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Functional Description
D15343-003 137
6.4.8 Video Functionality
The GMCH supports MPEG-2 decoding hardware, sub-picture support and DTV.
6.4.8.1 MPEG-2 Decoding
The GMCH MPEG2 Decoding supports Hardware Motion Compensation (HWMC). The GMCH
can accelerate video decoding for the following video coding standards:
• MPEG-2 support
• MPEG-1: Full feature support
• H.263 support
• MPEG-4: Only supports some features in the simple profile
The HWMC interface supports Hardware Video Acceleration Compatible API’s (HVA).
6.4.8.2 Hardware Motion Compensation
The HWMC process consists of reconstructing a new picture by predicting (either forward,
backward, or bi-directional) the resulting pixel colors from one or more reference pictures. The
GMCH receives the video stream and implements Motion Compensation and subsequent steps in
hardware. Performing Motion Compensation in hardware reduces the processor demand of
software-based MPEG-2 decoding, and thus improves system performance.
6.4.8.3 Sub-picture Support
Sub-picture is used for two purposes: Subtitles for movie captions, which are superimposed on a
main picture, and for menus to provide some visual operation environments for the user.
DVD allows movie subtitles to be recorded as sub-pictures. On a DVD disc, it is called subtitle
because it has been prepared for storing captions. Since the disc can have a maximum of 32 tracks
for subtitles, they can be used for various applications, for example, as Subtitles in different
languages.
There are two kinds of menus, the System menus and other In-Title menus. First, the System
menus are displayed and operated at startup of or during the playback of the disc or from the stop
state. Second, In-Title menus can be programmed as a combination of Sub-picture and Highlight
commands to be displayed during playback of the disc.
The GMCH supports sub-picture for DVD by mixing the two video streams via alpha blending.
Unlike color keying, alpha blending provides a softer effect and each pixel that is displayed is a
composite between the two video stream pixels. The GMCH can utilize four methods when dealing
with sub-pictures. This flexibility means that the GMCH can work with all sub-picture formats.