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Don't use hybrid "weak transients" on vowels

--- a/celt/celt_encoder.c

+++ b/celt/celt_encoder.c

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

 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 allow_weak_transients,

                               int *weak_transient)

 {

    int i;

@@ -262,7 +262,7 @@

       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

       partial collapse. */

-   if (low_rate)

+   if (allow_weak_transients)

    {

 #ifdef FIXED_POINT

       forward_shift = 5;

@@ -385,7 +385,7 @@

    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) {

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

       is_transient = 0;

       *weak_transient = 1;

    }

@@ -1612,8 +1612,12 @@

    shortBlocks = 0;

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

    {

+      /* Reduces the likelihood of energy instability on fricatives at low bitrate

+         in hybrid mode. It seems like we still want to have real transients on vowels

+         though (small SILK quantization offset value). */

+      int allow_weak_transients = hybrid && effectiveBytes<15 && st->silk_info.offset >= 100;

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

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

+            &tf_estimate, &tf_chan, allow_weak_transients, &weak_transient);

    }

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

    {