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.
structure of a DV file.
All frames are complete (i frames).
of an MPEG-2 file.
Complete i frames are interspersed with
incomplete b and p frames.
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
"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.