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Chapter 1 Working with HDV 13
About MPEG Compression
HD video requires significantly more data than SD video. A single HD video frame can
require up to six times more data than an SD frame. To record such large images with
such a low data rate, HDV uses long-GOP MPEG compression. MPEG compression
reduces the data rate by removing redundant visual information, both on a per-frame
basis and also across multiple frames.
Note: HDV specifically employs MPEG-2 compression, but the concepts of long-GOP
and I-frame-only compression discussed below apply to all versions of the MPEG
standard: MPEG-1, MPEG-2, and MPEG-4 (including AVC/H.264). For the purposes of this
general explanation, the term MPEG here may refer to any of these formats.
Spatial (Intraframe) Compression
Within a single frame, areas of similar color and texture can be coded with fewer bits
than the original frame, thus reducing the data rate with a 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).
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 bipredictive 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 subsequent 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 is at least one P- or B-frame for each I-frame.
Standard Definition Recording with an HDV Camcorder
In addition to recording HD video, most HDV camcorders can also record standard
definition DV video. You can capture, edit, and output this DV video just as you would
any other DV video.
Important: You should avoid recording DV and HDV video on the same tape. (You
should also avoid recording HDV footage using different frame sizes and frame rates
on the same tape.) This can cause problems during capture and playback.
An additional format defined within the HDV specification, known as SD, is available
on some JVC camcorders. Final Cut Pro does not support this format.