“I’ve been working on a video for my band in Final Cut Pro (bought from Sweetwater), and now I’m ready to export it for DVD and video tape reproduction. I applied some chapter markers so I can reference them in iDVD, but there are also special markers available called Compression Markers. What are these for?”
First, if you are using iDVD for DVD production the chapter markers you are creating are going to be useless. The current version of iDVD doesn’t recognize them. More professional solutions, such as DVD Studio Pro will allow you to take advantage of them. Now, on to your question…
MPEG video compression looks at groups of video frames as a related unit for the sake of determining how to best compress them. Within each group certain frames are designated differently, depending (usually) upon where they lie in the group. There are all kinds of tricky things going on here and there are many subtle distinctions that are beyond the scope of this tip. Suffice to say that one of the methods MPEG uses to compress video is to measure what changes from frame to frame. If a certain part of an image is static across multiple frames M-PEG can figure out that it doesn’t need to recreate that data over and over again, which results in a huge reduction in the overall quantity of data that needs to be stored or transmitted.
This compression can cause problems, however, when the compression algorithm makes a mistake. Let’s say a dramatic scene change happens to fall somewhere in the middle of a GOP (Group of Pictures). The codec may not completely get it right on the first frame of the change, which can result in some visible pixelation of the image for a frame or two while it resolves the changes. Adding a compression marker in FCP will trigger a new GOP where the marker is, which generally solves this problem. In practice I’ve rarely found a need for compression markers, and I suspect many other users have not either because there is not very much information about their use readily available.