Why is scene description information separate from audio-visual objects?
Scene description information is a property of the scene’s structure rather than of particular AV objects. Consequently, it is transmitted as a separate stream. This is an important feature for bitstream editing and one of the essential content based functionalities in MPEG-4. For bitstream editing, one can change the composition of AV objects without having to decode their bitstreams and change their content. If the position of the object were part of the object’s bitstream, this would become very difficult.
