Er, .mkvs can be ecnoded in any mpeg4 format (divx, xvid, 3ivx, so on). They are merely containers that can hold audio/vido/subtitles together. I've never seen an .mkv encoded with real in my life. There is no single codec but a filter is needed. .MKV is a relatively new format that allows for some pretty advanced features like variable bitrates, variable framerates, soft subs, chaptering, and dual audio capabilities. You'll need a good media player in order to play it (NOT WMP).