LOW BIT-RATE SPEECH CODERS FOR MULTIMEDIA COMMUNICATION

The International Telecommunications Union (ITU) has recently standard ized three speech coders which are applicable to low-bit-rate multimed ia communications. ITU Rec. G.729 8 kb/s CS-ACELP has a 15 ms algorith mic codec delay and provides network-quality speech. It was originally designed for wireless applications, but is applicable to multimedia c ommunications as well. Annex A of Rec. G.729 is a reduced-complexity v ersion of the CS-ACELP coder. It was designed explicitly for simultane ous voice and data applications that are prevalent in low-bit-rate mul timedia communications. These two coders use the same bitstream format and can interoperate. The ITU Rec. G.723.1 6.3 and 5.3 kb/s speech co der for multimedia communications was designed originally for low-bit- rate videophones. Its frame size of 30 ms and one-way algorithmic code c delay of 37.5 ms allow for a further reduction in bit rate compared to the G.729 coder. In applications where low delay is important, the delay of G.723.1 may be too large. However, if the delay is acceptable , G.723.1 provides a lower-complexity alternative to G.729 at the expe nse of a slight degradation in quality. This article describes the att ributes of speech coders such as bit rate, complexity, delay, and qual ity. Then it discusses the basic concepts of the three new ITU coders by comparing their specific attributes. The second part of this articl e describes the standardization process for each of these coders.