Fix the side frame conditional coding rules.

b24e5746 introduced changes to LastGainIndex which broke
 conditional coding for side frames after a mid-only frame (i.e.,
 in a 60 ms frame where the side is coded, not coded, then coded
 again).
These rules were a mess in general, however, because the side
 channel state kept a different nFramesDecoded count from the mid
 channel state, and had no way to tell if the prior side frame was
 coded.

This patch attempts to rationalize them by moving the conditional
 coding decision up to the top level, where all this information is
 available.
The first coded side frame after an uncoded side frame now always
 uses independent coding.
If such a frame is also not the first side frame in an Opus frame,
 then it doesn't include an LTP scaling parameter (because the LTP
 state is well-defined).

1 files changed, 26 insertions, 2 deletions

diff --git a/silk/dec_API.c b/silk/dec_API.c
index ade060f9..018173ef 100644
--- a/silk/dec_API.c
+++ b/silk/dec_API.c
@@ -182,13 +182,21 @@ opus_int silk_Decode(
                 for( n = 0; n < decControl->nChannelsInternal; n++ ) {
                     if( channel_state[ n ].LBRR_flags[ i ] ) {
                         opus_int pulses[ MAX_FRAME_LENGTH ];
+                        opus_int condCoding;
+
                         if( decControl->nChannelsInternal == 2 && n == 0 ) {
                             silk_stereo_decode_pred( psRangeDec, MS_pred_Q13 );
                             if( channel_state[ 1 ].LBRR_flags[ i ] == 0 ) {
                                 silk_stereo_decode_mid_only( psRangeDec, &decode_only_middle );
                             }
                         }
-                        silk_decode_indices( &channel_state[ n ], psRangeDec, i, 1 );
+                        /* Use conditional coding if previous frame available */
+                        if( i > 0 && channel_state[ n ].LBRR_flags[ i - 1 ] ) {
+                            condCoding = CODE_CONDITIONALLY;
+                        } else {
+                            condCoding = CODE_INDEPENDENTLY;
+                        }
+                        silk_decode_indices( &channel_state[ n ], psRangeDec, i, 1, condCoding );
                         silk_decode_pulses( psRangeDec, pulses, channel_state[ n ].indices.signalType,
                             channel_state[ n ].indices.quantOffsetType, channel_state[ n ].frame_length );
                     }
@@ -230,7 +238,23 @@ opus_int silk_Decode(
     /* Call decoder for one frame */
     for( n = 0; n < decControl->nChannelsInternal; n++ ) {
         if( n == 0 || ( ( lostFlag != FLAG_PACKET_LOST ? decode_only_middle : psDec->prev_decode_only_middle ) == 0 ) ) {
-            ret += silk_decode_frame( &channel_state[ n ], psRangeDec, &samplesOut1_tmp[ n ][ 2 + delay ], &nSamplesOutDec, lostFlag );
+            opus_int FrameIndex;
+            opus_int condCoding;
+
+            FrameIndex = channel_state[ 0 ].nFramesDecoded - n;
+            /* Use independent coding if no previous frame available */
+            if( FrameIndex <= 0 ) {
+                condCoding = CODE_INDEPENDENTLY;
+            } else if( lostFlag == FLAG_DECODE_LBRR ) {
+                condCoding = channel_state[ n ].LBRR_flags[ FrameIndex - 1 ] ? CODE_CONDITIONALLY : CODE_INDEPENDENTLY;
+            } else if( n > 0 && psDec->prev_decode_only_middle ) {
+                /* If we skipped a side frame in this packet, we don't
+                   need LTP scaling; the LTP state is well-defined. */
+                condCoding = CODE_INDEPENDENTLY_NO_LTP_SCALING;
+            } else {
+                condCoding = CODE_CONDITIONALLY;
+            }
+            ret += silk_decode_frame( &channel_state[ n ], psRangeDec, &samplesOut1_tmp[ n ][ 2 + delay ], &nSamplesOutDec, lostFlag, condCoding);
         } else {
             silk_memset( &samplesOut1_tmp[ n ][ 2 + delay ], 0, nSamplesOutDec * sizeof( opus_int16 ) );
         }