Re: Export MPEG-2 without recompression

There is no software that can take long-GOP MPEG and do any editing to it without recompression.

These clips are encoded originally as "Groups of Pictures" that are a given number of frames on length...12, 15, 20, etc. The first frame (the 'I' frame) is the only complete frame in the entire group. The rest of the images are derived from the I frame.

When you do even the most rudimentary cuts, you are shifting these groups and cutting pieces of them off.

For an easy example, let's say you lay 2 seconds of 29.97 HDV onto an edit timeline. There would be four 15 frame groups in this clip. For you master edit, you decide you need 10 frames of black, then a 7 frame fade up...simple stuff. Now you want to export back to MPEG2...

However, now the first frame of the first GOP is not the first frame of your video...it's a black frame, and it's 10 frames earlier than you original first GOP I frame...which makes the original first I frame of the clip, frame 10 of the first 15 frame GOP...not to mention that every GOP thereafter is shifted and needs to be completely rebuilt taking frames that were not I frames and using them as I frames in the new GOPs that create the new exported clip.

...keep in mind I didn't take out any frames or add any transitions. All I did was add ten frames of black and a fade to the beginning and recompression now has to take place.

So, the answer is, no...there is no software than can actually create edits in an MPEG stream and then re-export it to the same format without recompressing it. This is why intermediate codecs like CineForm and ProRes are popular with editors.

You can do it with DV and other intra-frame compressed formats because each frame is complete and separate and cutting the clips won't change each frame's structure, although even in these cases, any frames that have some effect or color correction applied, or are involved in a transition are still re-compressed.




TimK,
Director, Consultant
Kolb Productions,