Adaptive Multi-Rate (AMR) is used in speech coding and involves audio data compression. Audio data compression allows for more storage on voice files. ".amt" is a popular filename extension for AMR. Many cell phones use AMR for speech recording storage.
AMR is especially associated with the Global System for Mobile Communications (GSM). GSM, a circuit-switched system, originated in Europe in the 1980s and is used in many European countries. Several cellular companies in the United States and other non-European countries also use GSM technology.
Speech coding uses a circuit speech codec, or voice codec, that digitally transforms speech. Music can be encoded and decoded in a voice codec, but the quality of the music produced is usually quite poor. The speech or voice codec is specifically made to analyze human speech rather than to offer quality sound in other audio forms such as music.
AMR used in cellular technology is said to provide the highest quality of speech clarity for remote mobile phone communication at the lowest possible cost. AMR is thought to be a very adaptable speech codec for many different radio channels. The AMR can adjust itself by providing extra bits to help correct errors when the channel conditions are bad, while still providing only a minimum amount of bits during good channel conditions. This quality means that the AMR has more bits available for speech coding than other speech codecs.
AMR is not the first voice codec used for GSM, but the fourth. The Enhanced Full-Rate (EFR) codec and the other two speech codecs differ from AMR as they can not adjust their rate of bits produced. AMR is said to offer the best voice quality of the four speech codecs. Cellular telephone manufacturers continue to develop AMR codec technology by striving for the most natural sounding voice quality possible. These manufacturers often conduct consumer tests to test for the clearest sounding voice transmission on cellular phones.
Voice Activity Detection (VAD), Comfort Noise Generation (CNG) and Discontinuous Transmission (DTX) are some important features of AMR. VAD differentiates speech from silence to aid in speech processing and the VAD in AMR helps improve the quality of speech coding. CNG is low-level background static added on purpose to help minimize some negative effects of silence such as sudden swings in sound levels from voice to silence. DTX technology controls the transmitter switch during wireless conversations so that the battery or amplifiers are not used unnecessarily during silent periods when there is no speech.