ac3enc: select bandwidth based on bit rate, sample rate, and number of

full-bandwidth channels.

This reduces high-frequency artifacts and improves the quality of the lower
frequency audio at low bit rates.

1 files changed, 31 insertions, 3 deletions

diff --git a/libavcodec/ac3enc.c b/libavcodec/ac3enc.c
index 372f019f64..e0806b3081 100644
--- a/libavcodec/ac3enc.c
+++ b/libavcodec/ac3enc.c
@@ -330,6 +330,36 @@ 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.
  */
@@ -2095,9 +2125,7 @@ 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 */