What is AMR-NB (Adaptive Multi-Rate Narrowband)?

AMR-NB (Narrowband) is an adaptive multi-rate speech coder. It is utilized in Third Generation Partnership Project (3GPP) mobile communication. AMR speech codec comprises a multi-rate narrow band speech codec which encodes narrowband signals of range 200–3400 Hz at variable bit rates ranging from 4.75 to 12.2 kbit/s with toll quality speech beginning at 7.4 kbit/s.