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.

libavcodec/ac3enc.c

@@ -329,6 +329,36 @@ 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 */

tests/ref/acodec/ac3_fixed

-408f47ee5a60866fc751f7bc2314cbd6 *./tests/data/acodec/ac3.rm
+5ddb6d25dd117db29627f9d286153a7a *./tests/data/acodec/ac3.rm
 98751 ./tests/data/acodec/ac3.rm

tests/ref/lavf/rm

-da74fe749c2eb21bbead7de81d2f3078 *./tests/data/lavf/lavf.rm
+a1c71456f21d5459d2824d75bbdcc80c *./tests/data/lavf/lavf.rm
 346706 ./tests/data/lavf/lavf.rm

tests/ref/seek/ac3_rm

@@ -5,7 +5,8 @@ ret:-1         st:-1 flags:1  ts: 1.894167
 ret:-1         st: 0 flags:0  ts: 0.788000
 ret: 0         st: 0 flags:1  ts:-0.317000
 ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:    271 size:   556
-ret:-1         st:-1 flags:0  ts: 2.576668
+ret: 0         st:-1 flags:0  ts: 2.576668
+ret: 0         st: 0 flags:1 dts: 2.577000 pts: 2.577000 pos:  42397 size:   558
 ret:-1         st:-1 flags:1  ts: 1.470835
 ret:-1         st: 0 flags:0  ts: 0.365000
 ret: 0         st: 0 flags:1  ts:-0.741000
@@ -19,7 +20,8 @@ ret:-1         st:-1 flags:0  ts: 1.730004
 ret:-1         st:-1 flags:1  ts: 0.624171
 ret: 0         st: 0 flags:0  ts:-0.482000
 ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:    271 size:   556
-ret:-1         st: 0 flags:1  ts: 2.413000
+ret: 0         st: 0 flags:1  ts: 2.413000
+ret: 0         st: 0 flags:1 dts: 2.368000 pts: 2.368000 pos:  38981 size:   558
 ret:-1         st:-1 flags:0  ts: 1.306672
 ret:-1         st:-1 flags:1  ts: 0.200839
 ret: 0         st: 0 flags:0  ts:-0.905000
@@ -28,7 +30,8 @@ ret:-1         st: 0 flags:1  ts: 1.989000
 ret:-1         st:-1 flags:0  ts: 0.883340
 ret: 0         st:-1 flags:1  ts:-0.222493
 ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:    271 size:   556
-ret:-1         st: 0 flags:0  ts: 2.672000
+ret: 0         st: 0 flags:0  ts: 2.672000
+ret: 0         st: 0 flags:1 dts: 2.821000 pts: 2.821000 pos:  46383 size:   556
 ret:-1         st: 0 flags:1  ts: 1.566000
 ret:-1         st:-1 flags:0  ts: 0.460008
 ret: 0         st:-1 flags:1  ts:-0.645825