This reduces high-frequency artifacts and improves the quality of the lower
frequency audio at low bit rates.
---
The values in the table are based on reported bandwidth for Dolby's AC-3
encoder. All AC-3 samples I have from commercial sources match the values in
the table. Most of them also use channel coupling, but the table value
corresponds to the high end of the coupling range and to the bandwidth code
that is used when coupling is suppressed due to transients.
libavcodec/ac3enc.c | 34 +++++++++++++++++++++++++++++++---
tests/ref/acodec/ac3_fixed | 2 +-
tests/ref/lavf/rm | 2 +-
3 files changed, 33 insertions(+), 5 deletions(-)
diff --git a/libavcodec/ac3enc.c b/libavcodec/ac3enc.c
index f7a366b..23564a5 100644
--- a/libavcodec/ac3enc.c
+++ b/libavcodec/ac3enc.c
@@ -322,6 +322,36 @@ static const int64_t ac3_channel_layouts[] = {
/**
+ * LUT to select the bandwidth code based on the bit rate, sample rate, and
+ * number of full-bandwidth channels.
+ * bandwidth_tab[fbw_channels-1][sample rate code][bit rate code]
+ */
+static const uint8_t ac3_bandwidth_tab[5][3][19] = {
+// 32 40 48 56 64 80 96 112 128 160 192 224 256 320 384 448 512 576 640
+
+ { { 0, 0, 0, 12, 16, 32, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48 },
+ { 0, 0, 0, 16, 20, 36, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56 },
+ { 0, 0, 0, 32, 40, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60 } },
+
+ { { 0, 0, 0, 0, 0, 0, 0, 20, 24, 32, 48, 48, 48, 48, 48, 48, 48, 48, 48 },
+ { 0, 0, 0, 0, 0, 0, 4, 24, 28, 36, 56, 56, 56, 56, 56, 56, 56, 56, 56 },
+ { 0, 0, 0, 0, 0, 0, 20, 44, 52, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60 } },
+
+ { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 16, 24, 32, 40, 48, 48, 48, 48, 48, 48 },
+ { 0, 0, 0, 0, 0, 0, 0, 0, 4, 20, 28, 36, 44, 56, 56, 56, 56, 56, 56 },
+ { 0, 0, 0, 0, 0, 0, 0, 0, 20, 40, 48, 60, 60, 60, 60, 60, 60, 60, 60 } },
+
+ { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 12, 24, 32, 48, 48, 48, 48, 48, 48 },
+ { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 16, 28, 36, 56, 56, 56, 56, 56, 56 },
+ { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 32, 48, 60, 60, 60, 60, 60, 60, 60 } },
+
+ { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 8, 20, 32, 40, 48, 48, 48, 48 },
+ { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 12, 24, 36, 44, 56, 56, 56, 56 },
+ { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 28, 44, 60, 60, 60, 60, 60, 60 } }
+};
+
+
+/**
* Adjust the frame size to make the average bit rate match the target bit rate.
* This is only needed for 11025, 22050, and 44100 sample rates.
*/
@@ -2113,9 +2143,7 @@ static av_cold void set_bandwidth(AC3EncodeContext *s)
bw_code = av_clip((fbw_coeffs - 73) / 3, 0, 60);
} else {
/* use default bandwidth setting */
- /* XXX: should compute the bandwidth according to the frame
- size, so that we avoid annoying high frequency artifacts */
- bw_code = 50;
+ bw_code = ac3_bandwidth_tab[s->fbw_channels-1][s->bit_alloc.sr_code][s->frame_size_code/2];
}
/* set number of coefficients for each channel */
diff --git a/tests/ref/acodec/ac3_fixed b/tests/ref/acodec/ac3_fixed
index d4ade32..c131d62 100644
--- a/tests/ref/acodec/ac3_fixed
+++ b/tests/ref/acodec/ac3_fixed
@@ -1,2 +1,2 @@
-346073c97eada69330f61e103a170ca1 *./tests/data/acodec/ac3.rm
+7b42d544d09a0f38dbdbb48ea8514e7f *./tests/data/acodec/ac3.rm
98751 ./tests/data/acodec/ac3.rm
diff --git a/tests/ref/lavf/rm b/tests/ref/lavf/rm
index bb0b1db..f07a4c8 100644
--- a/tests/ref/lavf/rm
+++ b/tests/ref/lavf/rm
@@ -1,2 +1,2 @@
-7b7ede9548a09346675edad36acfbf19 *./tests/data/lavf/lavf.rm
+4dff0436d250a01e01b08f557eada2d2 *./tests/data/lavf/lavf.rm
346706 ./tests/data/lavf/lavf.rm
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