Apple Final Cut Pro 7 - Manual (Page 1909)

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).
1909 Appendix B Video Formats