I've just recently had a file format error with a few of the .mkv files that I've tried to open. It first happened while using video converter 6.3, so I upgraded to 7.0 thinking that would fix the problem. No luck. Only the ones made with mkvmerge v4.3.0 (Escape from the Island) and later. My older .mkv files still work fine in the video converter and the newer files play fine on the computer. Has anyone else had this problem? I'm somewhat suprised, AVS video converter has even opened files not listed in the open file box in the past. I've tried a freeware program (Free Video to iPod Converter 4.2.11) and it can open and convert the newer .mkv files. But , the program is very limited and slow. Do I need to download a codec from somewhere?