1102 Adaptive Multi-Rate Audio AMR-NB, GSM-AMR Audio The Adaptive Multi-Rate (AMR or 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 speech starting at 7.4 kbit/s. Text 1 Digital Preservation Department / The National Archives 17 Jun 2011 17 Jun 2011 audio/amr MIME fmt/356 PUID amr File extension 524 Adaptive Multi-Rate Audio Header (ASCII): #!AMR. 1 Absolute from BOF 0 2321414D520A Can be contained by 1103 3GPP Audio/Video File