Video Editing

If you wish to edit MPEG files, it is important to understand the basics of MPEG compression and the main differences between “frame editing” and “Group of Picture (GOP) editing.”

Let’s first start by stating that MPEG compression was not developed with the intention of supporting precise editing. Instead, MPEG serves as a high-powered compression tool made for pre-edited video. Nevertheless, with some fundamental knowledge and appropriate tools, it is possible to perform highly accurate edits of MPEG files.

Fundamentals of MPEG Compression

The main characteristic that complicates editing of MPEG video is the way in which MPEG encodes a compressed file. MPEG uses what’s referred to as “reference frames” and “difference frames.” For every 15 frames of movie footage, the normal standard of compression is 1 reference frame for every 14 difference frames. The difference frames only encode changes such as foreground movement, and therefore much less storage space than the preceding reference frames. This is where MPEG's high compression comes from.

Since each set of difference frames depends on a previous reference frame in order to be decoded, the only place in a MPEG stream that you can do cuts is at the reference frame. If you were to cut the video at a difference frame, that difference frame and all subsequent difference frames would refer to a preceding reference frame that doesn't exist any more. Such a stream of frames would then fail to decode properly at run-time, producing nonsensical movie playback. In everyday terms, the movie playback would be extremely choppy. This is the inherent weakness in editing MPEG.

Group of Picture (GOP) Editing

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. GOP is adequate for cutting commercials out of a recorded television program, for example, but not accurate enough for serious video editing.

Pixela’s Pixe VRF Browser falls within this category of editors; however, it boasts additional capabilities that competing products in the category do not:

Pixela's Pixe VRF Browser supports viewing and editing of popular DVD formats not supported by Mac. This makes viewing any movie recorded in the Video Recording Standard readable.

Furthermore, while most editing software only supports GOP (Group of Picture) unit editing, VRF Browser also supports highly precise frame editing with the use of frame markers.

Frame Editing

To create precise cuts and smooth transitions, it is necessary to move beyond GOP editing to frame-by-frame granularity. In the case of MPEG data streams, this requires enhancing the editing software with the power to decode an MPEG file’s GOPs into a sequence of uncompressed frames, which are then edited by the user as desired, then re-encoded into GOPs so that the advantages of MPEG compression are regained.

The ability to re-encode split GOPs allows you to do frame-accurate cuts. 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.

Pixela’s Capty MPEG Edit EX resides in this more powerful category of editor.

"With the advanced editing features of Capty MPEG Edit EX software, you can create professional quality DVDs at an affordable price," say Richard Yates of Pixela Corporation. "High performance frame by frame video and sound track editing features, with Dolby Digital support, enable you to tell your story the way you want it, without compromise."

Richard Yates is the Marketing and E-Commerce Manager for Pixela Corporation.