This document specifies the voice coding method for the Project 25 System and Standard Definition IS102 (originally published as a TSB). It describes the functional requirements for the transmission and reception of voice information using digital communication media described in the standard. This document is specifically intended to define the conversion of voice from an analog representation to a digital representation that consists of a net bit rate of 4.4 kbps for voice information, and a gross bit rate of 7.2 kbps after error control coding. This standard is compatible with the requirement for voice communication over the Project 25 Common Air Interface (TIA Document No. 102BAAA).

The voice coder (or vocoder) presented in this document is intended to be used throughout Project 25 in any equipment that requires an analog-to-digital or digital-to-analog voice interface. Specifically, mobile and portable radios as well as console equipment and gateways to voice networks may contain the vocoder described in this document. The reader is referred to the Project 25 Shell Standard for additional information on the integration of the vocoder into the overall communication system.