[opus] [PATCH] Add static bitrate allocation and force CELT-only for ambisonics encoding

[Michael Graczyk]

This patch adds a static bitrate allocation for ambisonics encoded with the
multistream API. We allocate 4 bits to the mono channel (channel index 0)
 for every 3 bits allocated to other channels. The allocation is static
because it does not depend on the input audio.

This patch also forces the underlying encoders to use CELT only for
ambisonics.

-- 

Thanks,
Michael Graczyk
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From f8ad3ed7f2c45c2f54dc87617dad02f6586f34d9 Mon Sep 17 00:00:00 2001
From: Michael Graczyk <mgraczyk at google.com>
Date: Mon, 4 Apr 2016 19:31:04 -0700
Subject: [PATCH] Add static bitrate allocation and force CELT-only for
 ambisonics encoding

---
 src/opus_multistream_encoder.c | 103 +++++++++++++++++++++++++++++++++++++----
 1 file changed, 93 insertions(+), 10 deletions(-)

diff --git a/src/opus_multistream_encoder.c b/src/opus_multistream_encoder.c
index f08789e..1c0db8b 100644
--- a/src/opus_multistream_encoder.c
+++ b/src/opus_multistream_encoder.c
@@ -512,7 +512,8 @@ int opus_multistream_surround_encoder_init(
       int application
 )
 {
-   int mapping_type;
+   MappingType mapping_type;
+
    if ((channels>255) || (channels<1))
       return OPUS_BAD_ARG;
    st->lfe_stream = -1;
@@ -657,24 +658,19 @@ OpusMSEncoder *opus_multistream_surround_encoder_create(
    return st;
 }
 
-static opus_int32 surround_rate_allocation(
+static void surround_rate_allocation(
       OpusMSEncoder *st,
       opus_int32 *rate,
-      int frame_size
+      int frame_size,
+      opus_int32 Fs
       )
 {
    int i;
    opus_int32 channel_rate;
-   opus_int32 Fs;
-   char *ptr;
    int stream_offset;
    int lfe_offset;
    int coupled_ratio; /* Q8 */
    int lfe_ratio;     /* Q8 */
-   opus_int32 rate_sum=0;
-
-   ptr = (char*)st + align(sizeof(OpusMSEncoder));
-   opus_encoder_ctl((OpusEncoder*)ptr, OPUS_GET_SAMPLE_RATE(&Fs));
 
    if (st->bitrate_bps > st->layout.nb_channels*40000)
       stream_offset = 20000;
@@ -727,6 +723,88 @@ static opus_int32 surround_rate_allocation(
          rate[i] = stream_offset+channel_rate;
       else
          rate[i] = lfe_offset+(channel_rate*lfe_ratio>>8);
+   }
+}
+
+#ifdef ENABLE_EXPERIMENTAL_AMBISONICS
+static void ambisonics_rate_allocation(
+      OpusMSEncoder *st,
+      opus_int32 *rate,
+      int frame_size,
+      opus_int32 Fs
+      )
+{
+   int i;
+   int non_mono_rate;
+   int total_rate;
+
+   /* The mono channel gets (rate_ratio_num / rate_ratio_den) times as many bits
+    * as all other channels */
+   const int rate_ratio_num = 4;
+   const int rate_ratio_den = 3;
+   const int num_channels = st->layout.nb_streams;
+
+   if (st->bitrate_bps==OPUS_AUTO)
+   {
+      total_rate = num_channels * (20000 + st->layout.nb_streams*(Fs+60*Fs/frame_size));
+   } else if (st->bitrate_bps==OPUS_BITRATE_MAX)
+   {
+      total_rate = num_channels * 320000;
+   } else {
+      total_rate = st->bitrate_bps;
+   }
+
+   /* Let y be the non-mono rate and let p, q be integers such that the mono
+    * channel rate is (p/q) * y.
+    * Also let T be the total bitrate to allocate. Then
+    *   (n - 1) y + (p/q) y = T
+    *   y = (T q) / (qn - q + p)
+    */
+   non_mono_rate =
+         total_rate * rate_ratio_den
+         / (rate_ratio_den*num_channels + rate_ratio_num - rate_ratio_den);
+
+#ifndef FIXED_POINT
+   if (st->variable_duration==OPUS_FRAMESIZE_VARIABLE && frame_size != Fs/50)
+   {
+      opus_int32 bonus = 60*(Fs/frame_size-50);
+      non_mono_rate += bonus;
+   }
+#endif
+
+   rate[0] = total_rate - (num_channels - 1) * non_mono_rate;
+   for (i=1;i<st->layout.nb_streams;i++)
+   {
+      rate[i] = non_mono_rate;
+   }
+}
+#endif // ENABLE_EXPERIMENTAL_AMBISONICS
+
+static opus_int32 rate_allocation(
+      OpusMSEncoder *st,
+      opus_int32 *rate,
+      int frame_size
+      )
+{
+   int i;
+   opus_int32 rate_sum=0;
+   opus_int32 Fs;
+   char *ptr;
+
+   ptr = (char*)st + align(sizeof(OpusMSEncoder));
+   opus_encoder_ctl((OpusEncoder*)ptr, OPUS_GET_SAMPLE_RATE(&Fs));
+
+#ifdef ENABLE_EXPERIMENTAL_AMBISONICS
+   if (st->mapping_type == MAPPING_TYPE_AMBISONICS) {
+     ambisonics_rate_allocation(st, rate, frame_size, Fs);
+   } else
+#endif
+   {
+     surround_rate_allocation(st, rate, frame_size, Fs);
+   }
+
+   for (i=0;i<st->layout.nb_streams;i++)
+   {
       rate[i] = IMAX(rate[i], 500);
       rate_sum += rate[i];
    }
@@ -829,7 +907,7 @@ static int opus_multistream_encode_native
    }
 
    /* Compute bitrate allocation between streams (this could be a lot better) */
-   rate_sum = surround_rate_allocation(st, bitrates, frame_size);
+   rate_sum = rate_allocation(st, bitrates, frame_size);
 
    if (!vbr)
    {
@@ -873,6 +951,11 @@ static int opus_multistream_encode_native
             opus_encoder_ctl(enc, OPUS_SET_FORCE_CHANNELS(2));
          }
       }
+#ifdef ENABLE_EXPERIMENTAL_AMBISONICS
+      else if (st->mapping_type == MAPPING_TYPE_AMBISONICS) {
+        opus_encoder_ctl(enc, OPUS_SET_FORCE_MODE(MODE_CELT_ONLY));
+      }
+#endif
    }
 
    ptr = (char*)st + align(sizeof(OpusMSEncoder));
-- 
2.8.0.rc3.226.g39d4020