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Revisit surround rate allocation

Stereo now gets twice the "marginal allocation" compared to mono, but has
a 20 kb/s offset. This should be more realistic across a wide range of
bitrates.

--- a/src/opus_multistream_encoder.c

+++ b/src/opus_multistream_encoder.c

@@ -362,6 +362,8 @@

    opus_int32 channel_rate;

    opus_int32 Fs;

    char *ptr;

+   int stream_offset;

+   int lfe_offset;

    int coupled_ratio; /* Q8 */

    int lfe_ratio;     /* Q8 */

@@ -368,9 +370,14 @@

    ptr = (char*)st + align(sizeof(OpusMSEncoder));

    opus_encoder_ctl((OpusEncoder*)ptr, OPUS_GET_SAMPLE_RATE(&Fs));

-   /* Should depend on the bitrate, for now we assume coupled streams get 60% more bits than mono */

-   coupled_ratio = 410;

-   /* Should depend on the bitrate, for now we assume LFE gets 1/12 the bits of mono */

+   /* We start by giving each stream (coupled or uncoupled) the same bitrate.

+      This models the main saving of coupled channels over uncoupled. */

+   stream_offset = 20000;

+   /* The LFE stream is an exception to the above and gets fewer bits. */

+   lfe_offset = 3500;

+   /* Coupled streams get twice the mono rate after the first 20 kb/s. */

+   coupled_ratio = 512;

+   /* Should depend on the bitrate, for now we assume LFE gets 1/8 the bits of mono */

    lfe_ratio = 32;

    /* Compute bitrate allocation between streams */

@@ -381,10 +388,17 @@

    {

       channel_rate = 300000;

    } else {

-      int total = ((st->layout.nb_streams-st->layout.nb_coupled_streams-(st->lfe_stream!=-1))<<8) /* mono */

-            + coupled_ratio*st->layout.nb_coupled_streams /* stereo */

-            + (st->lfe_stream!=-1)*lfe_ratio;

-      channel_rate = 256*(st->bitrate_bps-2000)/total;

+      int nb_lfe;

+      int nb_uncoupled;

+      int nb_coupled;

+      int total;

+      nb_lfe = (st->lfe_stream!=-1);

+      nb_coupled = st->layout.nb_coupled_streams;

+      nb_uncoupled = st->layout.nb_streams-nb_coupled-nb_lfe;

+      total = (nb_uncoupled<<8)         /* mono */

+            + coupled_ratio*nb_coupled /* stereo */

+            + nb_lfe*lfe_ratio;

+      channel_rate = 256*(st->bitrate_bps-lfe_offset*nb_lfe-stream_offset*(nb_coupled+nb_uncoupled))/total;

    }

 #ifndef FIXED_POINT

    if (st->variable_duration==OPUS_FRAMESIZE_VARIABLE && frame_size != Fs/50)

@@ -398,14 +412,12 @@

    for (i=0;i<st->layout.nb_streams;i++)

    {

       if (i<st->layout.nb_coupled_streams)

-         rate[i] = channel_rate*coupled_ratio>>8;

+         rate[i] = stream_offset+(channel_rate*coupled_ratio>>8);

       else if (i!=st->lfe_stream)

-         rate[i] = channel_rate;

+         rate[i] = stream_offset+channel_rate;

       else

-         rate[i] = 2000+(channel_rate*lfe_ratio>>8);

+         rate[i] = lfe_offset+(channel_rate*lfe_ratio>>8);

    }

-

-

 }

 /* Max size in case the encoder decides to return three frames */