From Hydrogenaudio Knowledgebase
MPEG-4, introduced in 1998, is the designation for a group of audio and video coding standards agreed upon by the MPEG (Moving Picture Experts Group). MPEG-4 is primarily designed to handle low bit rate content, from 4800 bit/s to approximately 4 Mbit/s. The primary uses for the MPEG-4 standard are web (streaming media) and Compact disc distribution, conversational (videophone) uses, and broadcast television.
MPEG-4 absorbs many of the features of MPEG-1 and MPEG-2, adding new features such as (extended) VRML support for 3D rendering, object-oriented composite files (including audio, video and VRML objects), support for Digital Rights Management and various types of interactivity.
Most of the features included in MPEG-4 are left to individual developers to implement. This means that there are very few complete implementations of the MPEG-4 standard. Anticipating this, the developers added the concept of "Profiles," allowing various capabilities to be grouped together.
MPEG-4 consists of several standards — termed "Layers" — as follows.
- Layer 1 describes synchronization and multiplexing of video and audio.
- Layer 2 is a compression codec for video signals.
- Layer 3 is a compression codec for perceptual coding of audio signals.
- Layer 4 describes procedures for testing compliance.
- Layer 5 describes systems for Software simulation.
- Layer 6 describes Delivery Multimedia Integration Framework (DMIF).
- Layer 10 is an advanced codec for video signals also called H.264.
The designated container for MPEG-4 content is MP4, which was also defined within MPEG-4.
MPEG-7 and MPEG-21 serve other purposes.