Apple Final Cut Express 4 - Manual (Page 235)

Chapter 17 Working with HDV 235
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 the Apple Intermediate Codec
Your HDV video is transcoded by 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 native 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.