A codec defines how the data is compressed, whereas a wrapper defines how the compressed data is packaged, as the name implies. It's important to distinguish between codecs and wrappers. QuickTime, AVI, Windows Media, and Real are all wrapper formats. Wrappers can (and often do) contain multiple streams or tracks of information: a video track, two or more audio tracks, and a timecode track are common. Wrapper formats can carry information about audio sample rates, timecode, and similar information that makes it much easier for FCP to read the file correctly and keep its media synchronized. Wrapper formats add metadatadata about datato the raw video stream, making it easier for programs to manipulate video and audio information. Most programs, though, want to see a more structured file format, and that's where "wrappers" come in. It's possible to record a raw stream of compressed video data to disk, and iMovie does just that: it takes DV data coming across FireWire and stores it in DV stream files.
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