Fix for flutter with FEC

The bug was caused by an improper feedback of the per-frame bitrate, causing
the bitrate to jump up and down from frame to frame, within a packet.
The patch avoids this, and also gives a slight improvement in general for
multi-frame packets, even without FEC.

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

1 files changed, 11 insertions, 4 deletions

diff --git a/silk/enc_API.c b/silk/enc_API.c
index f1993ada..f8060286 100644
--- a/silk/enc_API.c
+++ b/silk/enc_API.c
@@ -376,26 +376,33 @@ opus_int silk_Encode(                                   /* O    Returns error co
                 for( n = 0; n < encControl->nChannelsInternal; n++ ) {
                     silk_memset( psEnc->state_Fxx[ n ].sCmn.LBRR_flags, 0, sizeof( psEnc->state_Fxx[ n ].sCmn.LBRR_flags ) );
                 }
+
+                psEnc->nBitsUsedLBRR = ec_tell( psRangeEnc );
             }
 
             silk_HP_variable_cutoff( psEnc->state_Fxx );
 
             /* Total target bits for packet */
             nBits = silk_DIV32_16( silk_MUL( encControl->bitRate, encControl->payloadSize_ms ), 1000 );
-            /* Subtract half of the bits already used */
+            /* Subtract bits used for LBRR */
             if( !prefillFlag ) {
-                nBits -= ec_tell( psRangeEnc ) >> 1;
+                nBits -= psEnc->nBitsUsedLBRR;
             }
             /* Divide by number of uncoded frames left in packet */
-            nBits = silk_DIV32_16( nBits, psEnc->state_Fxx[ 0 ].sCmn.nFramesPerPacket - psEnc->state_Fxx[ 0 ].sCmn.nFramesEncoded );
+            nBits = silk_DIV32_16( nBits, psEnc->state_Fxx[ 0 ].sCmn.nFramesPerPacket );
             /* Convert to bits/second */
             if( encControl->payloadSize_ms == 10 ) {
                 TargetRate_bps = silk_SMULBB( nBits, 100 );
             } else {
                 TargetRate_bps = silk_SMULBB( nBits, 50 );
             }
-            /* Subtract fraction of bits in excess of target in previous packets */
+            /* Subtract fraction of bits in excess of target in previous frames and packets */
             TargetRate_bps -= silk_DIV32_16( silk_MUL( psEnc->nBitsExceeded, 1000 ), BITRESERVOIR_DECAY_TIME_MS );
+            if( !prefillFlag && psEnc->state_Fxx[ 0 ].sCmn.nFramesEncoded > 0 ) {
+                /* Compare actual vs target bits so far in this packet */
+                opus_int32 bitsBalance = ec_tell( psRangeEnc ) - psEnc->nBitsUsedLBRR - nBits * psEnc->state_Fxx[ 0 ].sCmn.nFramesEncoded;
+                TargetRate_bps -= silk_DIV32_16( silk_MUL( bitsBalance, 1000 ), BITRESERVOIR_DECAY_TIME_MS );
+            }
             /* Never exceed input bitrate */
             TargetRate_bps = silk_LIMIT( TargetRate_bps, encControl->bitRate, 5000 );