Apple MA261Z/A User Manual - Page 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. 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. 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. 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. Chapter 16 Working With HDV 209