SAF, the aggregation of LASeR and audiovisual material

SAF (Simple Aggregation Format) is part of the LASeR standard defining tools to fulfill all the requirements of rich-media service design at the interface between scene representation and transport mechanisms. SAF features the following functionality:
- simple aggregation of any type of media streams (MPEG or non-MPEG streams), resulting in a SAF stream with a low overhead multiplexing schema for low bandwidth networks,
- and possibility to cache SAF streams.

The result of the multiplexing of media streams is a SAF stream which can be delivered over any delivery mechanism: download-and-play, progressive download, streaming or broadcasting.
The purpose of the LASeR Systems decoder model is to provide an abstract view of the behaviour of the terminal. It may be used by the sender to predict how the receiving terminal will behave in terms of buffer management and synchronization when decoding data received in the form of elementary streams. The LASeR systems decoder model includes a timing model and a buffer model. The LASeR systems decoder model specifies:
- the conceptual interface for accessing data streams (Delivery Layer),
- decoding buffers for coded data for each elementary stream,
- the behavior of elementary stream decoders,
- composition memory for decoded data from each decoder, and
- the output behavior of composition memory towards the compositor.

Each elementary stream is attached to one single decoding buffer.
A multimedia presentation is a collection of a scene description and media (zero, one or more). A media is an individual audiovisual content of the following type: image (still picture), video (moving pictures), audio and by extension, font data. A scene description is constituted of text, graphics, animation, interactivity and spatial, audio and temporal layout. The sequence of a scene description and its timed modifications is called a scene description stream. A scene description stream is called a LASeR Stream.
Modifications to the scenes are called LASeR Commands. A command is used to act on elements or attributes of the scene at a given instant in time. LASeR Commands that need to be executed at the same time are grouped into one LASeR Access Unit (AU)

A scene description specifies four aspects of a presentation:
- how the scene elements (media or graphics) are organised spatially, e.g. the - spatial layout of the visual elements;
- how the scene elements (media or graphics) are organised temporally, i.e. if and how they are synchronised, when they start or end;
- how to interact with the elements in the scene (media or graphics), e.g. when a user clicks on an image;
- and if the scene is changing, how the scene changes happen.

Mattia Donna Bianco