AAC Low Delay, or AAC-LD) is audio compression standard designed to combine the advantages of perceptual audio coding with the low delay necessary for two-way communication.
[6][7] AAC-LD uses a version of the modified discrete cosine transform (MDCT) audio coding technique called the LD-MDCT.
[8] AAC-LD is widely used by Apple as the voice-over-IP (VoIP) speech codec in FaceTime.
Two-way communication with AAC-LD is possible on usual analog telephone lines and via ISDN connections.
Unlike speech coders, however, the achieved coding quality scales up with bitrate.