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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>

--- a/silk/control_SNR.c

+++ b/silk/control_SNR.c

@@ -70,11 +70,6 @@

                 break;

             }

         }

-

-        /* Reduce coding quality whenever LBRR is enabled, to free up some bits */

-        if( psEncC->LBRR_enabled ) {

-            psEncC->SNR_dB_Q7 = silk_SMLABB( psEncC->SNR_dB_Q7, 12 - psEncC->LBRR_GainIncreases, SILK_FIX_CONST( -0.25, 7 ) );

-        }

     }

     return ret;

--- a/silk/control_codec.c

+++ b/silk/control_codec.c

@@ -397,9 +397,10 @@

     const opus_int32            TargetRate_bps      /* I                        */

 )

 {

-    opus_int   ret = SILK_NO_ERROR;

+    opus_int   LBRR_in_previous_packet, ret = SILK_NO_ERROR;

     opus_int32 LBRR_rate_thres_bps;

+    LBRR_in_previous_packet = psEncC->LBRR_enabled;

     psEncC->LBRR_enabled = 0;

     if( psEncC->useInBandFEC && psEncC->PacketLoss_perc > 0 ) {

         if( psEncC->fs_kHz == 8 ) {

@@ -413,8 +414,13 @@

         if( TargetRate_bps > LBRR_rate_thres_bps ) {

             /* Set gain increase for coding LBRR excitation */

+            if( LBRR_in_previous_packet == 0 ) {

+                /* Previous packet did not have LBRR, and was therefore coded at a higher bitrate */

+                psEncC->LBRR_GainIncreases = 7;

+            } else {

+                psEncC->LBRR_GainIncreases = silk_max_int( 7 - silk_SMULWB( (opus_int32)psEncC->PacketLoss_perc, SILK_FIX_CONST( 0.4, 16 ) ), 2 );

+            }

             psEncC->LBRR_enabled = 1;

-            psEncC->LBRR_GainIncreases = silk_max_int( 7 - silk_SMULWB( (opus_int32)psEncC->PacketLoss_perc, SILK_FIX_CONST( 0.4, 16 ) ), 2 );

         }

     }

--- a/silk/enc_API.c

+++ b/silk/enc_API.c

@@ -376,6 +376,8 @@

                 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 );

@@ -382,12 +384,12 @@

             /* 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 );

@@ -394,8 +396,13 @@

             } 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 );

--- a/silk/fixed/structs_FIX.h

+++ b/silk/fixed/structs_FIX.h

@@ -116,6 +116,7 @@

 typedef struct {

     silk_encoder_state_FIX      state_Fxx[ ENCODER_NUM_CHANNELS ];

     stereo_enc_state            sStereo;

+    opus_int32                  nBitsUsedLBRR;

     opus_int32                  nBitsExceeded;

     opus_int                    nChannelsAPI;

     opus_int                    nChannelsInternal;

--- a/silk/float/structs_FLP.h

+++ b/silk/float/structs_FLP.h

@@ -115,6 +115,7 @@

 typedef struct {

     silk_encoder_state_FLP      state_Fxx[ ENCODER_NUM_CHANNELS ];

     stereo_enc_state            sStereo;

+    opus_int32                  nBitsUsedLBRR;

     opus_int32                  nBitsExceeded;

     opus_int                    nChannelsAPI;

     opus_int                    nChannelsInternal;