Apple Final Cut Express HD - Manual (Page 209)

Chapter 16 Working With HDV 209
IV
Temporal (Interframe) Compression
Instead of storing complete frames, temporal compression stores only what has
changed from one frame to the next, which dramatically reduces the amount of data
that needs to be stored while still achieving high-quality images. Video is stored in
three types of frames: a standalone I-frame that contains a complete image, and then
predictive P-frames and B-frames that store subsequent changes in the image. Every
half second or so, a new I-frame is introduced to provide a complete image on which
the P- and B-frames are based. Together, a group of I-, P-, and B-frames is called a group
of pictures, or GOP. HDV uses a long-GOP pattern, which means that there are several
P- or B- frames for each I-frame.
For example, suppose you record some typical “talking head” footage, such as an
interview in which a seated person moves very little throughout the shot. Most of the
person’s body stays still, so most of the visual information is stored in an I-frame; the
subsequent P- and B-frames store only the changes from one frame to the next.
Because P- and B-frames depend on other frames to create a meaningful image, your
computer spends more processing power decoding HDV frames for display than it does
when displaying intraframe-only formats such as DV, uncompressed video, or the Apple
Intermediate Codec.
More About Long-GOP Video
The term “long” refers to the fact that P- and B-frames are used between I-frame
intervals. At the other end of the spectrum, the opposite of “long-GOP MPEG-2” is
I-frame–only MPEG-2, in which only I-frames are used. Formats such as IMX use
I-frame–only MPEG-2, which reduces temporal artifacts and improves editing
performance. However, I-frame–only formats have a significantly higher data rate
because each frame must store enough data to be completely self-contained.
Therefore, while the decoding demands on your computer are decreased, there is a
greater demand for scratch disk speed and capacity.
Editing HDV Using Apple Intermediate Codec
Instead of working with native MPEG-2 HDV video, you can transcode your HDV video
to the Apple Intermediate Codec during capture. The Apple Intermediate Codec is a
high-quality video codec optimized for playback performance and quality. Although
the data rate of the Apple Intermediate Codec is three to four times higher than the
data rate of the native MPEG-2 HDV, the processing requirements to play back your
video are less. Unlike MPEG-2 HDV, the Apple Intermediate Codec does not use
temporal compression, so every frame can be decoded and displayed immediately,
without first decoding other frames.