3GA (3GPP Audio File)
The 3GA Audio file format
3GA file containers (created by the 3rd Generation Partnership Project) represent the Adaptive Multi-Rate (AMR, AMR-NB, or GSM-AMR) audio codec. This codec is meant for the compression of speech coding using link adaptation to choose from 8 different bit rates. 3GPP adopted AMR in 1999 and today those files are used by GSM and UMTS. Mobile devices often use this file type for speech recording (like memos, recorded phone calls, etc) though the playback on the device is not always possible.
Technical details of 3GA files
AMR speech codecs contain multi-rate narrowband speech codecs which encode 200-3400 Hz signals at bit rates between 4.75 and 122 kbits/s with a speech quality at 7.4 kbits/s. 3GA files are part of the MPEG-4 Part 14 system (MP4). 3GA files reduce the overhead of storage and bandwidth of the file for ease of storage on smartphones and similar mobile devices. AMR files are hybrids of speech parameters and waveform signals with a delay of 20 ms per frame. The 3GA container can use various codecs including AMR-NB, AMR-WB, AMR-WB+, AAC-LC, HE-AAC v1 and HE-AAC v2.