmp_Q15 );
89 }
90 }
91 }
92