to capture from minidv use:
$ dvgrab --autosplit --format raw --size 0 --opendml capture-
to convert .dv file into .mkv (x264) with ffmpeg use:
$ ffmpeg -i input.dv -acodec libmp3lame -aq 4 -vcodec libx264 -preset normal -crf 24 -s 720x576 -aspect 4:3 -deinterlace -threads 0 output.mkv