Apple Final Cut Express HD - Manual (Page 1041)

Appendix A Video Formats 1041
XIII
Run-length encoding is lossless, because all the information is retained after decoding.
This technique is particularly useful for computer graphics applications, because there
are often large fields of identical colors.
Note: If each bit in the original image were to alternate between 0 and 1, run-length
encoding would not only be ineffective, it could actually make the overall data rate
higher! Each codec is designed to anticipate and compress different kinds of data
patterns. For example, a codec designed for audio compression is not useful for video
compression, which has very different data patterns.
Lossy Codecs
Most video codecs are necessarily lossy, because it is usually impractical to store and
transmit uncompressed video signals. Even though most codecs lose some information
in the video signal, the goal is to make this information loss visually imperceptible.
When codec algorithms are developed, they are fine-tuned based on analyses of
human vision and perception. For example, if the human eye cannot differentiate
between lots of subtle variation in the red channel, a codec may throw away some of
that information and viewers may never notice.
Many formats, including JPEG and all varieties of DV, use a fairly complicated
algorithm called DCT encoding. Another method, called wavelet compression, is
starting to be used for popular codecs, such as the Apple Pixlet Video codec. DVDs,
modern digital television, and formats such as HDV use MPEG-2 compression, which
not only encodes single frames (intraframe, or spatial compression), but encodes
multiple frames at once (interframe, or temporal compression) by throwing away
data that is visually redundant over time.
Uncompressed Video
Video that has no compression applied can be unwieldy, so it is only used for the
highest quality video work, such as special effects and color correction at the last
stage of a project. Most professional projects have an offline phase that uses
compressed video and then an online, finishing phase that uses uncompressed video
recaptured at full resolution. Uncompressed video requires expensive VTRs and large,
high-speed hard disks.