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Improved quality of small frame sizes at low bitrate.

Adding a temporal energy floor to prevent extremely small values when there's
only one bin in the band.

--- a/libcelt/celt.c

+++ b/libcelt/celt.c

@@ -717,6 +717,7 @@

    int LM, M;

    int tf_select;

    celt_int32 vbr_rate=0;

+   celt_word16 max_decay;

    int nbFilledBytes, nbAvailableBytes;

    SAVE_STACK;

@@ -952,7 +953,13 @@

    /* Bit allocation */

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

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

+

+#ifdef FIXED_POINT

+      max_decay = MIN32(QCONST16(16,DB_SHIFT), SHL32(EXTEND32(nbAvailableBytes),DB_SHIFT-3));

+#else

+   max_decay = .125*nbAvailableBytes;

+#endif

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

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

    if (coarse_needed > nbAvailableBytes)

       coarse_needed = nbAvailableBytes;

--- a/libcelt/quant_bands.c

+++ b/libcelt/quant_bands.c

@@ -84,7 +84,7 @@

    celt_free(freq);

 }

-unsigned quant_coarse_energy(const CELTMode *m, int start, celt_word16 *eBands, celt_word16 *oldEBands, int budget, int intra, int *prob, celt_word16 *error, ec_enc *enc, int _C)

+unsigned quant_coarse_energy(const CELTMode *m, int start, const celt_word16 *eBands, celt_word16 *oldEBands, int budget, int intra, int *prob, celt_word16 *error, ec_enc *enc, int _C, celt_word16 max_decay)

 {

    int i, c;

    unsigned bits_used = 0;

@@ -121,6 +121,12 @@

          /* Rounding to nearest integer here is really important! */

          qi = (int)floor(.5f+f);

 #endif

+         if (qi < 0 && x < oldEBands[i+c*m->nbEBands]-max_decay)

+         {

+            qi += SHR16(oldEBands[i+c*m->nbEBands]-max_decay-x, DB_SHIFT);

+            if (qi > 0)

+               qi = 0;

+         }

          /* If we don't have enough bits to encode all the energy, just assume something safe.

             We allow slightly busting the budget here */

          bits_used=ec_enc_tell(enc, 0);

--- a/libcelt/quant_bands.h

+++ b/libcelt/quant_bands.h

@@ -56,7 +56,7 @@

 int intra_decision(celt_word16 *eBands, celt_word16 *oldEBands, int len);

-unsigned quant_coarse_energy(const CELTMode *m, int start, celt_word16 *eBands, celt_word16 *oldEBands, int budget, int intra, int *prob, celt_word16 *error, ec_enc *enc, int _C);

+unsigned quant_coarse_energy(const CELTMode *m, int start, const celt_word16 *eBands, celt_word16 *oldEBands, int budget, int intra, int *prob, celt_word16 *error, ec_enc *enc, int _C, celt_word16 max_decay);

 void quant_fine_energy(const CELTMode *m, int start, celt_ener *eBands, celt_word16 *oldEBands, celt_word16 *error, int *fine_quant, ec_enc *enc, int _C);