The MPEG-4 AAC LTP object type is similar to the AAC Main object type but replaces the MPEG-2 AAC predictor with the long-term predictor to provide the same efficiency at a lower implementation cost. The AAC Scalable object type allows a large number of scalable combinations such as combinations with TwinVQ and CELP coder tools as the core coders. It supports only mono or 2-channel stereo sound. 6. The TwinVQ (Transform domain Weighted Interleave Vector Quantization) object type is based on fixed rate vector quantization instead of AAC's Huffman coding. It operates at lower bitrates than AAC and supports both mono and stereo sound. MPEG-4 includes two different algorithms for coding speech; each operates at different bitrates, plus a Text-to-Speech Interface. MPEG Family of Specifications The CELP (Code Excited Linear Prediction) object type supports 8 kHz and 16 kHz sampling rates at 4-24 Kbps. CELP bitstreams can be coded in a scalable way through bit rate scalability and bandwidth scalability. The HVXC (Harmonic Vector Excitation Coding) object type gives a parametric representation of 8 kHz, mono speech at fixed bitrates between 2-4 Kbps and less than 2 Kbps using a variable bitrate mode, and supports pitch and speed changes. The TTSI (Text-to-Speech Interface) object type gives an extremely low- bitrate phonemic representation of speech. The actual text-to-speech synthesis is not specified; only the interface is defined. Bit rates range from 0.2 to 1.2 Kbps. The synthesized speech can be synchronized with a facial animation object (see below). Lastly, a number of different object types exist for synthetic sound. The Main Synthetic object type collects all MPEG-4 Structured Audio tools. Structured Audio is a way to describe methods of synthesis. It supports flexible, high-quality algorithmic synthesis through the Structured Audio Orchestra Language (SAOL) music-synthesis language, efficient Wavetable synthesis with the Structured Audio Sample-Bank Format (SASBF), and enables the use of high-quality mixing and