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Tunnel Mode Playback (Applicable to 4K @ 60 FPS Fire TV Devices Only)

The hardware decoder for some Fire TV devices support playback of 4K @ 60 FPS. To play a video at such high resolution and frame rate, the timing requirement of media pipeline is very aggressive and the app may not be able to render 4K frames at 16 msec interval due to thread and process scheduling limitations of the kernel. This may cause frame drops and a sub-par movie experience. To get the best out of the hardware, use Tunnel Mode playback.

How to Enable Tunnel Mode Playback

The main changes required to enable Tunnel Mode playback are as follows:

  1. Query the AudioTrack session ID from AudioManager:

    AudioManager audioManager = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);
    int audioSessionId = audioManager.generateAudioSessionId();
    
  2. Configure Video Decoder for Tunnel mode:

    format.setFeatureEnabled(MediaCodecInfo.CodecCapabilities.FEATURE_TunneledPlayback, true);
    
  3. Configure Video decoder with the AudioTrack session ID.

    format.setInteger(android.media.MediaFormat.KEY_AUDIO_SESSION_ID, audioSessionId);
    
  4. Create AudioTrack with the above queried session ID and set the AudioAttributes flag FLAG_HW_AV_SYNC.

    AudioAttributes attributes = new AudioAttributes.Builder()
      .setLegacyStreamType(AudioManager.STREAM_MUSIC)
      .setFlags(AudioAttributes.FLAG_HW_AV_SYNC)
      .build();
     AudioFormat format = new AudioFormat.Builder()
      .setEncoding(targetEncoding)
      .setSampleRate(sampleRate)
      .setChannelMask(channelConfig)
      .build();
     int mode = AudioTrack.MODE_STREAM;
     AudioTrack audioTrack = new AudioTrack(attributes, format, bufferSize, mode, audioSessionId);
    
  5. Do not call dequeueOutputBuffer and releaseOutputBuffer for video decoder.
  6. Insert an 16-byte AV Sync header to the audio data being written to AudioTrack.

    private ByteBuffer avSyncHeader = ByteBuffer.allocate(16);
    avSyncHeader.order(ByteOrder.BIG_ENDIAN);
    avSyncHeader.position(0);
    avSyncHeader.putInt(AVSYNC_START_CODE);
    avSyncHeader.putInt(bufferBytesRemaining);
    avSyncHeader.putLong(presentationTimeUs * 1000);
    

This header contains a start code, presentation time (in nano secs), and size of data. Write this data before you write the audio data to audio track.

More Details about Tunnel Mode Playback

For more details refer to the Tunnel Mode Playback implementation in Exoplayer 2: