The MPEG-4 Low Delay Audio Coder (aka AAC Low Delay, or AAC-LD) is audio compression format designed to combine the advantages of perceptual audio coding with the low delay necessary for two-way communication. It is closely derived from the MPEG-2 Advanced Audio Coding (AAC) format. It was published in MPEG-4 Audio Version 2 (ISO/IEC 14496-3:1999/Amd 1:2000)
Real time CODEC requirements
The most stringent requirements are a maximum algorithmic delay of only 20 ms and a good audio quality for all kind of audio signals including speech and music.
The AAC-LD coding scheme bridges the gap between speech coding schemes and high quality audio coding schemes.