Better rate allocation for stereo SILK in hybrid mode
SILK was being allocated too few bits for stereo hybrid, often resulting
in forced narrowing of the width. We now allocate more bits to SILK
and reduce the threshold for narrowing. This improves quality enough that
the bitrate threshold for switching to SILK can be increased to 44 kb/s.
diff --git a/silk/stereo_LR_to_MS.c b/silk/stereo_LR_to_MS.c
index dda0298..c822666 100644
--- a/silk/stereo_LR_to_MS.c
+++ b/silk/stereo_LR_to_MS.c
@@ -109,7 +109,7 @@
if( total_rate_bps < 1 ) {
total_rate_bps = 1;
}
- min_mid_rate_bps = silk_SMLABB( 2000, fs_kHz, 900 );
+ min_mid_rate_bps = silk_SMLABB( 2000, fs_kHz, 600 );
silk_assert( min_mid_rate_bps < 32767 );
/* Default bitrate distribution: 8 parts for Mid and (5+3*frac) parts for Side. so: mid_rate = ( 8 / ( 13 + 3 * frac ) ) * total_ rate */
frac_3_Q16 = silk_MUL( 3, frac_Q16 );
diff --git a/src/opus_encoder.c b/src/opus_encoder.c
index 9f4134d..0494170 100644
--- a/src/opus_encoder.c
+++ b/src/opus_encoder.c
@@ -154,7 +154,7 @@
static const opus_int32 mode_thresholds[2][2] = {
/* voice */ /* music */
{ 64000, 16000}, /* mono */
- { 36000, 16000}, /* stereo */
+ { 44000, 16000}, /* stereo */
};
static const opus_int32 fec_thresholds[] = {
@@ -762,7 +762,7 @@
return 0;
}
-static int compute_silk_rate_for_hybrid(int rate, int bandwidth, int frame20ms, int vbr, int fec) {
+static int compute_silk_rate_for_hybrid(int rate, int bandwidth, int frame20ms, int vbr, int fec, int channels) {
int entry;
int i;
int N;
@@ -779,6 +779,8 @@
{32000, 22000, 22000, 28000, 28000},
{64000, 38000, 38000, 50000, 50000}
};
+ /* Do the allocation per-channel. */
+ rate /= channels;
entry = 1 + frame20ms + 2*fec;
N = sizeof(rate_table)/sizeof(rate_table[0]);
for (i=1;i<N;i++)
@@ -805,6 +807,10 @@
}
if (bandwidth==OPUS_BANDWIDTH_SUPERWIDEBAND)
silk_rate += 300;
+ silk_rate *= channels;
+ /* The CELT layer saves a bit more than SILK for stereo, so we boost SILK. */
+ if (channels == 2 && rate >= 12000)
+ silk_rate += 1000;
return silk_rate;
}
@@ -1682,7 +1688,8 @@
if( st->mode == MODE_HYBRID ) {
/* 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, st->silk_mode.LBRR_coded);
+ curr_bandwidth, st->Fs == 50 * frame_size, st->use_vbr, st->silk_mode.LBRR_coded,
+ st->stream_channels);
if (!st->energy_masking)
{
/* Increasingly attenuate high band when it gets allocated fewer bits */
@@ -1797,7 +1804,8 @@
{
/* Compute SILK bitrate corresponding to the max total bits available */
opus_int32 maxBitRate = compute_silk_rate_for_hybrid(st->silk_mode.maxBits*st->Fs / frame_size,
- curr_bandwidth, st->Fs == 50 * frame_size, st->use_vbr, st->silk_mode.LBRR_coded);
+ curr_bandwidth, st->Fs == 50 * frame_size, st->use_vbr, st->silk_mode.LBRR_coded,
+ st->stream_channels);
st->silk_mode.maxBits = maxBitRate * frame_size / st->Fs;
}
}
