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Intensity stereo update, partially based on Monty's listening

Intensity stereo is now based on equiv_rate and should no longer oscillate
in hybrid mode. Also, there is now a separate set of thresholds for
surround tuned by Monty. The non-surround thresholds were also updated at
low bitrate to be closer to what Monty found for surround.

--- a/celt/celt_encoder.c

+++ b/celt/celt_encoder.c

@@ -1804,25 +1804,21 @@

    if (C==2)

    {

-      int effectiveRate;

-

-      static const opus_val16 intensity_thresholds[21]=

+      int surround;

+      static const opus_val16 intensity_thresholds[2][21]=

       /* 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19  20  off*/

-        { 16,21,23,25,27,29,31,33,35,38,42,46,50,54,58,63,68,75,84,102,130};

+       {{  1, 2, 3, 4, 5, 6, 7,20,26,30,36,44,50,56,62,67,72,79,88,106,134},

+        {  1, 2, 3, 4, 5, 6, 7,12,16,18,24,30,36,42,48,54,60,66,80, 98,120}};

       static const opus_val16 intensity_histeresis[21]=

-        {  2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 4, 5, 6,  8, 12};

+        {  1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 3, 3, 4, 5, 6,  8, 8};

       /* Always use MS for 2.5 ms frames until we can do a better analysis */

       if (LM!=0)

          dual_stereo = stereo_analysis(mode, X, LM, N);

-      /* Account for coarse energy */

-      effectiveRate = (8*effectiveBytes - 80)>>LM;

-

-      /* effectiveRate in kb/s */

-      effectiveRate = 2*effectiveRate/5;

-

-      st->intensity = hysteresis_decision((opus_val16)effectiveRate, intensity_thresholds, intensity_histeresis, 21, st->intensity);

+      surround = st->energy_mask!=NULL;

+      st->intensity = hysteresis_decision((opus_val16)equiv_rate/1000,

+            intensity_thresholds[surround], intensity_histeresis, 21, st->intensity);

       st->intensity = IMIN(st->end,IMAX(st->start, st->intensity));

    }