AMR is a speech codec used in GSM technology for audio data compression, providing high-quality speech clarity at low cost. It is adaptable to different radio channels and has features such as VAD, CNG, and DTX. AMR is used for voice recording storage on many cell phones.
Adaptive Multi-Rate (AMR) is used in speech coding and involves compression of audio data. Audio data compression allows for more storage space on voice files. “.amt” is a popular file extension for AMR. Many cell phones use AMR for voice recording storage.
AMR is particularly 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 mobile phone companies in the United States and other non-European countries also use GSM technology.
Speech coding uses a speech circuit codec, or speech codec, that digitally transforms speech. Music can be encoded and decoded in a speech codec, but the quality of the music produced is usually quite poor. Speech or speech codec is made specifically for analyzing human speech rather than delivering quality sound in other audio forms such as music.
The AMR used in cellular technology is said to provide the highest quality speech clarity for cellular phone remote communication at the lowest possible cost. AMR is thought to be a very adaptable speech codec for many different radio channels. The AMR can self-regulate by providing extra bits to help correct errors when channel conditions are bad, while providing only a minimal amount of bits during good channel conditions. This quality means that 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 in that they cannot adjust the bit rate they produce. AMR is said to offer the best voice quality out of the four voice codecs. Mobile phone manufacturers continue to develop AMR codec technology in an effort to achieve the most natural voice quality possible. These manufacturers often conduct consumer tests to verify the clearest sounding voice transmission on cell phones.
Voice Activity Detection (VAD), Comfort Noise Generation (CNG) and Discontinuous Transmission (DTX) are some prominent 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 a low-level background static added to help minimize some negative effects of silence, such as sudden swings in sound levels from voice to silence. DTX technology controls the switch of the transmitter during wireless conversations so that the battery or amplifiers are not used unnecessarily during periods of silence when there is no speaking.
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