User Guide

Chapter 1 Working With HDV 13
About MPEG-2 Compression
High definition video requires significantly more data than standard definition 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 MPEG-2 compression.
MPEG compression reduces the data rate by removing redundant visual information,
both on a per-frame basis and also across multiple frames.
Spatial (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 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 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.
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.