I was experimenting with transcoding video with ffmpeg. using the command outputs the .avi into .mkv, but the output quality is very, very bad, while the original is dvd-quality.

I didn't see any options that would let me change this. What am I doing wrong?

ffmpeg has extremely low default bitrate settings (only 200k for video and 64k for audio, although it might depend on the codec used). Try adding something like -b 2000k and -ab 192k to boost these to more reasonable levels.

An always pay attention to the output a program is giving you. You'll often find clues as to what's going on. In this case, ffmpeg will clearly display the characteristics for both input file and the output target, so it should be easy to see the changes being made.

It generally also helps to post such details here when asking questions.