AMR-NB

Adaptive Multi-Rate Narrow band

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 speech starting at 7.4 kbit/s.

AMR was adopted as the standard speech codec by 3GPP in October 1999 and is now widely used in GSM[4] and UMTS. It uses link adaptation to select from one of eight different bit rates based on link conditions.