The present document 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 [2]), Voice Activity Detector
(GSM 06.42 [6]), comfort noise (GSM 06.22 [4]), Discontinuous Transmission (GSM 06.41 [5]) and example solutions
for substituting and muting of lost frames (GSM 06.21 [3]).