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Re-enabling high-band attenuation for hybrid mode

The new attenuation is much smaller than previously.

--- a/src/opus_encoder.c

+++ b/src/opus_encoder.c

@@ -1545,7 +1545,6 @@

         /* Distribute bits between SILK and CELT */

         total_bitRate = 8 * bytes_target * frame_rate;

         if( st->mode == MODE_HYBRID ) {

-            int HB_gain_ref;

             /* Base rate for SILK */

             st->silk_mode.bitRate = compute_silk_rate_for_hybrid(total_bitRate,

                   curr_bandwidth, st->Fs == 50 * frame_size, st->use_vbr);

@@ -1553,10 +1552,7 @@

             {

                /* Increasingly attenuate high band when it gets allocated fewer bits */

                celt_rate = total_bitRate - st->silk_mode.bitRate;

-               HB_gain_ref = (curr_bandwidth == OPUS_BANDWIDTH_SUPERWIDEBAND) ? 3000 : 3600;

-               HB_gain = SHL32((opus_val32)celt_rate, 9) / SHR32((opus_val32)celt_rate + st->stream_channels * HB_gain_ref, 6);

-               HB_gain = HB_gain < (opus_val32)Q15ONE*6/7 ? HB_gain + Q15ONE/7 : Q15ONE;

-               HB_gain = Q15ONE;

+               HB_gain = Q15ONE - SHR32(celt_exp2(-celt_rate * QCONST16(1.f/1024, 10)), 1);

             }

         } else {

             /* SILK gets all bits */