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Smooth out the LBRR rate estimate

Reduces fluctuations in the non-FEC target bitrate.

--- a/silk/enc_API.c

+++ b/silk/enc_API.c

@@ -270,6 +270,7 @@

                        psEnc->state_Fxx[ 0 ].sCmn.fs_kHz * 1000 );

     ALLOC( buf, nSamplesFromInputMax, opus_int16 );

     while( 1 ) {

+        int curr_nBitsUsedLBRR = 0;

         nSamplesToBuffer  = psEnc->state_Fxx[ 0 ].sCmn.frame_length - psEnc->state_Fxx[ 0 ].sCmn.inputBufIx;

         nSamplesToBuffer  = silk_min( nSamplesToBuffer, nSamplesToBufferMax );

         nSamplesFromInput = silk_DIV32_16( nSamplesToBuffer * psEnc->state_Fxx[ 0 ].sCmn.API_fs_Hz, psEnc->state_Fxx[ 0 ].sCmn.fs_kHz * 1000 );

@@ -342,6 +343,7 @@

                 opus_uint8 iCDF[ 2 ] = { 0, 0 };

                 iCDF[ 0 ] = 256 - silk_RSHIFT( 256, ( psEnc->state_Fxx[ 0 ].sCmn.nFramesPerPacket + 1 ) * encControl->nChannelsInternal );

                 ec_enc_icdf( psRangeEnc, 0, iCDF, 8 );

+                curr_nBitsUsedLBRR = ec_tell( psRangeEnc );

                 /* Encode any LBRR data from previous packet */

                 /* Encode LBRR flags */

@@ -386,8 +388,7 @@

                 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 );

+                curr_nBitsUsedLBRR = ec_tell( psRangeEnc ) - curr_nBitsUsedLBRR;

             }

             silk_HP_variable_cutoff( psEnc->state_Fxx );

@@ -396,6 +397,16 @@

             nBits = silk_DIV32_16( silk_MUL( encControl->bitRate, encControl->payloadSize_ms ), 1000 );

             /* Subtract bits used for LBRR */

             if( !prefillFlag ) {

+                /* psEnc->nBitsUsedLBRR is an exponential moving average of the LBRR usage,

+                   except that for the first LBRR frame it does no averaging and for the first

+                   frame after after LBRR, it goes back to zero immediately. */

+                if ( curr_nBitsUsedLBRR < 10 ) {

+                    psEnc->nBitsUsedLBRR = 0;

+                } else if ( psEnc->nBitsUsedLBRR < 10) {

+                    psEnc->nBitsUsedLBRR = curr_nBitsUsedLBRR;

+                } else {

+                    psEnc->nBitsUsedLBRR = ( psEnc->nBitsUsedLBRR + curr_nBitsUsedLBRR ) / 2;

+                }

                 nBits -= psEnc->nBitsUsedLBRR;

             }

             /* Divide by number of uncoded frames left in packet */