+ AVCodecContext* _audio_codec_context; ///< may be 0 if there is no audio
+ AVCodec* _audio_codec; ///< may be 0 if there is no audio
+ AVCodecContext* _subtitle_codec_context; ///< may be 0 if there is no subtitle
+ AVCodec* _subtitle_codec; ///< may be 0 if there is no subtitle