units. The Synthetic profile groups the synthetic object types. The target applications are defined by the need for good quality sound at very low data rates. There are three levels that define the amount of memory for data, the sampling rates, the amount of TTSI objects, and some further processing restrictions. The Main profile includes all object types. It is useful in environments in which processing power is available to create rich, highest quality audio scenes that may combine organic sources with synthetic ones. Two applications are the DVD and multimedia broadcast. This profile has four levels that are defined in terms of complexity units. There are two different types of complexity units: processor complexity units (PCU), which are specified in millions of operations per second, and RAM complexity units (RCU), that are specified in terms of the number of kWords. The standard also specifies the complexity units required for each object type. This provides authors with maximum freedom in choosing the right object types and allocating resources among them. For example, a profile could contain main AAC and Wavetable synthesis object types, and a level could specify a maximum of two of each. This would prevent the resources reserved for the AAC objects to be used for a third and fourth Wavetable synthesis object even though it would not break the decoder. Complexity units enable the author freedom to use decoder resources for any combination of profile-supported object types. How MPEG-4 DIFFERS FROM MPEG-2 The difference between MPEG-4 and MPEG-2 is not improvement in video or audio reproduction for a single application (as MPEG-2 specifically supports DTV). MPEG- 4 represents the establishment of an adaptable set of tools for combining various types of video, audio, and interaction to provide environments beyond what anyone may now imagine. The essence of MPEG-4 is an object-based audiovisual representation model that allows authors to build scenes using individual objects MPEG Family of Specifications that