The two main reasons why it's not advisable to edit the video from a DVD:
1) your video quality will significantly diminish, because MPEG-2 video is a compressed format
2) extracting and converting video from a DVD is not an easy process.
If you look at the actual files that are on a DVD in your computer, you will see a folder called "VIDEO_TS", which contains files that have the extensions VOB, IFO, and BUP. The video on a DVD resides in those "VOB" files. A VOB is simply a container file that holds MPEG-2 video.
MPEG-2 is the format that is used on every DVD disc. MPEG-2 is a 720 pixel wide by 480 pixel high video that is heavily compressed, like online videos. When a video is compressed, quality is sacrificed for the sake of a smaller file.
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Frame structure of a DV file.
All frames are complete (i frames).
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Frame structure of an MPEG-2 file.
Complete i frames are interspersed with
incomplete b and p frames.
For example, a digital video (DV) video file contains complete data for every single frame of video. This is why DV files are 13 GB for one hour of footage..
But when a DV file is encoded to MPEG-2, the picture data for some of the frames is altered. An MPEG-2 file only contains full data for a few frames (i frames), while the others are saved with partial information (b and p frames). This encoding makes it possible to fit a 2-hour video on a DVD disc. The "bp" sequence occurs 7 times before another compelete "i" frame occurs (see right).
Therefore, this is the first reason you shouldn't edit from MPEG-2 files: Not all the picture data for every frame exists, due to the compression inherent to the MPEG-2 file format..
Furthermore, in order to access the MPEG-2 video on your DVD, you would need to extract the video from the VOB files. While you can technically just copy and edit a VOB (with an MPEG-friendly program), this is not the best thing because VOBs are not just video files, but container files that carry other data as well.
Once you managed to extract the MPEG-2 video to computer, you would need to convert it to AVI or MOV, since few video editing programs can interpret MPEG-2 files. Since you are dealing with compressed video, this results in a DV AVI or MOV that can be poor quality. And if you edit it and recompress it for use on a new DVD, the quality diminishes even further.