MPEG-4 Systems Layer
A major advancement of MPEG-4 over MPEG-1 and MPEG-2 is its "systems" layers. A cursory look at MPEG-4 systems can yield a better understanding of the importance of MPEG-4 now and in the future. Many think that MPEG-4 is simply a better compression standard for both audio and video. While this is certainly true, MPEG-4 has many other features that exist within the standard that will be relevant to the video broadcast industry for years to come. In addition to audio and video compression, MPEG-4 adds the ability to code animations, 3D environments and images, provide for stream interactivity, provide digital rights management, and allow for content distribution on a variety of networks, including IP and MPEG-2 transport streams (DVB). Hundreds of companies have participated in the creation of the MPEG-4 standard and, in fact, MPEG-4 encompasses 20 International Standards Organization (ISO) specifications, as well as several industry organization standards such as 3GPP (http://www.3gpp.org) and the Internet Streaming Media Alliance or ISMA (http://www.isma.tv). For a visual representation of the MPEG-4 standard, see Figure 6.
According to the MPEG-4 working group (http://www.chiariglione.org/mpeg/) MPEG-4 "systems" were developed to "provide the necessary facilities for specifying how audiovisual objects can be composed together in an MPEG-4 terminal to form complete scenes, how a user can interact with the content, as well as how the streams should be managed for transmission or storage."
Also according to the working group, "Much of the functionality that MPEG-4 provides comes from the systems part. As systems take care of (among other issues) streams management and scene description, it acts as a 'wrapper' to the source coding technology."
Figure 6: MPEG-4 container capabilities
Part of the MPEG-4 systems layer is its file or "container" format. A MPEG-4 video elementary stream, such as produced by the Fujitsu MPEG-4 core, cannot stand alone. It must be placed inside of a "container" in order to be processed and decoded by a MPEG-4 decoder. This means that MPEG-4 video, prior to being transmitted from one mobile phone to another, must be placed inside of a container (referred to as a MP4 file). This MP4 file describes what is inside of the MP4 container and how the video and audio should be played by the receiver of the MP4 file. Without this framework, the receiver cannot identify the data it has received. A properly formatted MP4 file is critical to MPEG-4 interoperability between terminals. For more information on MPEG-4 systems see http://www.chiariglione.org/mpeg/faq/mp4-sys/mp4-sys.htm.
The Fujitsu MMPs produce a MPEG-4 video elementary stream (ES) also known as an .M4V. This video ES when coupled with a MPEG-4 audio ES, sometimes called an .M4A file, constitutes a "watchable" video known as an .MP4 file. So where does the "coupling" take place and who does it? The answer for Fujitsu MMPs is the Fujitsu MP4 software library. The Fujitsu MPEG-4 CODEC core coupled with the Fujitsu MP4 library allows for the creation of MPEG-4 compliant files that are interchangeable across virtually all MPEG-4 terminals. MPEG-4 video elementary streams that are not encapsulated inside of MP4 files, however, cannot be guaranteed to play on all MPEG-4 compliant terminals.
There are a variety of custom MPEG-4 implementations on the market today. DIVX, the popular Internet video compression technology consists of MPEG-4 video elementary streams encapsulated inside of an .AVI file format. Microsoft stores their MPEG-4 video elementary streams inside of a custom container format called Advanced Systems Format or ASF. These are clear examples of companies seeking to take advantage of the improved video compression that MPEG-4 offers, but without making a commitment to the entire MPEG-4 standard. The advantage to using the MPEG-4 file format is the ability to transmit seamlessly and interchangeably across MPEG-4 terminals be it mobile devices, desktop devices or simply MPEG-4 TV terminals regardless of make or model. Other container formats including Microsoft's Windows Media, cannot offer anywhere near the interchangeability of MPEG-4. (see http://all-streaming-media.com/streaming-media-faq/faq-mp4-streaming.htm).
About the author
Rick Tewell oversees software development strategy for Fujitsu Microelectronics' family of Graphic Processing Units. Previously, he founded Sequoia Advanced Technologies, Inc. which developed one of the first digital video implementations for the Windows and PC platforms using IEEE-1394 so users can connect digital camcorders to PCs for capturing and editing. In 2000, when Ligos Corporation acquired Sequoia, his team engineered a technology solution for converting video from digital camcorders directly to the MPEG-2 (DVD) format. He can be reached at attewell@fma.fujitsu.com.
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