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Adding a decision mechanism for turning folding on or off depending on the
signal characteristics

--- a/libcelt/bands.c

+++ b/libcelt/bands.c

@@ -397,6 +397,69 @@

    RESTORE_STACK;

 }

+int folding_decision(const CELTMode *m, celt_norm_t *X, celt_word16_t *average, int *last_decision)

+{

+   int i;

+   int NR=0;

+   celt_word32_t ratio = EPSILON;

+   const celt_int16_t * restrict eBands = m->eBands;

+   for (i=0;i<m->nbEBands;i++)

+   {

+      int j, N;

+      int max_i=0;

+      celt_word16_t max_val=EPSILON;

+      celt_word32_t floor_ener=EPSILON;

+      celt_norm_t * restrict x = X+eBands[i];

+      N = eBands[i+1]-eBands[i];

+      for (j=0;j<N;j++)

+      {

+         if (ABS16(x[j])>max_val)

+         {

+            max_val = ABS16(x[j]);

+            max_i = j;

+         }

+      }

+#if 0

+      for (j=0;j<N;j++)

+      {

+         if (abs(j-max_i)>2)

+            floor_ener += x[j]*x[j];

+      }

+#else

+      floor_ener = QCONST32(1.,28)-MULT16_16(max_val,max_val);

+      if (max_i < N-1)

+         floor_ener -= MULT16_16(x[max_i+1], x[max_i+1]);

+      if (max_i < N-2)

+         floor_ener -= MULT16_16(x[max_i+2], x[max_i+2]);

+      if (max_i > 0)

+         floor_ener -= MULT16_16(x[max_i-1], x[max_i-1]);

+      if (max_i > 1)

+         floor_ener -= MULT16_16(x[max_i-2], x[max_i-2]);

+      floor_ener = MAX32(floor_ener, EPSILON);

+#endif

+      if (N>7 && eBands[i] >= m->pitchEnd)

+      {

+         celt_word16_t r;

+         celt_word16_t den = celt_sqrt(floor_ener);

+         den = MAX32(QCONST16(.02, 15), den);

+         r = DIV32_16(SHL32(EXTEND32(max_val),8),den);

+         ratio = ADD32(ratio, EXTEND32(r));

+         NR++;

+      }

+   }

+   if (NR>0)

+      ratio = DIV32_16(ratio, NR);

+   ratio = ADD32(HALF32(ratio), HALF32(*average));

+   if (!*last_decision)

+   {

+      *last_decision = (ratio < QCONST16(1.8,8));

+   } else {

+      *last_decision = (ratio < QCONST16(3.,8));

+   }

+   *average = EXTRACT16(ratio);

+   return *last_decision;

+}

+

 /* Quantisation of the residual */

 void quant_bands(const CELTMode *m, celt_norm_t * restrict X, celt_norm_t *P, celt_mask_t *W, int pitch_used, celt_pgain_t *pgains, const celt_ener_t *bandE, int *pulses, int shortBlocks, int fold, int total_bits, ec_enc *enc)

 {

--- a/libcelt/bands.h

+++ b/libcelt/bands.h

@@ -73,6 +73,8 @@

  */

 int compute_pitch_gain(const CELTMode *m, const celt_norm_t *X, const celt_norm_t *P, celt_pgain_t *gains);

+int folding_decision(const CELTMode *m, celt_norm_t *X, celt_word16_t *average, int *last_decision);

+

 /** Quantisation/encoding of the residual spectrum

  * @param m Mode data 

  * @param X Residual (normalised)

--- a/libcelt/celt.c

+++ b/libcelt/celt.c

@@ -78,6 +78,9 @@

    int pitch_enabled;

    int pitch_available;

    int delayedIntra;

+   celt_word16_t tonal_average;

+   int fold_decision;

+

    int VBR_rate; /* Target number of 16th bits per frame */

    celt_word16_t * restrict preemph_memE; /* Input is 16-bit, so why bother with 32 */

    celt_sig_t    * restrict preemph_memD;

@@ -113,6 +116,8 @@

    st->pitch_enabled = 1;

    st->pitch_available = 1;

    st->delayedIntra = 1;

+   st->tonal_average = QCONST16(1.,8);

+   st->fold_decision = 1;

    st->in_mem = celt_alloc(st->overlap*C*sizeof(celt_sig_t));

    st->out_mem = celt_alloc((MAX_PERIOD+st->overlap)*C*sizeof(celt_sig_t));

@@ -652,7 +657,8 @@

    /* Band normalisation */

    normalise_bands(st->mode, freq, X, bandE);

-

+   if (!shortBlocks && !folding_decision(st->mode, X, &st->tonal_average, &st->fold_decision))

+      has_fold = 0;

 #ifdef EXP_PSY

    ALLOC(bandN,C*st->mode->nbEBands, celt_ener_t);

    ALLOC(bandM,st->mode->nbEBands, celt_ener_t);