This European Telecommunication Standard (ETS) is an introduction to the other parts of this ETS dealing with the speech processing functions in the TETRA system. In addition to this introduction@ this ETS contains the full specification of the speech CODEC including provisions for specific cases of operation and optional functionalities plus CODEC conformance testing. In this part@ a general overview of the speech processing parts is given with reference to the ETSs where each part is specified in detail.