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Fixes problems with unstable filter detection

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LPC_inverse_pred_gain_QA() was failing to detect unstable filters
because of insufficient numerical precision. Precision was increased
to Q24 for LPC coefs and the reciprocal now uses all the precision
available (variable shift right from the start).
This commit is contained in:
J-M Valin (rewritten by T.B. Terriberry) 2011-09-30 18:12:13 -04:00 • committed by Jean-Marc Valin
parent efe16c5d47
commit b23a6ca5cf
1 changed files with 14 additions and 13 deletions
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27
silk/LPC_inv_pred_gain.c
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@ -31,9 +31,11 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#include "SigProc_FIX.h" #include "SigProc_FIX.h"
#define QA 16 #define QA 24
#define A_LIMIT SILK_FIX_CONST( 0.99975, QA ) #define A_LIMIT SILK_FIX_CONST( 0.99975, QA )
#define MUL32_FRAC_Q(a32, b32, Q) ((opus_int32)(silk_RSHIFT_ROUND64(silk_SMULL(a32, b32), Q)))
/* Compute inverse of LPC prediction gain, and */ /* Compute inverse of LPC prediction gain, and */
/* test if LPC coefficients are stable (all poles within unit circle) */ /* test if LPC coefficients are stable (all poles within unit circle) */
static opus_int LPC_inverse_pred_gain_QA( /* O: Returns 1 if unstable, otherwise 0 */ static opus_int LPC_inverse_pred_gain_QA( /* O: Returns 1 if unstable, otherwise 0 */
@ -43,8 +45,8 @@ static opus_int LPC_inverse_pred_gain_QA( /* O: Returns 1 if unstable,
const opus_int order /* I: Prediction order */ const opus_int order /* I: Prediction order */
) )
{ {
opus_int k, n, headrm; opus_int k, n, mult2Q;
opus_int32 rc_Q31, rc_mult1_Q30, rc_mult2_Q16, tmp_QA; opus_int32 rc_Q31, rc_mult1_Q30, rc_mult2, tmp_QA;
opus_int32 *Aold_QA, *Anew_QA; opus_int32 *Aold_QA, *Anew_QA;
Anew_QA = A_QA[ order & 1 ]; Anew_QA = A_QA[ order & 1 ];
@ -59,13 +61,14 @@ static opus_int LPC_inverse_pred_gain_QA( /* O: Returns 1 if unstable,
/* Set RC equal to negated AR coef */ /* Set RC equal to negated AR coef */
rc_Q31 = -silk_LSHIFT( Anew_QA[ k ], 31 - QA ); rc_Q31 = -silk_LSHIFT( Anew_QA[ k ], 31 - QA );
/* rc_mult1_Q30 range: [ 1 : 2^30-1 ] */ /* rc_mult1_Q30 range: [ 1 : 2^30 ] */
rc_mult1_Q30 = ( silk_int32_MAX >> 1 ) - silk_SMMUL( rc_Q31, rc_Q31 ); rc_mult1_Q30 = ( (opus_int32)1 << 30 ) - silk_SMMUL( rc_Q31, rc_Q31 );
silk_assert( rc_mult1_Q30 > ( 1 << 15 ) ); /* reduce A_LIMIT if fails */ silk_assert( rc_mult1_Q30 > ( 1 << 15 ) ); /* reduce A_LIMIT if fails */
silk_assert( rc_mult1_Q30 < ( 1 << 30 ) ); silk_assert( rc_mult1_Q30 < ( 1 << 30 ) );
/* rc_mult2_Q16 range: [ 2^16 : silk_int32_MAX ] */ /* rc_mult2 range: [ 2^30 : silk_int32_MAX ] */
rc_mult2_Q16 = silk_INVERSE32_varQ( rc_mult1_Q30, 46 ); /* 16 = 46 - 30 */ mult2Q = 32 - silk_CLZ32( silk_abs( rc_mult1_Q30 ) );
rc_mult2 = silk_INVERSE32_varQ( rc_mult1_Q30, mult2Q + 30 );
/* Update inverse gain */ /* Update inverse gain */
/* invGain_Q30 range: [ 0 : 2^30 ] */ /* invGain_Q30 range: [ 0 : 2^30 ] */
@ -78,11 +81,9 @@ static opus_int LPC_inverse_pred_gain_QA( /* O: Returns 1 if unstable,
Anew_QA = A_QA[ k & 1 ]; Anew_QA = A_QA[ k & 1 ];
/* Update AR coefficient */ /* Update AR coefficient */
headrm = silk_CLZ32( rc_mult2_Q16 ) - 1;
rc_mult2_Q16 = silk_LSHIFT( rc_mult2_Q16, headrm ); /* Q: 16 + headrm */
for( n = 0; n < k; n++ ) { for( n = 0; n < k; n++ ) {
tmp_QA = Aold_QA[ n ] - silk_LSHIFT( silk_SMMUL( Aold_QA[ k - n - 1 ], rc_Q31 ), 1 ); tmp_QA = Aold_QA[ n ] - MUL32_FRAC_Q( Aold_QA[ k - n - 1 ], rc_Q31, 31 );
Anew_QA[ n ] = silk_LSHIFT( silk_SMMUL( tmp_QA, rc_mult2_Q16 ), 16 - headrm ); Anew_QA[ n ] = MUL32_FRAC_Q( tmp_QA, rc_mult2 , mult2Q );
} }
} }
@ -95,7 +96,7 @@ static opus_int LPC_inverse_pred_gain_QA( /* O: Returns 1 if unstable,
rc_Q31 = -silk_LSHIFT( Anew_QA[ 0 ], 31 - QA ); rc_Q31 = -silk_LSHIFT( Anew_QA[ 0 ], 31 - QA );
/* Range: [ 1 : 2^30 ] */ /* Range: [ 1 : 2^30 ] */
rc_mult1_Q30 = ( silk_int32_MAX >> 1 ) - silk_SMMUL( rc_Q31, rc_Q31 ); rc_mult1_Q30 = ( (opus_int32)1 << 30 ) - silk_SMMUL( rc_Q31, rc_Q31 );
/* Update inverse gain */ /* Update inverse gain */
/* Range: [ 0 : 2^30 ] */ /* Range: [ 0 : 2^30 ] */
@ -142,7 +143,7 @@ opus_int silk_LPC_inverse_pred_gain_Q24( /* O: Returns 1 if unstable,
/* Increase Q domain of the AR coefficients */ /* Increase Q domain of the AR coefficients */
for( k = 0; k < order; k++ ) { for( k = 0; k < order; k++ ) {
Anew_QA[ k ] = silk_RSHIFT_ROUND( A_Q24[ k ], 24 - QA ); Anew_QA[ k ] = silk_RSHIFT( A_Q24[ k ], 24 - QA );
} }
return LPC_inverse_pred_gain_QA( invGain_Q30, Atmp_QA, order ); return LPC_inverse_pred_gain_QA( invGain_Q30, Atmp_QA, order );