The Moving Picture Experts Group

Which layers are passed, before MPEG-4 objects are composed?

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There are three layers in an audio-visual terminal that should be mentioned: delivery layer, sync layer and compression layer.

  • The delivery layer encapsulates all the transport and multiplex functionality. It is mostly defined outside the MPEG-4 Systems standard. Some adaptation to (arbitrary) transport layers are defined in the scope of the Delivery Multimedia Integration Format (DMIF), which is published as part 6 to the MPEG-4 standard. The interface to the delivery layer is called DMIF application interface (DAI). MPEG-4 Systems just specifies an optional multiplex tool, called FlexMux, that may be used in the adaptation to an existing or future transport layer. This structure provides significant flexibility in deploying MPEG-4 in a variety of communications environments.
  • The sync layer adapts elementary stream data for communication across the DMIF application interface, providing timing and synchronization information, as well as fragmentation and random access information. The sync layer on the receiver side extracts this timing information to enable synchronized decoding and, subsequently, composition of the elementary stream data. Its syntax is configurable and can also be empty.
  • The compression layer recovers data from its encoded format (elementary stream) and performs the necessary operations to reconstruct the original information. It incorporates the audio-visual object decoders. The decoded information is then used by the terminal's composition, rendering and presentation subsystems.