Adaptive Multi-Rate (AMR) is a lossy Audio data compression scheme optimized for speech coding. AMR is adopted as the standard speech codec by 3GPP.
The codec has eight bit rates, 12.2, 10.2, 7.95, 7.40, 6.70, 5.90, 5.15 and 4.75 kbit/s. The bitstream is 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 Confort Noise Generation (CNG).
The idea to use various modes arises from the network conditions. If the channel is bad source coding is reduced and channel coding is increased. This improves the quality and robustness of the network. In particular case of AMR this improvement is somewhere around 4-6 dB S/N for useable communication. The usage of AMR requires optimized link adaptation that selects the best codec mode to meet the local radio channel and capacity requirements. The new intelligent system allows the network operator to prioritize capacity or quality per base station.
See also AMR-WB