Apple Final Cut Pro 6 - Manual (Page 1876)

Appendix A Video Formats 393
V
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.
About 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.
About MPEG Compression
MPEG encoding is based on eliminating redundant video information, not only within
a frame but over a period of time. In a shot where there is little motion, such as an
interview, most of the video content does not change from frame to frame, and MPEG
encoding can compress the video by a huge ratio with little or no perceptible quality loss.
MPEG compression reduces video data rates in two ways:
 Spatial (intraframe) compression: Compresses individual frames.
 Temporal (interframe) compression: Compresses groups of frames together by
eliminating redundant visual data across multiple frames.
Intraframe Compression
Within a single frame, areas of similar color and texture can be coded with fewer bits
than the original, thus reducing the data rate with minimal loss in noticeable visual
quality. JPEG compression works in a similar way to compress still images. Intraframe
compression is used to create standalone video frames called I-frames (short
for intraframe).