MPEG Editors

Why MPEG Editing is Hard

Before you buy an MPEG editor, you must realize that MPEG was not designed to be edited. MPEG is a high-powered compression algorithm made for pre-edited video that is simply played back as-is and never changed.

The main thing that makes it difficult to edit MPEG video is that an MPEG encoder only puts out one full "reference" frame for every 14 or so "difference" frames. These difference frames only encode what changed since the reference frame. This is where MPEG's high compression comes from: difference frames are much smaller than reference frames. (This is an oversimplified explanation, but it suffices.)

Since difference frames depend on a previous reference frame in order to be decoded, the only place in an MPEG stream that you can do simple cuts is right before a reference frame. If you were to cut the video before a difference frame, that difference frame and all subsequent difference frames would refer to a preceding reference frame that doesn't exist any more. That stream would then fail to decode properly.

Types of MPEG Editors

The simplest type of MPEG editor doesn't try to work around this problem. So-called "GOP-accurate editors" only do cuts on a "group of pictures" boundary. A GOP is a self-contained set of frames that starts with a reference frame. The problem with GOP-accurate editors is that GOPs are typically 12-18 frames long, meaning that you may be forced to make a cut up to 11-17 frames before or after the most optimal cut point; that's means your clips can be about half a second too long or too short. This is adequate for cutting commercials out of a television program you recorded, but not accurate enough for serious video editing.

The more powerful MPEG editor type is one that can re-encode the stream as necessary. This allows the editor to do frame-accurate cuts, because it can re-encode split GOPs. This type of MPEG editor can also offer tools that change the frames themselves (e.g., titling and transitions) because it can re-encode all the GOPs that contain changed frames. You may wish to read my re-DCTing article to learn what you can expect from an editor that re-encodes MPEGs.

Where to Get MPEG Editors

There are two main types of MPEG editors these days: standalone editors, and traditional video editing software (NLEs).

The standalone software is mostly a throwback to the days when few NLE packages would read MPEG files, but they still have some value. Being dedicated programs, they may sometimes work better than generic NLE software. For example, Apple's highly-regarded Final Cut Pro will let you edit MPEG files, but it will re-encode the entire stream. A dedicated MPEG editor will either give you GOP-level cuts or GOP re-encoding, so the entire stream isn't re-encoded. In the past I've used Womble's MPEG2VCR, TMPGEnc's editing features, and Vitec Multimedia's MPEG editors. It's been a long time since I used any of these, though, so I've removed the reviews of these programs that used to be here.

Most NLE software these days offers some form of MPEG editing. You will need to investigate it to see how capable it is. As with the anecdote about Final Cut, you may find that it will edit the MPEG stream, but there will be some serious limitation that might send you looking for a dedicated MPEG editor.

Conclusion

The "high road" of MPEG editing is to not do it at all. When making MPEG files from AVIs, always keep the AVI until you're sure you are happy with the final MPEG. Editing the MPEG is much harder than editing the AVI, and you won't be able to use all the video editing tools you're accustomed to. If you absolutely must edit an MPEG file, don't expect miracles. Always remember that by editing MPEGs, you're swimming upstream.