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Define "weak transients" for low bitrate hybrid

These rely on TF rather than short windows to avoid partial collapse.

--- a/celt/celt_encoder.c

+++ b/celt/celt_encoder.c

@@ -226,7 +226,8 @@

 static int transient_analysis(const opus_val32 * OPUS_RESTRICT in, int len, int C,

-                              opus_val16 *tf_estimate, int *tf_chan, int low_rate)

+                              opus_val16 *tf_estimate, int *tf_chan, int low_rate,

+                              int *weak_transient)

 {

    int i;

    VARDECL(opus_val16, tmp);

@@ -256,6 +257,7 @@

    SAVE_STACK;

    ALLOC(tmp, len, opus_val16);

+   *weak_transient = 0;

    /* For lower bitrates, let's be more conservative and have a forward masking

       decay of 3.3 dB/ms. This avoids having to code transients at very low

       bitrate (mostly for hybrid), which can result in unstable energy and/or

@@ -381,7 +383,12 @@

       }

    }

    is_transient = mask_metric>200;

-

+   /* For low bitrates, define "weak transients" that need to be

+      handled differently to avoid partial collapse. */

+   if (low_rate && is_transient && mask_metric<600) {

+      is_transient = 0;

+      *weak_transient = 1;

+   }

    /* Arbitrary metric for VBR boost */

    tf_max = MAX16(0,celt_sqrt(27*mask_metric)-42);

    /* *tf_estimate = 1 + MIN16(1, sqrt(MAX16(0, tf_max-30))/20); */

@@ -1388,6 +1395,7 @@

    opus_val16 surround_trim = 0;

    opus_int32 equiv_rate;

    int hybrid;

+   int weak_transient = 0;

    VARDECL(opus_val16, surround_dynalloc);

    ALLOC_STACK;

@@ -1605,7 +1613,7 @@

    if (st->complexity >= 1 && !st->lfe)

    {

       isTransient = transient_analysis(in, N+overlap, CC,

-            &tf_estimate, &tf_chan, effectiveBytes<15);

+            &tf_estimate, &tf_chan, effectiveBytes<15, &weak_transient);

    }

    if (LM>0 && ec_tell(enc)+3<=total_bits)

    {

@@ -1784,6 +1792,14 @@

       tf_select = tf_analysis(mode, effEnd, isTransient, tf_res, lambda, X, N, LM, tf_estimate, tf_chan);

       for (i=effEnd;i<end;i++)

          tf_res[i] = tf_res[effEnd-1];

+   } else if (hybrid && weak_transient)

+   {

+      /* For weak transients, we rely on the fact that improving time resolution using

+         TF on a long window is imperfect and will not result in an energy collapse at

+         low bitrate. */

+      for (i=0;i<end;i++)

+         tf_res[i] = 1;

+      tf_select=0;

    } else if (hybrid && effectiveBytes<15)

    {

       /* For low bitrate hybrid, we force temporal resolution to 5 ms rather than 2.5 ms. */