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This document specifies the procedures to be employed to test the different enhancements of TIA-102.BABA compatible speech codecs in environmental noise conditions. The original baseline speech codec from 1992 is the Improved Multi-Band Excitation (IMBETM)1 system described in TIA-102.BABA, Project 25 Vocoder Description. The IMBE speech codec is used to digitally encode the speech signal and provide forward error control for transmission at a data rate of 7200 b/s. The baseline speech codec is defined as the reference codec for the Project 25 MOS Conformance Test, reference [2].

The testing of the codecs is based upon subjective tests which employ listening tests to judge overall speech quality.

This document takes the approach of comparing the baseline speech codec with enhanced implementations that are now available. The baseline speech codec is based upon the Digital Voice Systems Incorporated implementation of the IMBETM algorithm on the VC-20 Project 25 hardware card, or the equivalent software version. The software version is of primary interest in this test. The experiment compares the vocoders with various channel conditions as might occur on a land mobile radio channel.