DM 800 + mkv ....

Use ffmpeg ([Only registered and activated users can see links. Click Here To Register...]) to create .mkv files which are directly viewable in DM800 and/or on PC (at least in GNU/Linux-based operating systems):

ffmpeg -deinterlace -y -i "<input_file_name>" -f matroska -acodec ac3 -ac 2 -ar 48000 -ab 128k -vcodec libx264 -level 41 -r 25 -aspect 4:3 -coder 1 -g 250 -b 1024k -bt 1024k -bf 16 "<output_file_name.mkv>"

Of course, you can other ffmpeg options, such as -threads 2, -flags +loop, etc. to improve the quality of MPEG-4 (H.264) video.

For MAC OS X, I think there is ffmpegX ([Only registered and activated users can see links. Click Here To Register...]), you can try that.

[Only registered and activated users can see links. Click Here To Register...]