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Quality: removes VBR attenuation at low bitrate

Turns out that even low bitrates benefit from VBR

--- a/celt/celt_encoder.c

+++ b/celt/celt_encoder.c

@@ -1279,21 +1279,11 @@

       /*printf("%f %d\n", maxDepth, floor_depth);*/

    }

-   if ((!has_surround_mask||lfe) && (constrained_vbr || bitrate<64000))

+   /* Make VBR less aggressive for constrained VBR because we can't keep a higher bitrate

+      for long. Needs tuning. */

+   if ((!has_surround_mask||lfe) && constrained_vbr)

    {

-      opus_val16 rate_factor = Q15ONE;

-      if (bitrate < 64000)

-      {

-#ifdef FIXED_POINT

-         rate_factor = MAX16(0,(bitrate-32000));

-#else

-         rate_factor = MAX16(0,(1.f/32768)*(bitrate-32000));

-#endif

-      }

-      if (constrained_vbr)

-         rate_factor = MIN16(rate_factor, QCONST16(0.67f, 15));

-      target = base_target + (opus_int32)MULT16_32_Q15(rate_factor, target-base_target);

-

+      target = base_target + (opus_int32)MULT16_32_Q15(QCONST16(0.67f, 15), target-base_target);

    }

    if (!has_surround_mask && tf_estimate < QCONST16(.2f, 14))