MPEG-4

ISO/IEC 14496. MPEG-4 covers three areas, digital television, interactive graphics applications (synthetic content) and interactive multimedia (Web distribution and access to content). It provides the standardized technological elements enabling the integration of the production, distribution and content access of the three fields.

Since its first publication in 1999, MPEG-4 video compression achieved quality targets with ever-lower bit rates. Like MPEG-2 the compression is DCT-based and uses inter- and intra-field compression but implements many refinements, such as a choice of block sizes and motion compensation accuracy of one-eighth of a pixel against MPEG-2’s half pixel.

MPEG-4 is guilty of generating too many names and versions. The highest quality MPEG compression technology is known by ISO and IEC as MPEG-4 AVC (Advanced Video Coding). It is also know by the ITU-T as H.264 or MPEG-4 part 10. Notable predecessors are MPEG-4 part 2 (ASP) and H.263. Significantly, MPEG-4 AVC achieves up to a 64 percent bit rate reduction over MPEG-2 for the same quality and it opened possibilities for HD DVDs and transmission, etc., as well as room to offer more SD DTV channels, or more quality. MPEG-4 also specifies low bit rates (5-64 kb/s) for mobile and Internet applications with frame rates up to 15 Hz, and images up to 352 x 288 pixels.

MPEG-4 AVC video coding and decoding are far more complex than MPEG-2 but Moore’s Law absorbed that technical challenge. QuickTime and RealPlayer were among early adopters of MPEG-4. While established systems need to stick to their MPEG-2, most if not all later video services use MPEG-4.

The interactive multimedia side of MPEG-4 includes storage, access and communication as well as viewer interaction and 3D broadcasting. Aural and visual objects (AVOs) represent the content which may be natural – from cameras or microphones, or synthetic – generated by computers. Their composition is described by the Binary Format for Scene description (BIFS) – scene construction information to form composite audiovisual scenes from the AVOs. Hence, a weather forecast could require relatively little data – a fixed background image with a number of cloud, sun, etc, symbols appearing and moving, audio objects to describe the action and a video ‘talking head’ all composed and choreographed as defined by the BIFS. Viewer interactivity is provided by the selection and movement of objects or the overall point of view – both visually and aurally.

Audio: This builds on previous MPEG standards and includes High Efficiency Advanced Audio Coding (HE-AAC). This nearly doubled the efficiency of MPEG-4 Audio, improving on the original AAC and offers better quality for the same bit rate as the ubiquitous MP3 codec (from MPEG-2). Stereo CD-quality at 48 kb/s and excellent quality at 32 kb/s is reported. This is not a replacement for AAC, but rather a superset which extends the reach of high-quality MPEG-4 audio to much lower bit rates. High Efficiency AAC decoders will decode both types of AAC for backward compatibility.

DVB has approved two MPEG-4 codecs for use for broadcast transport streams: H.264/AVC video codec (MPEG-4 Part 10) and the High Efficiency Advanced Audio Coding (HE-AAC) audio codec. This mandates support of Main Profile for H.264/AVC SDTV receivers, with an option for the use of High Profile. The support of High Profile is mandated for H.264/AVC HDTV receivers.

Websites:
www.chiariglione.org/mpeg
www.m4if.org