The Adaptive Multi-Rate (AMR, AMR-NB, or GSM-AMR) audio codec is represented by 3GA file containers (developed by the 3rd Generation Partnership Project). This codec is designed to compress speech using link adaptation at one of eight available bit rates. AMR was adopted by 3GPP in 1999, and GSM and UMTS currently use those files. Although playing on the device is not always available, mobile devices frequently employ this file type for speech recording (such memos, recorded phone calls, etc.).
Multi-rate narrowband speech codecs in AMR speech codecs are used to encrypt signals between 200 and 3400 Hz at bit rates ranging from 4.75 to 122 kbits/s with a voice quality of 7.4 kbits/s. A component of the MPEG-4 Part 14 system are 3GA files (MP4). For easier storage on smartphones and other mobile devices, 3GA files minimise the file’s bandwidth and overhead. AMR files combine waveform signals and speech characteristics with a 20 ms delay each frame. Several codecs, including AMR-NB, AMR-WB, AMR-WB+, AAC-LC, HE-AAC v1 and v2, are supported by the 3GA container.
list of programs that can open 3GA documents:
- Windows Media Player
- QuickTime Player
- VLC Media Player