Fixes CNG to address the DTX issue reported by Gonzalo Mariano

- Bug fix: the CNG synthesis filter output consisted of only the prediction
  values, now added the input to that.
- CNG excitation level now takes into account the level from the PLC (since
  both PLC and CNG can be active at the same time).
- CNG is now called before PLC_glue() to smoothen the transition out of CNG.

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>

silk/CNG.c

@@ -34,7 +34,7 @@ POSSIBILITY OF SUCH DAMAGE.
 
 /* Generates excitation for CNG LPC synthesis */
 static OPUS_INLINE void silk_CNG_exc(
-    opus_int32                       residual_Q10[],     /* O    CNG residual signal Q10                     */
+    opus_int32                       exc_Q10[],          /* O    CNG excitation signal Q10                   */
     opus_int32                       exc_buf_Q14[],      /* I    Random samples buffer Q10                   */
     opus_int32                       Gain_Q16,           /* I    Gain to apply                               */
     opus_int                         length,             /* I    Length                                      */
@@ -55,7 +55,7 @@ static OPUS_INLINE void silk_CNG_exc(
         idx = (opus_int)( silk_RSHIFT( seed, 24 ) & exc_mask );
         silk_assert( idx >= 0 );
         silk_assert( idx <= CNG_BUF_MASK_MAX );
-        residual_Q10[ i ] = (opus_int16)silk_SAT16( silk_SMULWW( exc_buf_Q14[ idx ], Gain_Q16 >> 4 ) );
+        exc_Q10[ i ] = (opus_int16)silk_SAT16( silk_SMULWW( exc_buf_Q14[ idx ], Gain_Q16 >> 4 ) );
     }
     *rand_seed = seed;
 }
@@ -85,7 +85,7 @@ void silk_CNG(
 )
 {
     opus_int   i, subfr;
-    opus_int32 sum_Q6, max_Gain_Q16;
+    opus_int32 sum_Q6, max_Gain_Q16, gain_Q16;
     opus_int16 A_Q12[ MAX_LPC_ORDER ];
     silk_CNG_struct *psCNG = &psDec->sCNG;
     SAVE_STACK;
@@ -129,7 +129,12 @@ void silk_CNG(
         ALLOC( CNG_sig_Q10, length + MAX_LPC_ORDER, opus_int32 );
 
         /* Generate CNG excitation */
-        silk_CNG_exc( CNG_sig_Q10 + MAX_LPC_ORDER, psCNG->CNG_exc_buf_Q14, psCNG->CNG_smth_Gain_Q16, length, &psCNG->rand_seed );
+		gain_Q16 = silk_SMULWW( psDec->sPLC.randScale_Q14, psDec->sPLC.prevGain_Q16[1] );
+		gain_Q16 = silk_SMULWW( gain_Q16, gain_Q16 );
+		gain_Q16 = silk_SUB_LSHIFT32(silk_SMULWW( psCNG->CNG_smth_Gain_Q16, psCNG->CNG_smth_Gain_Q16 ), gain_Q16, 5 );
+		gain_Q16 = silk_max_32( gain_Q16, 0 );
+		gain_Q16 = silk_LSHIFT32( silk_SQRT_APPROX( gain_Q16 ), 8 );
+        silk_CNG_exc( CNG_sig_Q10 + MAX_LPC_ORDER, psCNG->CNG_exc_buf_Q14, gain_Q16, length, &psCNG->rand_seed );
 
         /* Convert CNG NLSF to filter representation */
         silk_NLSF2A( A_Q12, psCNG->CNG_smth_NLSF_Q15, psDec->LPC_order );
@@ -162,7 +167,7 @@ void silk_CNG(
             /* Update states */
             CNG_sig_Q10[ MAX_LPC_ORDER + i ] = silk_ADD_LSHIFT( CNG_sig_Q10[ MAX_LPC_ORDER + i ], sum_Q6, 4 );
 
-            frame[ i ] = silk_ADD_SAT16( frame[ i ], silk_RSHIFT_ROUND( sum_Q6, 6 ) );
+            frame[ i ] = silk_ADD_SAT16( frame[ i ], silk_RSHIFT_ROUND( CNG_sig_Q10[ MAX_LPC_ORDER + i ], 10 ) );
         }
         silk_memcpy( psCNG->CNG_synth_state, &CNG_sig_Q10[ length ], MAX_LPC_ORDER * sizeof( opus_int32 ) );
     } else {

silk/decode_frame.c

@@ -107,16 +107,16 @@ opus_int silk_decode_frame(
     silk_memmove( psDec->outBuf, &psDec->outBuf[ psDec->frame_length ], mv_len * sizeof(opus_int16) );
     silk_memcpy( &psDec->outBuf[ mv_len ], pOut, psDec->frame_length * sizeof( opus_int16 ) );
 
-    /****************************************************************/
-    /* Ensure smooth connection of extrapolated and good frames     */
-    /****************************************************************/
-    silk_PLC_glue_frames( psDec, pOut, L );
-
     /************************************************/
     /* Comfort noise generation / estimation        */
     /************************************************/
     silk_CNG( psDec, psDecCtrl, pOut, L );
 
+    /****************************************************************/
+    /* Ensure smooth connection of extrapolated and good frames     */
+    /****************************************************************/
+    silk_PLC_glue_frames( psDec, pOut, L );
+
     /* Update some decoder state variables */
     psDec->lagPrev = psDecCtrl->pitchL[ psDec->nb_subfr - 1 ];