GOP vs. Frame

mfa

New member
For a newbie, would you please explain the difference between "GOP accurate" editing and "Frame accurate" editing and when you would use one over the other? [Or point me to a reference that does.]
 

Danr

Administrator
Staff member
For a newbie, would you please explain the difference between "GOP accurate" editing and "Frame accurate" editing and when you would use one over the other?
I was looking for a link to an MPEG2 overview, but can't seem to find one quickly, so I'll try to give you a brief and way overly simplified explination.

GOP stands for "Group of Pictures". When your video is encoded and compressed into an MPEG format, each frame becomes part of a group of pictures. The first frame of that group is called an I-Frame (also called keyframe by some) and while its compressed, the original material can be reconstructed from the data in the frame. The rest of the frames in the GOP are difference frames. They are coded with the difference between the I-Frame picture and the current picture. That way only the material that changes from frame to frame needs to be re-encoded. You cannot reconstruct the frames in the GOP without first processing the I-Frame and the previous difference frames.

A GOP is usually 15 frames, but that's just a convention its can be any number of frames (DVD limits GOPs to 18 frames). In the US, which broadcasts at 30 FPS (29.97 actually) a GOP is 0.5 seconds of video.

When you use VideoReDo in GOP mode, all cut points will be rounded to the next GOP boundary. In Frame Accurate (FA) mode, the cut point will be on the exact frame you specify.

When to use GOP or FA mode?
For most people, FA mode is best. Its more accurate, therefore your cuts will appear exactly where you want them. However, to create frame accurate cuts without re-encoding your entire file, VideoReDo has to create shortened GOPs at the cut points. Some programs might not like the shortened GOPs even though they are MPEG2 compliant. If you come across such a program, then switch to GOP mode.

Hope this helps. If I come across some good MPEG overview links, I'll post them on this site.
 
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