Why do I need compression on my video?

Video compression is used by nearly every computer-based editing system. Many systems can edit uncompressed digital video. But because of storage space requirements and equipment expenses, the vast majority of desktop editing systems use compression.

Compression also allows video to find its way into many lower-bandwidth applications. For example, Web-based video and CD-ROM video would be impossible without sophisticated forms of video compression that dramatically reduce file sizes while maintaining acceptable levels of quality.

The enabling technology of video compression is the codec. Codecs are extensions used by QuickTime to create and play back different formats of digital video. A codec mathematically describes how a video signal can be squeezed down into less space.

Uncompressed video can be as much as 32 MB per second (including an alpha channel, which is not normally used for regular video playback). However, video compressed with the M-JPEG codec, at a ratio of approximately 2:1 (considered to be exceptionally high quality), is as little as 10 MB per second, depending on the capture card being used. Offline quality M-JPEG video can be as little as 1 MB per second and Web video, compressed with the Sorenson codec, is often 300K per second or smaller.

Note: The expression of megabytes (MB) or kilobytes (K) per second or per frame is referred to as the video clip’s data rate. Knowing the data rate and codec of a QuickTime movie gives you some idea of how much it has been compressed.

Codecs are either lossless or lossy. Lossless codecs reduce the file size of a video clip without affecting the image quality. Lossy codecs reduce the file size of a video clip by discarding actual bits of information from the video clip, reducing image quality by an extent that depends on the data rate of the clip and the codec used.

Codecs use different mathematical models of video compression. Different codecs may have different perceived visual quality at the same data rate. The two primary ways to compress video are using spatial compression and temporal compression. Most codecs use variations of both of these compression methods in combination.

Different codecs require different amounts of processor power to decompress a digital video file into viewable form in real time. Some codecs use your video capture and playback hardware to help with this decompression. Other codecs are optimized to play back in real time using only software and your computer’s built-in processor.

QuickTime comes with a variety of codecs useful for different purposes. By purchasing or downloading additional codecs, you can expand QuickTime’s ability to encode and decode video clips using different kinds of video compression to new formats. In Final Cut Pro, QuickTime capabilities are available within the Export to QuickTime compression settings.

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