Adaptive Multi-Rate (AMR) is an audio data compression scheme optimized for speech coding. AMR was adopted as the standard speech codec by 3GPP in October 1998 and is now widely used in GSM. It uses link adaptation to select from one of eight different bit rates based on link conditions.
The bit rates 12.2, 10.2, 7.95, 7.40, 6.70, 5.90, 5.15 and 4.75 kbit/s are based on frames which contain 160 samples and are 20 milliseconds long. AMR uses different techniques, such as Algebraic Code Excited Linear Prediction (ACELP), Discontinuous Transmission (DTX), voice activity detection (VAD) and comfort noise generation (CNG). The usage of AMR requires optimized link adaptation that selects the best codec mode to meet the local radio channel and capacity requirements. If the radio conditions are bad, source coding is reduced and channel coding is increased. This improves the quality and robustness of the network connection while sacrificing some voice clarity. In the particular case of AMR this improvement is somewhere around 4-6 dB S/N for useable communication. The new intelligent system allows the network operator to prioritize capacity or quality per base station.