private void writeToPlaybackBuffer(ByteBuffer audioData, long presentationTimeUs) {
        int size = audioData.remaining();
        if(mTransferBuffer == null || mTransferBuffer.length < size) {
            mTransferBuffer = new byte[size];
        }
        audioData.get(mTransferBuffer, 0, size);

        //Log.d(TAG, "audio write / chunk count " + mPlaybackBufferChunkCount);
        mLastPresentationTimeUs = presentationTimeUs;
        mAudioTrack.write(mTransferBuffer, 0, size);
    }
Returns the current PTS of the playback head or PTS_NOT_SET if the PTS cannot be reliably
calculated yet.
For this method to return a PTS, audio samples need to be written before ({@link #write(ByteBuffer, long)}.
@return the PTS at the playback head or PTS_NOT_SET if unknown
MediaPlayer/src/main/java/net/protyposis/android/mediaplayer/AudioPlayback.java