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The '''Adaptive Multi-Rate''' ('''AMR''', '''AMR-NB''' or '''GSM-AMR''') '''audio codec''' is an [[audio compression format]] optimized for [[speech coding]]. AMR speech codec consists of a multi-rate [[narrowband]] speech codec that encodes narrowband (200–3400 Hz) signals at variable bit rates ranging from 4.75 to 12.2 kbit/s with toll quality<ref>{{cite web |title=What's toll-quality voice? |url=https://www.itworld.com/article/2783105/what-s-toll-quality-voice-.html |website=ITworld |date=13 December 2000 |access-date=26 July 2019}}</ref> speech starting at 7.4 kbit/s.<ref>[http://tools.ietf.org/html/rfc4867#page-5 RFC 4867 - RTP Payload Format and File Storage Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs] Page 35</ref>
AMR was adopted as the standard speech [[codec]] by [[3GPP]] in October 1999 and is now widely used in [[GSM]]<ref>http://www.eetimes.com/electronics-news/4139026/Sorting-Through-GSM-Codecs-A-Tutorial</ref> and [[UMTS]]. It uses [[link adaptation]] to select from one of eight different bit rates based on link conditions.
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