10.4. MPEG-4 HVXC Coding
HVXC allows coding of speech signals at very low bit rates with reasonable quality—that is, communication quality to near toll quality. In fact, two bit rates are accepted: 2 and 4 kbit/s, defining the so-called 2- and 4-kbit/s modes. This is possible due to the efficient representation of the LPC residuals, in which harmonic coding for voiced segments and VXC for unvoiced segments are employed. Major algorithmic features follow:
 Toll quality is the speech quality of public switched telephone networks, and communication quality is that of 2G cell phone networks.