AMR (Adaptive Multi-Rate) is the family of speech codecs built for 2G and 3G mobile telephony. AMR-NB (narrowband) was the GSM/UMTS voice codec, switching among eight bitrates from 4.75 to 12.2 kbps to trade quality for error protection as radio conditions change; AMR-WB (wideband, standardized as G.722.2) added wider bandwidth for clearer 'HD Voice' on mobile. Their adaptivity — dropping bitrate to spend bits on channel coding when the signal is weak — was key to surviving the cellular radio environment. Both still appear in VoLTE calls, voicemail systems, and the .amr recordings phones produce, though VoLTE/VoNR increasingly use the newer EVS codec.
