 postproduction in the Systems Audio BIFS tool set. Sound can be described at '0 Kbps' (i.e., sound continues without input until it is stopped by an explicit command) to 3-4 Kbps for extremely expressive sounds in the MPEG-4 Structured Audio format. The Wavetable Synthesis object type is a subset of the Main Synthetic object type that makes use of the SASBF format and the widely used MIDI (Musical Instrument Digital Interface) Wavetable format tools to provide simple sampling synthesis. The General MIDI object type gives interoperability with existing content (see above). Unlike the Main Synthetic or Wavetable Synthesis object types, it does not provide completely predictable (i.e., normative) sound quality and decoder behavior. 10. The Algorithmic Synthesis and AudioFX object type provides SAOL-based synthesis capabilities for very low-bitrate terminals (FX stands for effects). 11. The NULL object type provides the possibility to feed raw PCM data directly to the MPEG-4 audio compositor to allow local sound enhancement at the decoder. Support for this object type is in the compositor, not in the decoder. Video Communications: The Whole Picture MPEG-4 Profiles Although there are numerous object types in the audio area, there are only four distinct profiles (as defined below). Codec builders can claim conformance to profiles at a certain level, but cannot claim conformance to object types. Two levels are defined, determining whether either one or a maximum of 20 objects can be present in the (audio) scene. The Scalable profile was primarily defined to allow good quality, reasonable complexity, low bitrate audio on the Internet (an environment in which bitrate varies between users and over time). Scalability enables optimal use of limited and of varying bandwidth without encoding and storing the material multiple times. The scalable profile has four levels that restrict the amount of objects in the scene, the total amount of channels, and the sampling frequency. The highest level employs the concept of complexity 