Move AC-3 bitrate calculation into Ac3Util.

This is in preparation for removing bitrate from MediaFormat.

library/src/main/java/com/google/android/exoplayer/audio/AudioTrack.java

@@ -16,6 +16,7 @@
 package com.google.android.exoplayer.audio;
 
 import com.google.android.exoplayer.C;
+import com.google.android.exoplayer.util.Ac3Util;
 import com.google.android.exoplayer.util.Assertions;
 import com.google.android.exoplayer.util.Util;
 
@@ -433,11 +434,7 @@ public final class AudioTrack {
     int result = 0;
     if (temporaryBufferSize == 0) {
       if (isAc3 && ac3Bitrate == UNKNOWN_AC3_BITRATE) {
-        // Each AC-3 buffer contains 1536 frames of audio, so the AudioTrack playback position
+        ac3Bitrate = Ac3Util.getBitrate(size, sampleRate);
-        // advances by 1536 per buffer (32 ms at 48 kHz). Calculate the bitrate in kbit/s.
-        int unscaledAc3Bitrate = size * 8 * sampleRate;
-        int divisor = 1000 * 1536;
-        ac3Bitrate = (unscaledAc3Bitrate + divisor / 2) / divisor;
       }
 
       // This is the first time we've seen this {@code buffer}.

library/src/main/java/com/google/android/exoplayer/extractor/ts/Ac3Reader.java

@@ -46,6 +46,7 @@ import com.google.android.exoplayer.util.ParsableByteArray;
   private long frameDurationUs;
   private MediaFormat mediaFormat;
   private int sampleSize;
+  private int bitrate;
 
   // Used when reading the samples.
   private long timeUs;
@@ -149,14 +150,15 @@ import com.google.android.exoplayer.util.ParsableByteArray;
    * Parses the sample header.
    */
   private void parseHeader() {
+    headerScratchBits.setPosition(0);
+    sampleSize = Ac3Util.parseFrameSize(headerScratchBits);
     if (mediaFormat == null) {
       headerScratchBits.setPosition(0);
       mediaFormat = Ac3Util.parseFrameAc3Format(headerScratchBits);
       output.format(mediaFormat);
+      bitrate = Ac3Util.getBitrate(sampleSize, mediaFormat.sampleRate);
     }
-    headerScratchBits.setPosition(0);
+    frameDurationUs = (int) (1000L * 8 * sampleSize / bitrate);
-    sampleSize = Ac3Util.parseFrameSize(headerScratchBits);
-    frameDurationUs = (int) (1000000L * 8 * sampleSize / mediaFormat.bitrate);
   }
 
 }

library/src/main/java/com/google/android/exoplayer/util/Ac3Util.java

@@ -50,10 +50,8 @@ public final class Ac3Util {
     if ((nextByte & 0x04) != 0) {
       channelCount++;
     }
-    // Map bit_rate_code onto a bitrate in bit/s.
-    int bitrate = BITRATES[((nextByte & 0x03) << 3) + (data.readUnsignedByte() >> 5)] * 1000;
     return MediaFormat.createAudioFormat(MimeTypes.AUDIO_AC3, MediaFormat.NO_VALUE,
-        MediaFormat.NO_VALUE, channelCount, sampleRate, bitrate, Collections.<byte[]>emptyList());
+        MediaFormat.NO_VALUE, channelCount, sampleRate, Collections.<byte[]>emptyList());
   }
 
   /**
@@ -91,8 +89,7 @@ public final class Ac3Util {
     data.skipBits(4 * 8);
 
     int fscod = data.readBits(2);
-    int frmsizecod = data.readBits(6);
+    data.skipBits(14); // frmsizecod(6) + bsid (5 bits) + bsmod (3 bits)
-    data.skipBits(8); // bsid (5 bits) + bsmod (3 bits)
     int acmod = data.readBits(3);
     if ((acmod & 0x01) != 0 && acmod != 1) {
       data.skipBits(2); // cmixlev
@@ -106,7 +103,7 @@ public final class Ac3Util {
     boolean lfeon = data.readBit();
     return MediaFormat.createAudioFormat(MimeTypes.AUDIO_AC3, MediaFormat.NO_VALUE,
         MediaFormat.NO_VALUE, CHANNEL_COUNTS[acmod] + (lfeon ? 1 : 0), SAMPLE_RATES[fscod],
-        BITRATES[frmsizecod / 2] * 1000, Collections.<byte[]>emptyList());
+        Collections.<byte[]>emptyList());
   }
 
   /**
@@ -133,6 +130,21 @@ public final class Ac3Util {
     }
   }
 
+  /**
+   * Returns the bitrate of AC-3 audio given the size of a buffer and the sample rate.
+   *
+   * @param bufferSize Size in bytes of a full buffer of samples.
+   * @param sampleRate Sample rate in hz.
+   * @return Bitrate of the audio stream in kbit/s.
+   */
+  public static int getBitrate(int bufferSize, int sampleRate) {
+    // Each AC-3 buffer contains 1536 frames of audio, so the AudioTrack playback position
+    // advances by 1536 per buffer (32 ms at 48 kHz).
+    int unscaledBitrate = bufferSize * 8 * sampleRate;
+    int divisor = 1000 * 1536;
+    return (unscaledBitrate + divisor / 2) / divisor;
+  }
+
   private Ac3Util() {
     // Prevent instantiation.
   }