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Better computation of the VBR rate upper bound and reducing the coarse energy
budget at low bitrate.

--- a/libcelt/celt.c

+++ b/libcelt/celt.c

@@ -764,16 +764,28 @@

    ALLOC(fine_quant, st->mode->nbEBands, int);

    ALLOC(pulses, st->mode->nbEBands, int);

+   /* Computes the max bit-rate allowed in VBR more to avoid busting the budget */

+   if (st->vbr_rate>0)

+   {

+      celt_int32 vbr_bound, max_allowed;

+

+      vbr_bound = st->vbr_rate;

+      max_allowed = (st->vbr_rate + vbr_bound - st->vbr_reservoir)>>(BITRES+3); 

+      if (max_allowed < nbCompressedBytes)

+         nbCompressedBytes = max_allowed;

+   }

+

    /* Bit allocation */

    ALLOC(error, C*st->mode->nbEBands, celt_word16);

-   coarse_needed = quant_coarse_energy(st->mode, bandLogE, st->oldBandE, nbCompressedBytes*8-16, intra_ener, st->mode->prob, error, &enc, C);

+   coarse_needed = quant_coarse_energy(st->mode, bandLogE, st->oldBandE, nbCompressedBytes*4-8, intra_ener, st->mode->prob, error, &enc, C);

    coarse_needed = ((coarse_needed*3-1)>>3)+1;

-

+   if (coarse_needed > nbCompressedBytes)

+      coarse_needed = nbCompressedBytes;

    /* Variable bitrate */

    if (st->vbr_rate>0)

    {

      celt_word16 alpha;

-     celt_int32 delta, vbr_bound;

+     celt_int32 delta;

      /* The target rate in 16th bits per frame */

      celt_int32 target=st->vbr_rate;

@@ -812,16 +824,8 @@

      /*printf ("%d\n", st->vbr_drift);*/

      /* We could use any multiple of vbr_rate as bound (depending on the delay) */

-     vbr_bound = st->vbr_rate;

-     if (st->vbr_reservoir > vbr_bound)

+     if (st->vbr_reservoir < 0)

      {

-        /* Busted the reservoir -- reduce the rate */

-        int adjust = 1+(st->vbr_reservoir-vbr_bound-1)/(8<<BITRES);

-        nbCompressedBytes -= adjust;

-        st->vbr_reservoir -= adjust*(8<<BITRES);

-        /*printf ("-%d\n", adjust);*/

-     } else if (st->vbr_reservoir < 0)

-     {

         /* We're under the min value -- increase rate */

         int adjust = 1-(st->vbr_reservoir-1)/(8<<BITRES);

         st->vbr_reservoir += adjust*(8<<BITRES);

@@ -1373,7 +1377,7 @@

    ALLOC(fine_quant, st->mode->nbEBands, int);

    /* Get band energies */

-   unquant_coarse_energy(st->mode, bandE, st->oldBandE, len*8-16, intra_ener, st->mode->prob, &dec, C);

+   unquant_coarse_energy(st->mode, bandE, st->oldBandE, len*4-8, intra_ener, st->mode->prob, &dec, C);

    ALLOC(pulses, st->mode->nbEBands, int);

    ALLOC(offsets, st->mode->nbEBands, int);