A DVD is a compressed format made for playback only whether on your computer or DVD player. You can “rip” the content if you are sufficiently knowledgeable about video format conversion and purchase software specific to that purpose.
When editing from a “ripped” DVD you may find that you have missing sound, missing data, or other anomalies. Worse, it may not even be playable. This is because you are trying to return to a format that has been compressed approximately six times back to its original format with a very low-quality result.
For the highest quality edit, Los Angeles Video Transfer Service recommends converting your DVD to either an .avi or .mp4 format.