This European Telecommunication Standard (ETS) contains an electronic copy of the ANSI-C code for the GSM half rate codec. The ANSI-C code is necessary for a bit exact implementation of the half rate speech transcoder (GSM 06.20 (ETS 300 969) [2])@ Voice Activity Detector (GSM 06.42 (ETS 300 973) [6])@ comfort noise (GSM 06.22 (ETS 300 971) [4])@ Discontinuous Transmission (GSM 06.41 (ETS 300 972) [5]) and example solutions for substituting and muting of lost frames (GSM 06.21 (ETS 300 970) [3])