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Adds surround masking to SILK

--- a/src/opus_encoder.c

+++ b/src/opus_encoder.c

@@ -95,7 +95,7 @@

     int          silk_bw_switch;

     /* Sampling rate (at the API level) */

     int          first;

-    int          energy_masking;

+    opus_val16 * energy_masking;

     StereoWidthState width_mem;

     opus_val16   delay_buffer[MAX_ENCODER_BUFFER*2];

 #ifndef DISABLE_FLOAT_API

@@ -1478,6 +1478,38 @@

             st->silk_mode.bitRate = total_bitRate;

         }

+        /* Surround masking for SILK */

+        if (st->energy_masking && st->use_vbr && !st->lfe)

+        {

+           opus_val32 mask_sum=0;

+           opus_val16 masking_depth;

+           opus_int32 rate_offset;

+           int c;

+           int end = 17;

+           opus_int16 srate = 16000;

+           if (st->bandwidth == OPUS_BANDWIDTH_NARROWBAND)

+           {

+              end = 13;

+              srate = 8000;

+           } else if (st->bandwidth == OPUS_BANDWIDTH_MEDIUMBAND)

+           {

+              end = 15;

+              srate = 12000;

+           }

+           for (c=0;c<st->channels;c++)

+           {

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

+                 mask_sum += st->energy_masking[21*c+i];

+           }

+           /* Conservative rate reduction, we cut the masking in half */

+           masking_depth = HALF16(mask_sum / end*st->channels);

+           masking_depth = MAX16(QCONST16(-2.f, DB_SHIFT), MIN16(masking_depth, QCONST16(0.f, DB_SHIFT)));

+           rate_offset = PSHR32(MULT16_16(srate, masking_depth), DB_SHIFT);

+           rate_offset = MAX32(rate_offset, -2*st->silk_mode.bitRate/3);

+           st->silk_mode.bitRate += rate_offset;

+           bytes_target += rate_offset * frame_size / (8 * st->Fs);

+        }

+

         st->silk_mode.payloadSize_ms = 1000 * frame_size / st->Fs;

         st->silk_mode.nChannelsAPI = st->channels;

         st->silk_mode.nChannelsInternal = st->stream_channels;

@@ -2338,7 +2370,7 @@

         case OPUS_SET_ENERGY_MASK_REQUEST:

         {

             opus_val16 *value = va_arg(ap, opus_val16*);

-            st->energy_masking = (value!=NULL);

+            st->energy_masking = value;

             ret = celt_encoder_ctl(celt_enc, OPUS_SET_ENERGY_MASK(value));

         }

         break;

--- a/src/opus_multistream_encoder.c

+++ b/src/opus_multistream_encoder.c

@@ -741,10 +741,13 @@

       opus_encoder_ctl(enc, OPUS_SET_BITRATE(bitrates[s]));

       if (st->surround)

       {

-         opus_encoder_ctl(enc, OPUS_SET_FORCE_MODE(MODE_CELT_ONLY));

          opus_encoder_ctl(enc, OPUS_SET_BANDWIDTH(OPUS_BANDWIDTH_FULLBAND));

          if (s < st->layout.nb_coupled_streams)

+         {

+            /* To preserve the spatial image, force stereo CELT on coupled streams */

+            opus_encoder_ctl(enc, OPUS_SET_FORCE_MODE(MODE_CELT_ONLY));

             opus_encoder_ctl(enc, OPUS_SET_FORCE_CHANNELS(2));

+         }

       }

    }