AAC-LD

The MPEG-4 Low Delay Audio Coder (aka AAC Low Delay, or AAC-LD) is 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.

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.

Two-way communication with AAC-LD is possible on usual analog telephone lines and via ISDN connections. Compared to known speech coders, the codec is capable of coding both music and speech signals with good quality. Unlike speech coders, however, the achieved coding quality scales up with bitrate. Transparent quality can be achieved.

AAC LD can also process stereo signals by using the advanced stereo coding tools of AAC. Thus it is possible to transmit a stereo signal with a bandwidth of 7 kHz via one ISDN line or with a bandwidth of 15 kHz via two ISDN lines.

See also

* Advanced Audio Coding (parent technology)
* MP3 (related older technology)
* G.729 (ITU-T's teleconferencing audio codec)