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'''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.
==See also==
* [[AMR-WB]]
* [[AMR-WBplus|AMR-WB+]]
* [[Half Rate]]
* [[Full Rate]]
* [[Sampling Rate]]
* [[IS-641]]
== External links ==
* [http://www.etsi.com/pressroom/previous/1998/smg%5F27%5Famr.htm Original Press Release]
* [http://www.3gpp.org/ftp/Specs/html-info/26-series.htm The official 3GPP website with AMR specifications]
* [http://www.3gpp.org/ftp/Specs/html-info/26073.htm Fixed-point ANSI-C code]
* [http://www.3gpp.org/ftp/Specs/html-info/26104.htm Floatingpoint ANSI-C code]
* [http://www.geocities.com/tvhuangsg/mobcast/ iPodder plug-in to convert Podcasts to AMR audio clips]
* [http://www.free-ringtones.ie/ringtone-converter/mp3-to-amr/ Online tool to convert MP3's to AMR]
* [http://www.faqs.org/rfcs/rfc3267.html RFC for AMR/AMR-WB Codec]
[[Category:Audio codecs]]
[[Category:Speech codecs]]
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