AT3 is a sound format used in Persona 1 PSP. It was developed by Sony Computer Entertainment for the Sony PlayStation Portable. It is succeeded by AT9 on the PlayStation Vita.
Usage ranges from non-looping sound effects and voices to looping background music, however, this game uses AT3's for exclusively music. To make AT3's, you should use a sample rate of 44100 hz, with 16 bit depth audio.
Command line tools to make AT3's. You can download them here.
Required to use AT3 Tools. Comes with .BAT files. Download AT3 Workspace here.