+ /** mutex to protect _video, _audio and _closed_caption for when we are clearing them and they all need to be
+ cleared together without any data being inserted in the interim;
+ XXX: is this necessary now that all butler output data is timestamped? Perhaps the locked clear-out
+ is only required if we guarantee that get_video() and get_audio() calls are in sync.
+ */
+ boost::mutex _buffers_mutex;