Subjective performance evaluation of the GSM half-rate coding algorithm (with voice signals)

The Pan-European cellular digital mobile radio system GSM uses a codec with a net bit rate of 13.0 kbit/s (gross bit rate including error protection 22.8 kbit/s), known as the "full rate" RPE-LTP (regular pulse excitation with long term prediction). GSM is now ready to dub channel capacity with the adoption of a new algorithm as an ETSI Recommendation, the candidate codec being called appropriately "half-rate" (gross bit rate 11.4 kbit/s). Internationally coordinated series of subjective listening experiments were planned and carried out during the exercise. Four main phases were necessary, called qualification, selection(s), optimisation and characterisation. The paper describes the tests performed and gives an outline of the performance of the codec with voice signals under realistic network conditions. The effects on the speech performance produced by the voice activity detector and related DTX system are not the main subject of the paper but information on this topic is given.